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@settitle GNAT Reference Manual
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GNAT Reference Manual , March 24, 2015

AdaCore

Copyright @copyright{} 2008-2015, Free Software Foundation
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@titlepage
@title GNAT Reference Manual
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@contents

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@emph{GNAT, The GNU Ada Development Environment}


@include gcc-common.texi
GCC version @value{version-GCC}@*
AdaCore

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being "GNAT Reference
Manual", and with no Back-Cover Texts.  A copy of the license is
included in the section entitled @ref{1,,GNU Free Documentation License}.

@menu
* About This Guide:: 
* Implementation Defined Pragmas:: 
* Implementation Defined Aspects:: 
* Implementation Defined Attributes:: 
* Standard and Implementation Defined Restrictions:: 
* Implementation Advice:: 
* Implementation Defined Characteristics:: 
* Intrinsic Subprograms:: 
* Representation Clauses and Pragmas:: 
* Standard Library Routines:: 
* The Implementation of Standard I/O:: 
* The GNAT Library:: 
* Interfacing to Other Languages:: 
* Specialized Needs Annexes:: 
* Implementation of Specific Ada Features:: 
* Implementation of Ada 2012 Features:: 
* Obsolescent Features:: 
* Compatibility and Porting Guide:: 
* GNU Free Documentation License:: 
* Index:: 

@detailmenu
 --- The Detailed Node Listing ---

About This Guide

* What This Reference Manual Contains:: 
* Conventions:: 
* Related Information:: 

Implementation Defined Pragmas

* Pragma Abort_Defer:: 
* Pragma Abstract_State:: 
* Pragma Ada_83:: 
* Pragma Ada_95:: 
* Pragma Ada_05:: 
* Pragma Ada_2005:: 
* Pragma Ada_12:: 
* Pragma Ada_2012:: 
* Pragma Allow_Integer_Address:: 
* Pragma Annotate:: 
* Pragma Assert:: 
* Pragma Assert_And_Cut:: 
* Pragma Assertion_Policy:: 
* Pragma Assume:: 
* Pragma Assume_No_Invalid_Values:: 
* Pragma Async_Readers:: 
* Pragma Async_Writers:: 
* Pragma Attribute_Definition:: 
* Pragma C_Pass_By_Copy:: 
* Pragma Check:: 
* Pragma Check_Float_Overflow:: 
* Pragma Check_Name:: 
* Pragma Check_Policy:: 
* Pragma CIL_Constructor:: 
* Pragma Comment:: 
* Pragma Common_Object:: 
* Pragma Compile_Time_Error:: 
* Pragma Compile_Time_Warning:: 
* Pragma Compiler_Unit:: 
* Pragma Compiler_Unit_Warning:: 
* Pragma Complete_Representation:: 
* Pragma Complex_Representation:: 
* Pragma Component_Alignment:: 
* Pragma Contract_Cases:: 
* Pragma Convention_Identifier:: 
* Pragma CPP_Class:: 
* Pragma CPP_Constructor:: 
* Pragma CPP_Virtual:: 
* Pragma CPP_Vtable:: 
* Pragma CPU:: 
* Pragma Debug:: 
* Pragma Debug_Policy:: 
* Pragma Default_Scalar_Storage_Order:: 
* Pragma Default_Storage_Pool:: 
* Pragma Depends:: 
* Pragma Detect_Blocking:: 
* Pragma Disable_Atomic_Synchronization:: 
* Pragma Dispatching_Domain:: 
* Pragma Effective_Reads:: 
* Pragma Effective_Writes:: 
* Pragma Elaboration_Checks:: 
* Pragma Eliminate:: 
* Pragma Enable_Atomic_Synchronization:: 
* Pragma Export_Function:: 
* Pragma Export_Object:: 
* Pragma Export_Procedure:: 
* Pragma Export_Value:: 
* Pragma Export_Valued_Procedure:: 
* Pragma Extend_System:: 
* Pragma Extensions_Allowed:: 
* Pragma External:: 
* Pragma External_Name_Casing:: 
* Pragma Fast_Math:: 
* Pragma Favor_Top_Level:: 
* Pragma Finalize_Storage_Only:: 
* Pragma Float_Representation:: 
* Pragma Global:: 
* Pragma Ident:: 
* Pragma Ignore_Pragma:: 
* Pragma Implementation_Defined:: 
* Pragma Implemented:: 
* Pragma Implicit_Packing:: 
* Pragma Import_Function:: 
* Pragma Import_Object:: 
* Pragma Import_Procedure:: 
* Pragma Import_Valued_Procedure:: 
* Pragma Independent:: 
* Pragma Independent_Components:: 
* Pragma Initial_Condition:: 
* Pragma Initialize_Scalars:: 
* Pragma Initializes:: 
* Pragma Inline_Always:: 
* Pragma Inline_Generic:: 
* Pragma Interface:: 
* Pragma Interface_Name:: 
* Pragma Interrupt_Handler:: 
* Pragma Interrupt_State:: 
* Pragma Invariant:: 
* Pragma Java_Constructor:: 
* Pragma Java_Interface:: 
* Pragma Keep_Names:: 
* Pragma License:: 
* Pragma Link_With:: 
* Pragma Linker_Alias:: 
* Pragma Linker_Constructor:: 
* Pragma Linker_Destructor:: 
* Pragma Linker_Section:: 
* Pragma Lock_Free:: 
* Pragma Loop_Invariant:: 
* Pragma Loop_Optimize:: 
* Pragma Loop_Variant:: 
* Pragma Machine_Attribute:: 
* Pragma Main:: 
* Pragma Main_Storage:: 
* Pragma No_Body:: 
* Pragma No_Elaboration_Code_All:: 
* Pragma No_Inline:: 
* Pragma No_Return:: 
* Pragma No_Run_Time:: 
* Pragma No_Strict_Aliasing:: 
* Pragma No_Tagged_Streams:: 
* Pragma Normalize_Scalars:: 
* Pragma Obsolescent:: 
* Pragma Optimize_Alignment:: 
* Pragma Ordered:: 
* Pragma Overflow_Mode:: 
* Pragma Overriding_Renamings:: 
* Pragma Partition_Elaboration_Policy:: 
* Pragma Part_Of:: 
* Pragma Passive:: 
* Pragma Persistent_BSS:: 
* Pragma Polling:: 
* Pragma Post:: 
* Pragma Postcondition:: 
* Pragma Post_Class:: 
* Pragma Pre:: 
* Pragma Precondition:: 
* Pragma Predicate:: 
* Pragma Preelaborable_Initialization:: 
* Pragma Prefix_Exception_Messages:: 
* Pragma Pre_Class:: 
* Pragma Priority_Specific_Dispatching:: 
* Pragma Profile:: 
* Pragma Profile_Warnings:: 
* Pragma Propagate_Exceptions:: 
* Pragma Provide_Shift_Operators:: 
* Pragma Psect_Object:: 
* Pragma Pure_Function:: 
* Pragma Rational:: 
* Pragma Ravenscar:: 
* Pragma Refined_Depends:: 
* Pragma Refined_Global:: 
* Pragma Refined_Post:: 
* Pragma Refined_State:: 
* Pragma Relative_Deadline:: 
* Pragma Remote_Access_Type:: 
* Pragma Restricted_Run_Time:: 
* Pragma Restriction_Warnings:: 
* Pragma Reviewable:: 
* Pragma Share_Generic:: 
* Pragma Shared:: 
* Pragma Short_Circuit_And_Or:: 
* Pragma Short_Descriptors:: 
* Pragma Simple_Storage_Pool_Type:: 
* Pragma Source_File_Name:: 
* Pragma Source_File_Name_Project:: 
* Pragma Source_Reference:: 
* Pragma SPARK_Mode:: 
* Pragma Static_Elaboration_Desired:: 
* Pragma Stream_Convert:: 
* Pragma Style_Checks:: 
* Pragma Subtitle:: 
* Pragma Suppress:: 
* Pragma Suppress_All:: 
* Pragma Suppress_Debug_Info:: 
* Pragma Suppress_Exception_Locations:: 
* Pragma Suppress_Initialization:: 
* Pragma Task_Name:: 
* Pragma Task_Storage:: 
* Pragma Test_Case:: 
* Pragma Thread_Local_Storage:: 
* Pragma Time_Slice:: 
* Pragma Title:: 
* Pragma Type_Invariant:: 
* Pragma Type_Invariant_Class:: 
* Pragma Unchecked_Union:: 
* Pragma Unevaluated_Use_Of_Old:: 
* Pragma Unimplemented_Unit:: 
* Pragma Universal_Aliasing:: 
* Pragma Universal_Data:: 
* Pragma Unmodified:: 
* Pragma Unreferenced:: 
* Pragma Unreferenced_Objects:: 
* Pragma Unreserve_All_Interrupts:: 
* Pragma Unsuppress:: 
* Pragma Use_VADS_Size:: 
* Pragma Validity_Checks:: 
* Pragma Volatile:: 
* Pragma Warning_As_Error:: 
* Pragma Warnings:: 
* Pragma Weak_External:: 
* Pragma Wide_Character_Encoding:: 

Implementation Defined Aspects

* Aspect Abstract_State:: 
* Annotate:: 
* Aspect Async_Readers:: 
* Aspect Async_Writers:: 
* Aspect Contract_Cases:: 
* Aspect Depends:: 
* Aspect Dimension:: 
* Aspect Dimension_System:: 
* Aspect Effective_Reads:: 
* Aspect Effective_Writes:: 
* Aspect Favor_Top_Level:: 
* Aspect Global:: 
* Aspect Initial_Condition:: 
* Aspect Initializes:: 
* Aspect Inline_Always:: 
* Aspect Invariant:: 
* Aspect Invariant'Class:: 
* Aspect Iterable:: 
* Aspect Linker_Section:: 
* Aspect Lock_Free:: 
* Aspect No_Elaboration_Code_All:: 
* Aspect No_Tagged_Streams:: 
* Aspect Object_Size:: 
* Aspect Obsolescent:: 
* Aspect Part_Of:: 
* Aspect Persistent_BSS:: 
* Aspect Predicate:: 
* Aspect Pure_Function:: 
* Aspect Refined_Depends:: 
* Aspect Refined_Global:: 
* Aspect Refined_Post:: 
* Aspect Refined_State:: 
* Aspect Remote_Access_Type:: 
* Aspect Scalar_Storage_Order:: 
* Aspect Shared:: 
* Aspect Simple_Storage_Pool:: 
* Aspect Simple_Storage_Pool_Type:: 
* Aspect SPARK_Mode:: 
* Aspect Suppress_Debug_Info:: 
* Aspect Suppress_Initialization:: 
* Aspect Test_Case:: 
* Aspect Thread_Local_Storage:: 
* Aspect Universal_Aliasing:: 
* Aspect Universal_Data:: 
* Aspect Unmodified:: 
* Aspect Unreferenced:: 
* Aspect Unreferenced_Objects:: 
* Aspect Value_Size:: 
* Aspect Warnings:: 

Implementation Defined Attributes

* Attribute Abort_Signal:: 
* Attribute Address_Size:: 
* Attribute Asm_Input:: 
* Attribute Asm_Output:: 
* Attribute Atomic_Always_Lock_Free:: 
* Attribute Bit:: 
* Attribute Bit_Position:: 
* Attribute Code_Address:: 
* Attribute Compiler_Version:: 
* Attribute Constrained:: 
* Attribute Default_Bit_Order:: 
* Attribute Default_Scalar_Storage_Order:: 
* Attribute Deref:: 
* Attribute Descriptor_Size:: 
* Attribute Elaborated:: 
* Attribute Elab_Body:: 
* Attribute Elab_Spec:: 
* Attribute Elab_Subp_Body:: 
* Attribute Emax:: 
* Attribute Enabled:: 
* Attribute Enum_Rep:: 
* Attribute Enum_Val:: 
* Attribute Epsilon:: 
* Attribute Fast_Math:: 
* Attribute Fixed_Value:: 
* Attribute From_Any:: 
* Attribute Has_Access_Values:: 
* Attribute Has_Discriminants:: 
* Attribute Img:: 
* Attribute Integer_Value:: 
* Attribute Invalid_Value:: 
* Attribute Iterable:: 
* Attribute Large:: 
* Attribute Library_Level:: 
* Attribute Lock_Free:: 
* Attribute Loop_Entry:: 
* Attribute Machine_Size:: 
* Attribute Mantissa:: 
* Attribute Maximum_Alignment:: 
* Attribute Mechanism_Code:: 
* Attribute Null_Parameter:: 
* Attribute Object_Size:: 
* Attribute Old:: 
* Attribute Passed_By_Reference:: 
* Attribute Pool_Address:: 
* Attribute Range_Length:: 
* Attribute Ref:: 
* Attribute Restriction_Set:: 
* Attribute Result:: 
* Attribute Safe_Emax:: 
* Attribute Safe_Large:: 
* Attribute Safe_Small:: 
* Attribute Scalar_Storage_Order:: 
* Attribute Simple_Storage_Pool:: 
* Attribute Small:: 
* Attribute Storage_Unit:: 
* Attribute Stub_Type:: 
* Attribute System_Allocator_Alignment:: 
* Attribute Target_Name:: 
* Attribute To_Address:: 
* Attribute To_Any:: 
* Attribute Type_Class:: 
* Attribute Type_Key:: 
* Attribute TypeCode:: 
* Attribute UET_Address:: 
* Attribute Unconstrained_Array:: 
* Attribute Universal_Literal_String:: 
* Attribute Unrestricted_Access:: 
* Attribute Update:: 
* Attribute Valid_Scalars:: 
* Attribute VADS_Size:: 
* Attribute Value_Size:: 
* Attribute Wchar_T_Size:: 
* Attribute Word_Size:: 

Standard and Implementation Defined Restrictions

* Partition-Wide Restrictions:: 
* Program Unit Level Restrictions:: 

Partition-Wide Restrictions

* Immediate_Reclamation:: 
* Max_Asynchronous_Select_Nesting:: 
* Max_Entry_Queue_Length:: 
* Max_Protected_Entries:: 
* Max_Select_Alternatives:: 
* Max_Storage_At_Blocking:: 
* Max_Task_Entries:: 
* Max_Tasks:: 
* No_Abort_Statements:: 
* No_Access_Parameter_Allocators:: 
* No_Access_Subprograms:: 
* No_Allocators:: 
* No_Anonymous_Allocators:: 
* No_Calendar:: 
* No_Coextensions:: 
* No_Default_Initialization:: 
* No_Delay:: 
* No_Dependence:: 
* No_Direct_Boolean_Operators:: 
* No_Dispatch:: 
* No_Dispatching_Calls:: 
* No_Dynamic_Attachment:: 
* No_Dynamic_Priorities:: 
* No_Entry_Calls_In_Elaboration_Code:: 
* No_Enumeration_Maps:: 
* No_Exception_Handlers:: 
* No_Exception_Propagation:: 
* No_Exception_Registration:: 
* No_Exceptions:: 
* No_Finalization:: 
* No_Fixed_Point:: 
* No_Floating_Point:: 
* No_Implicit_Conditionals:: 
* No_Implicit_Dynamic_Code:: 
* No_Implicit_Heap_Allocations:: 
* No_Implicit_Loops:: 
* No_Initialize_Scalars:: 
* No_IO:: 
* No_Local_Allocators:: 
* No_Local_Protected_Objects:: 
* No_Local_Timing_Events:: 
* No_Long_Long_Integers:: 
* No_Multiple_Elaboration:: 
* No_Nested_Finalization:: 
* No_Protected_Type_Allocators:: 
* No_Protected_Types:: 
* No_Recursion:: 
* No_Reentrancy:: 
* No_Relative_Delay:: 
* No_Requeue_Statements:: 
* No_Secondary_Stack:: 
* No_Select_Statements:: 
* No_Specific_Termination_Handlers:: 
* No_Specification_of_Aspect:: 
* No_Standard_Allocators_After_Elaboration:: 
* No_Standard_Storage_Pools:: 
* No_Stream_Optimizations:: 
* No_Streams:: 
* No_Task_Allocators:: 
* No_Task_Attributes_Package:: 
* No_Task_Hierarchy:: 
* No_Task_Termination:: 
* No_Tasking:: 
* No_Terminate_Alternatives:: 
* No_Unchecked_Access:: 
* No_Use_Of_Entity:: 
* Simple_Barriers:: 
* Static_Priorities:: 
* Static_Storage_Size:: 

Program Unit Level Restrictions

* No_Elaboration_Code:: 
* No_Entry_Queue:: 
* No_Implementation_Aspect_Specifications:: 
* No_Implementation_Attributes:: 
* No_Implementation_Identifiers:: 
* No_Implementation_Pragmas:: 
* No_Implementation_Restrictions:: 
* No_Implementation_Units:: 
* No_Implicit_Aliasing:: 
* No_Obsolescent_Features:: 
* No_Wide_Characters:: 
* SPARK_05:: 

Implementation Advice

* RM 1.1.3(20); Error Detection: RM 1 1 3 20 Error Detection. 
* RM 1.1.3(31); Child Units: RM 1 1 3 31 Child Units. 
* RM 1.1.5(12); Bounded Errors: RM 1 1 5 12 Bounded Errors. 
* RM 2.8(16); Pragmas: RM 2 8 16 Pragmas. 
* RM 2.8(17-19); Pragmas: RM 2 8 17-19 Pragmas. 
* RM 3.5.2(5); Alternative Character Sets: RM 3 5 2 5 Alternative Character Sets. 
* RM 3.5.4(28); Integer Types: RM 3 5 4 28 Integer Types. 
* RM 3.5.4(29); Integer Types: RM 3 5 4 29 Integer Types. 
* RM 3.5.5(8); Enumeration Values: RM 3 5 5 8 Enumeration Values. 
* RM 3.5.7(17); Float Types: RM 3 5 7 17 Float Types. 
* RM 3.6.2(11); Multidimensional Arrays: RM 3 6 2 11 Multidimensional Arrays. 
* RM 9.6(30-31); Duration'Small: RM 9 6 30-31 Duration'Small. 
* RM 10.2.1(12); Consistent Representation: RM 10 2 1 12 Consistent Representation. 
* RM 11.4.1(19); Exception Information: RM 11 4 1 19 Exception Information. 
* RM 11.5(28); Suppression of Checks: RM 11 5 28 Suppression of Checks. 
* RM 13.1 (21-24); Representation Clauses: RM 13 1 21-24 Representation Clauses. 
* RM 13.2(6-8); Packed Types: RM 13 2 6-8 Packed Types. 
* RM 13.3(14-19); Address Clauses: RM 13 3 14-19 Address Clauses. 
* RM 13.3(29-35); Alignment Clauses: RM 13 3 29-35 Alignment Clauses. 
* RM 13.3(42-43); Size Clauses: RM 13 3 42-43 Size Clauses. 
* RM 13.3(50-56); Size Clauses: RM 13 3 50-56 Size Clauses. 
* RM 13.3(71-73); Component Size Clauses: RM 13 3 71-73 Component Size Clauses. 
* RM 13.4(9-10); Enumeration Representation Clauses: RM 13 4 9-10 Enumeration Representation Clauses. 
* RM 13.5.1(17-22); Record Representation Clauses: RM 13 5 1 17-22 Record Representation Clauses. 
* RM 13.5.2(5); Storage Place Attributes: RM 13 5 2 5 Storage Place Attributes. 
* RM 13.5.3(7-8); Bit Ordering: RM 13 5 3 7-8 Bit Ordering. 
* RM 13.7(37); Address as Private: RM 13 7 37 Address as Private. 
* RM 13.7.1(16); Address Operations: RM 13 7 1 16 Address Operations. 
* RM 13.9(14-17); Unchecked Conversion: RM 13 9 14-17 Unchecked Conversion. 
* RM 13.11(23-25); Implicit Heap Usage: RM 13 11 23-25 Implicit Heap Usage. 
* RM 13.11.2(17); Unchecked Deallocation: RM 13 11 2 17 Unchecked Deallocation. 
* RM 13.13.2(17); Stream Oriented Attributes: RM 13 13 2 17 Stream Oriented Attributes. 
* RM A.1(52); Names of Predefined Numeric Types: RM A 1 52 Names of Predefined Numeric Types. 
* RM A.3.2(49); Ada.Characters.Handling: RM A 3 2 49 Ada Characters Handling. 
* RM A.4.4(106); Bounded-Length String Handling: RM A 4 4 106 Bounded-Length String Handling. 
* RM A.5.2(46-47); Random Number Generation: RM A 5 2 46-47 Random Number Generation. 
* RM A.10.7(23); Get_Immediate: RM A 10 7 23 Get_Immediate. 
* RM B.1(39-41); Pragma Export: RM B 1 39-41 Pragma Export. 
* RM B.2(12-13); Package Interfaces: RM B 2 12-13 Package Interfaces. 
* RM B.3(63-71); Interfacing with C: RM B 3 63-71 Interfacing with C. 
* RM B.4(95-98); Interfacing with COBOL: RM B 4 95-98 Interfacing with COBOL. 
* RM B.5(22-26); Interfacing with Fortran: RM B 5 22-26 Interfacing with Fortran. 
* RM C.1(3-5); Access to Machine Operations: RM C 1 3-5 Access to Machine Operations. 
* RM C.1(10-16); Access to Machine Operations: RM C 1 10-16 Access to Machine Operations. 
* RM C.3(28); Interrupt Support: RM C 3 28 Interrupt Support. 
* RM C.3.1(20-21); Protected Procedure Handlers: RM C 3 1 20-21 Protected Procedure Handlers. 
* RM C.3.2(25); Package Interrupts: RM C 3 2 25 Package Interrupts. 
* RM C.4(14); Pre-elaboration Requirements: RM C 4 14 Pre-elaboration Requirements. 
* RM C.5(8); Pragma Discard_Names: RM C 5 8 Pragma Discard_Names. 
* RM C.7.2(30); The Package Task_Attributes: RM C 7 2 30 The Package Task_Attributes. 
* RM D.3(17); Locking Policies: RM D 3 17 Locking Policies. 
* RM D.4(16); Entry Queuing Policies: RM D 4 16 Entry Queuing Policies. 
* RM D.6(9-10); Preemptive Abort: RM D 6 9-10 Preemptive Abort. 
* RM D.7(21); Tasking Restrictions: RM D 7 21 Tasking Restrictions. 
* RM D.8(47-49); Monotonic Time: RM D 8 47-49 Monotonic Time. 
* RM E.5(28-29); Partition Communication Subsystem: RM E 5 28-29 Partition Communication Subsystem. 
* RM F(7); COBOL Support: RM F 7 COBOL Support. 
* RM F.1(2); Decimal Radix Support: RM F 1 2 Decimal Radix Support. 
* RM G; Numerics: RM G Numerics. 
* RM G.1.1(56-58); Complex Types: RM G 1 1 56-58 Complex Types. 
* RM G.1.2(49); Complex Elementary Functions: RM G 1 2 49 Complex Elementary Functions. 
* RM G.2.4(19); Accuracy Requirements: RM G 2 4 19 Accuracy Requirements. 
* RM G.2.6(15); Complex Arithmetic Accuracy: RM G 2 6 15 Complex Arithmetic Accuracy. 
* RM H.6(15/2); Pragma Partition_Elaboration_Policy: RM H 6 15/2 Pragma Partition_Elaboration_Policy. 

Intrinsic Subprograms

* Intrinsic Operators:: 
* Compilation_Date:: 
* Compilation_Time:: 
* Enclosing_Entity:: 
* Exception_Information:: 
* Exception_Message:: 
* Exception_Name:: 
* File:: 
* Line:: 
* Shifts and Rotates:: 
* Source_Location:: 

Representation Clauses and Pragmas

* Alignment Clauses:: 
* Size Clauses:: 
* Storage_Size Clauses:: 
* Size of Variant Record Objects:: 
* Biased Representation:: 
* Value_Size and Object_Size Clauses:: 
* Component_Size Clauses:: 
* Bit_Order Clauses:: 
* Effect of Bit_Order on Byte Ordering:: 
* Pragma Pack for Arrays:: 
* Pragma Pack for Records:: 
* Record Representation Clauses:: 
* Handling of Records with Holes:: 
* Enumeration Clauses:: 
* Address Clauses:: 
* Use of Address Clauses for Memory-Mapped I/O:: 
* Effect of Convention on Representation:: 
* Conventions and Anonymous Access Types:: 
* Determining the Representations chosen by GNAT:: 

The Implementation of Standard I/O

* Standard I/O Packages:: 
* FORM Strings:: 
* Direct_IO:: 
* Sequential_IO:: 
* Text_IO:: 
* Wide_Text_IO:: 
* Wide_Wide_Text_IO:: 
* Stream_IO:: 
* Text Translation:: 
* Shared Files:: 
* Filenames encoding:: 
* File content encoding:: 
* Open Modes:: 
* Operations on C Streams:: 
* Interfacing to C Streams:: 

Text_IO

* Stream Pointer Positioning:: 
* Reading and Writing Non-Regular Files:: 
* Get_Immediate:: 
* Treating Text_IO Files as Streams:: 
* Text_IO Extensions:: 
* Text_IO Facilities for Unbounded Strings:: 

Wide_Text_IO

* Stream Pointer Positioning: Stream Pointer Positioning<2>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<2>. 

Wide_Wide_Text_IO

* Stream Pointer Positioning: Stream Pointer Positioning<3>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<3>. 

The GNAT Library

* Ada.Characters.Latin_9 (a-chlat9.ads): Ada Characters Latin_9 a-chlat9 ads. 
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads): Ada Characters Wide_Latin_1 a-cwila1 ads. 
* Ada.Characters.Wide_Latin_9 (a-cwila1.ads): Ada Characters Wide_Latin_9 a-cwila1 ads. 
* Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads): Ada Characters Wide_Wide_Latin_1 a-chzla1 ads. 
* Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads): Ada Characters Wide_Wide_Latin_9 a-chzla9 ads. 
* Ada.Containers.Formal_Doubly_Linked_Lists (a-cfdlli.ads): Ada Containers Formal_Doubly_Linked_Lists a-cfdlli ads. 
* Ada.Containers.Formal_Hashed_Maps (a-cfhama.ads): Ada Containers Formal_Hashed_Maps a-cfhama ads. 
* Ada.Containers.Formal_Hashed_Sets (a-cfhase.ads): Ada Containers Formal_Hashed_Sets a-cfhase ads. 
* Ada.Containers.Formal_Ordered_Maps (a-cforma.ads): Ada Containers Formal_Ordered_Maps a-cforma ads. 
* Ada.Containers.Formal_Ordered_Sets (a-cforse.ads): Ada Containers Formal_Ordered_Sets a-cforse ads. 
* Ada.Containers.Formal_Vectors (a-cofove.ads): Ada Containers Formal_Vectors a-cofove ads. 
* Ada.Containers.Formal_Indefinite_Vectors (a-cfinve.ads): Ada Containers Formal_Indefinite_Vectors a-cfinve ads. 
* Ada.Containers.Bounded_Holders (a-coboho.ads): Ada Containers Bounded_Holders a-coboho ads. 
* Ada.Command_Line.Environment (a-colien.ads): Ada Command_Line Environment a-colien ads. 
* Ada.Command_Line.Remove (a-colire.ads): Ada Command_Line Remove a-colire ads. 
* Ada.Command_Line.Response_File (a-clrefi.ads): Ada Command_Line Response_File a-clrefi ads. 
* Ada.Direct_IO.C_Streams (a-diocst.ads): Ada Direct_IO C_Streams a-diocst ads. 
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads): Ada Exceptions Is_Null_Occurrence a-einuoc ads. 
* Ada.Exceptions.Last_Chance_Handler (a-elchha.ads): Ada Exceptions Last_Chance_Handler a-elchha ads. 
* Ada.Exceptions.Traceback (a-exctra.ads): Ada Exceptions Traceback a-exctra ads. 
* Ada.Sequential_IO.C_Streams (a-siocst.ads): Ada Sequential_IO C_Streams a-siocst ads. 
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads): Ada Streams Stream_IO C_Streams a-ssicst ads. 
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads): Ada Strings Unbounded Text_IO a-suteio ads. 
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads): Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads. 
* Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads): Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads. 
* Ada.Text_IO.C_Streams (a-tiocst.ads): Ada Text_IO C_Streams a-tiocst ads. 
* Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads): Ada Text_IO Reset_Standard_Files a-tirsfi ads. 
* Ada.Wide_Characters.Unicode (a-wichun.ads): Ada Wide_Characters Unicode a-wichun ads. 
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads): Ada Wide_Text_IO C_Streams a-wtcstr ads. 
* Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads): Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads. 
* Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads): Ada Wide_Wide_Characters Unicode a-zchuni ads. 
* Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads): Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads. 
* Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads): Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads. 
* GNAT.Altivec (g-altive.ads): GNAT Altivec g-altive ads. 
* GNAT.Altivec.Conversions (g-altcon.ads): GNAT Altivec Conversions g-altcon ads. 
* GNAT.Altivec.Vector_Operations (g-alveop.ads): GNAT Altivec Vector_Operations g-alveop ads. 
* GNAT.Altivec.Vector_Types (g-alvety.ads): GNAT Altivec Vector_Types g-alvety ads. 
* GNAT.Altivec.Vector_Views (g-alvevi.ads): GNAT Altivec Vector_Views g-alvevi ads. 
* GNAT.Array_Split (g-arrspl.ads): GNAT Array_Split g-arrspl ads. 
* GNAT.AWK (g-awk.ads): GNAT AWK g-awk ads. 
* GNAT.Bounded_Buffers (g-boubuf.ads): GNAT Bounded_Buffers g-boubuf ads. 
* GNAT.Bounded_Mailboxes (g-boumai.ads): GNAT Bounded_Mailboxes g-boumai ads. 
* GNAT.Bubble_Sort (g-bubsor.ads): GNAT Bubble_Sort g-bubsor ads. 
* GNAT.Bubble_Sort_A (g-busora.ads): GNAT Bubble_Sort_A g-busora ads. 
* GNAT.Bubble_Sort_G (g-busorg.ads): GNAT Bubble_Sort_G g-busorg ads. 
* GNAT.Byte_Order_Mark (g-byorma.ads): GNAT Byte_Order_Mark g-byorma ads. 
* GNAT.Byte_Swapping (g-bytswa.ads): GNAT Byte_Swapping g-bytswa ads. 
* GNAT.Calendar (g-calend.ads): GNAT Calendar g-calend ads. 
* GNAT.Calendar.Time_IO (g-catiio.ads): GNAT Calendar Time_IO g-catiio ads. 
* GNAT.CRC32 (g-crc32.ads): GNAT CRC32 g-crc32 ads. 
* GNAT.Case_Util (g-casuti.ads): GNAT Case_Util g-casuti ads. 
* GNAT.CGI (g-cgi.ads): GNAT CGI g-cgi ads. 
* GNAT.CGI.Cookie (g-cgicoo.ads): GNAT CGI Cookie g-cgicoo ads. 
* GNAT.CGI.Debug (g-cgideb.ads): GNAT CGI Debug g-cgideb ads. 
* GNAT.Command_Line (g-comlin.ads): GNAT Command_Line g-comlin ads. 
* GNAT.Compiler_Version (g-comver.ads): GNAT Compiler_Version g-comver ads. 
* GNAT.Ctrl_C (g-ctrl_c.ads): GNAT Ctrl_C g-ctrl_c ads. 
* GNAT.Current_Exception (g-curexc.ads): GNAT Current_Exception g-curexc ads. 
* GNAT.Debug_Pools (g-debpoo.ads): GNAT Debug_Pools g-debpoo ads. 
* GNAT.Debug_Utilities (g-debuti.ads): GNAT Debug_Utilities g-debuti ads. 
* GNAT.Decode_String (g-decstr.ads): GNAT Decode_String g-decstr ads. 
* GNAT.Decode_UTF8_String (g-deutst.ads): GNAT Decode_UTF8_String g-deutst ads. 
* GNAT.Directory_Operations (g-dirope.ads): GNAT Directory_Operations g-dirope ads. 
* GNAT.Directory_Operations.Iteration (g-diopit.ads): GNAT Directory_Operations Iteration g-diopit ads. 
* GNAT.Dynamic_HTables (g-dynhta.ads): GNAT Dynamic_HTables g-dynhta ads. 
* GNAT.Dynamic_Tables (g-dyntab.ads): GNAT Dynamic_Tables g-dyntab ads. 
* GNAT.Encode_String (g-encstr.ads): GNAT Encode_String g-encstr ads. 
* GNAT.Encode_UTF8_String (g-enutst.ads): GNAT Encode_UTF8_String g-enutst ads. 
* GNAT.Exception_Actions (g-excact.ads): GNAT Exception_Actions g-excact ads. 
* GNAT.Exception_Traces (g-exctra.ads): GNAT Exception_Traces g-exctra ads. 
* GNAT.Exceptions (g-expect.ads): GNAT Exceptions g-expect ads. 
* GNAT.Expect (g-expect.ads): GNAT Expect g-expect ads. 
* GNAT.Expect.TTY (g-exptty.ads): GNAT Expect TTY g-exptty ads. 
* GNAT.Float_Control (g-flocon.ads): GNAT Float_Control g-flocon ads. 
* GNAT.Formatted_String (g-forstr.ads): GNAT Formatted_String g-forstr ads. 
* GNAT.Heap_Sort (g-heasor.ads): GNAT Heap_Sort g-heasor ads. 
* GNAT.Heap_Sort_A (g-hesora.ads): GNAT Heap_Sort_A g-hesora ads. 
* GNAT.Heap_Sort_G (g-hesorg.ads): GNAT Heap_Sort_G g-hesorg ads. 
* GNAT.HTable (g-htable.ads): GNAT HTable g-htable ads. 
* GNAT.IO (g-io.ads): GNAT IO g-io ads. 
* GNAT.IO_Aux (g-io_aux.ads): GNAT IO_Aux g-io_aux ads. 
* GNAT.Lock_Files (g-locfil.ads): GNAT Lock_Files g-locfil ads. 
* GNAT.MBBS_Discrete_Random (g-mbdira.ads): GNAT MBBS_Discrete_Random g-mbdira ads. 
* GNAT.MBBS_Float_Random (g-mbflra.ads): GNAT MBBS_Float_Random g-mbflra ads. 
* GNAT.MD5 (g-md5.ads): GNAT MD5 g-md5 ads. 
* GNAT.Memory_Dump (g-memdum.ads): GNAT Memory_Dump g-memdum ads. 
* GNAT.Most_Recent_Exception (g-moreex.ads): GNAT Most_Recent_Exception g-moreex ads. 
* GNAT.OS_Lib (g-os_lib.ads): GNAT OS_Lib g-os_lib ads. 
* GNAT.Perfect_Hash_Generators (g-pehage.ads): GNAT Perfect_Hash_Generators g-pehage ads. 
* GNAT.Random_Numbers (g-rannum.ads): GNAT Random_Numbers g-rannum ads. 
* GNAT.Regexp (g-regexp.ads): GNAT Regexp g-regexp ads. 
* GNAT.Registry (g-regist.ads): GNAT Registry g-regist ads. 
* GNAT.Regpat (g-regpat.ads): GNAT Regpat g-regpat ads. 
* GNAT.Rewrite_Data (g-rewdat.ads): GNAT Rewrite_Data g-rewdat ads. 
* GNAT.Secondary_Stack_Info (g-sestin.ads): GNAT Secondary_Stack_Info g-sestin ads. 
* GNAT.Semaphores (g-semaph.ads): GNAT Semaphores g-semaph ads. 
* GNAT.Serial_Communications (g-sercom.ads): GNAT Serial_Communications g-sercom ads. 
* GNAT.SHA1 (g-sha1.ads): GNAT SHA1 g-sha1 ads. 
* GNAT.SHA224 (g-sha224.ads): GNAT SHA224 g-sha224 ads. 
* GNAT.SHA256 (g-sha256.ads): GNAT SHA256 g-sha256 ads. 
* GNAT.SHA384 (g-sha384.ads): GNAT SHA384 g-sha384 ads. 
* GNAT.SHA512 (g-sha512.ads): GNAT SHA512 g-sha512 ads. 
* GNAT.Signals (g-signal.ads): GNAT Signals g-signal ads. 
* GNAT.Sockets (g-socket.ads): GNAT Sockets g-socket ads. 
* GNAT.Source_Info (g-souinf.ads): GNAT Source_Info g-souinf ads. 
* GNAT.Spelling_Checker (g-speche.ads): GNAT Spelling_Checker g-speche ads. 
* GNAT.Spelling_Checker_Generic (g-spchge.ads): GNAT Spelling_Checker_Generic g-spchge ads. 
* GNAT.Spitbol.Patterns (g-spipat.ads): GNAT Spitbol Patterns g-spipat ads. 
* GNAT.Spitbol (g-spitbo.ads): GNAT Spitbol g-spitbo ads. 
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads): GNAT Spitbol Table_Boolean g-sptabo ads. 
* GNAT.Spitbol.Table_Integer (g-sptain.ads): GNAT Spitbol Table_Integer g-sptain ads. 
* GNAT.Spitbol.Table_VString (g-sptavs.ads): GNAT Spitbol Table_VString g-sptavs ads. 
* GNAT.SSE (g-sse.ads): GNAT SSE g-sse ads. 
* GNAT.SSE.Vector_Types (g-ssvety.ads): GNAT SSE Vector_Types g-ssvety ads. 
* GNAT.Strings (g-string.ads): GNAT Strings g-string ads. 
* GNAT.String_Split (g-strspl.ads): GNAT String_Split g-strspl ads. 
* GNAT.Table (g-table.ads): GNAT Table g-table ads. 
* GNAT.Task_Lock (g-tasloc.ads): GNAT Task_Lock g-tasloc ads. 
* GNAT.Time_Stamp (g-timsta.ads): GNAT Time_Stamp g-timsta ads. 
* GNAT.Threads (g-thread.ads): GNAT Threads g-thread ads. 
* GNAT.Traceback (g-traceb.ads): GNAT Traceback g-traceb ads. 
* GNAT.Traceback.Symbolic (g-trasym.ads): GNAT Traceback Symbolic g-trasym ads. 
* GNAT.UTF_32 (g-table.ads): GNAT UTF_32 g-table ads. 
* GNAT.Wide_Spelling_Checker (g-u3spch.ads): GNAT Wide_Spelling_Checker g-u3spch ads. 
* GNAT.Wide_Spelling_Checker (g-wispch.ads): GNAT Wide_Spelling_Checker g-wispch ads. 
* GNAT.Wide_String_Split (g-wistsp.ads): GNAT Wide_String_Split g-wistsp ads. 
* GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads): GNAT Wide_Wide_Spelling_Checker g-zspche ads. 
* GNAT.Wide_Wide_String_Split (g-zistsp.ads): GNAT Wide_Wide_String_Split g-zistsp ads. 
* Interfaces.C.Extensions (i-cexten.ads): Interfaces C Extensions i-cexten ads. 
* Interfaces.C.Streams (i-cstrea.ads): Interfaces C Streams i-cstrea ads. 
* Interfaces.Packed_Decimal (i-pacdec.ads): Interfaces Packed_Decimal i-pacdec ads. 
* Interfaces.VxWorks (i-vxwork.ads): Interfaces VxWorks i-vxwork ads. 
* Interfaces.VxWorks.IO (i-vxwoio.ads): Interfaces VxWorks IO i-vxwoio ads. 
* System.Address_Image (s-addima.ads): System Address_Image s-addima ads. 
* System.Assertions (s-assert.ads): System Assertions s-assert ads. 
* System.Atomic_Counters (s-atocou.ads): System Atomic_Counters s-atocou ads. 
* System.Memory (s-memory.ads): System Memory s-memory ads. 
* System.Multiprocessors (s-multip.ads): System Multiprocessors s-multip ads. 
* System.Multiprocessors.Dispatching_Domains (s-mudido.ads): System Multiprocessors Dispatching_Domains s-mudido ads. 
* System.Partition_Interface (s-parint.ads): System Partition_Interface s-parint ads. 
* System.Pool_Global (s-pooglo.ads): System Pool_Global s-pooglo ads. 
* System.Pool_Local (s-pooloc.ads): System Pool_Local s-pooloc ads. 
* System.Restrictions (s-restri.ads): System Restrictions s-restri ads. 
* System.Rident (s-rident.ads): System Rident s-rident ads. 
* System.Strings.Stream_Ops (s-ststop.ads): System Strings Stream_Ops s-ststop ads. 
* System.Unsigned_Types (s-unstyp.ads): System Unsigned_Types s-unstyp ads. 
* System.Wch_Cnv (s-wchcnv.ads): System Wch_Cnv s-wchcnv ads. 
* System.Wch_Con (s-wchcon.ads): System Wch_Con s-wchcon ads. 

Interfacing to Other Languages

* Interfacing to C:: 
* Interfacing to C++:: 
* Interfacing to COBOL:: 
* Interfacing to Fortran:: 
* Interfacing to non-GNAT Ada code:: 

Implementation of Specific Ada Features

* Machine Code Insertions:: 
* GNAT Implementation of Tasking:: 
* GNAT Implementation of Shared Passive Packages:: 
* Code Generation for Array Aggregates:: 
* The Size of Discriminated Records with Default Discriminants:: 
* Strict Conformance to the Ada Reference Manual:: 

GNAT Implementation of Tasking

* Mapping Ada Tasks onto the Underlying Kernel Threads:: 
* Ensuring Compliance with the Real-Time Annex:: 

Code Generation for Array Aggregates

* Static constant aggregates with static bounds:: 
* Constant aggregates with unconstrained nominal types:: 
* Aggregates with static bounds:: 
* Aggregates with non-static bounds:: 
* Aggregates in assignment statements:: 

Obsolescent Features

* pragma No_Run_Time:: 
* pragma Ravenscar:: 
* pragma Restricted_Run_Time:: 
* pragma Task_Info:: 
* package System.Task_Info (s-tasinf.ads): package System Task_Info s-tasinf ads. 

Compatibility and Porting Guide

* Writing Portable Fixed-Point Declarations:: 
* Compatibility with Ada 83:: 
* Compatibility between Ada 95 and Ada 2005:: 
* Implementation-dependent characteristics:: 
* Compatibility with Other Ada Systems:: 
* Representation Clauses:: 
* Compatibility with HP Ada 83:: 

Compatibility with Ada 83

* Legal Ada 83 programs that are illegal in Ada 95:: 
* More deterministic semantics:: 
* Changed semantics:: 
* Other language compatibility issues:: 

Implementation-dependent characteristics

* Implementation-defined pragmas:: 
* Implementation-defined attributes:: 
* Libraries:: 
* Elaboration order:: 
* Target-specific aspects:: 

@end detailmenu
@end menu

@node About This Guide,Implementation Defined Pragmas,Top,Top
@anchor{gnat_rm/about_this_guide about-this-guide}@anchor{2}@anchor{gnat_rm/about_this_guide doc}@anchor{3}@anchor{gnat_rm/about_this_guide gnat-reference-manual}@anchor{4}@anchor{gnat_rm/about_this_guide id1}@anchor{5}
@chapter About This Guide



This manual contains useful information in writing programs using the
GNAT compiler.  It includes information on implementation dependent
characteristics of GNAT, including all the information required by
Annex M of the Ada language standard.

GNAT implements Ada 95, Ada 2005 and Ada 2012, and it may also be
invoked in Ada 83 compatibility mode.
By default, GNAT assumes Ada 2012,
but you can override with a compiler switch
to explicitly specify the language version.
(Please refer to the @emph{GNAT User's Guide} for details on these switches.)
Throughout this manual, references to 'Ada' without a year suffix
apply to all the Ada versions of the language.

Ada is designed to be highly portable.
In general, a program will have the same effect even when compiled by
different compilers on different platforms.
However, since Ada is designed to be used in a
wide variety of applications, it also contains a number of system
dependent features to be used in interfacing to the external world.
.. index:: Implementation-dependent features

@geindex Portability

Note: Any program that makes use of implementation-dependent features
may be non-portable.  You should follow good programming practice and
isolate and clearly document any sections of your program that make use
of these features in a non-portable manner.

@menu
* What This Reference Manual Contains:: 
* Conventions:: 
* Related Information:: 

@end menu

@node What This Reference Manual Contains,Conventions,,About This Guide
@anchor{gnat_rm/about_this_guide what-this-reference-manual-contains}@anchor{6}
@section What This Reference Manual Contains


This reference manual contains the following chapters:


@itemize *

@item 
@ref{7,,Implementation Defined Pragmas}, lists GNAT implementation-dependent
pragmas, which can be used to extend and enhance the functionality of the
compiler.

@item 
@ref{8,,Implementation Defined Attributes}, lists GNAT
implementation-dependent attributes, which can be used to extend and
enhance the functionality of the compiler.

@item 
@ref{9,,Standard and Implementation Defined Restrictions}, lists GNAT
implementation-dependent restrictions, which can be used to extend and
enhance the functionality of the compiler.

@item 
@ref{a,,Implementation Advice}, provides information on generally
desirable behavior which are not requirements that all compilers must
follow since it cannot be provided on all systems, or which may be
undesirable on some systems.

@item 
@ref{b,,Implementation Defined Characteristics}, provides a guide to
minimizing implementation dependent features.

@item 
@ref{c,,Intrinsic Subprograms}, describes the intrinsic subprograms
implemented by GNAT, and how they can be imported into user
application programs.

@item 
@ref{d,,Representation Clauses and Pragmas}, describes in detail the
way that GNAT represents data, and in particular the exact set
of representation clauses and pragmas that is accepted.

@item 
@ref{e,,Standard Library Routines}, provides a listing of packages and a
brief description of the functionality that is provided by Ada's
extensive set of standard library routines as implemented by GNAT.

@item 
@ref{f,,The Implementation of Standard I/O}, details how the GNAT
implementation of the input-output facilities.

@item 
@ref{10,,The GNAT Library}, is a catalog of packages that complement
the Ada predefined library.

@item 
@ref{11,,Interfacing to Other Languages}, describes how programs
written in Ada using GNAT can be interfaced to other programming
languages.

@item 
@ref{12,,Specialized Needs Annexes}, describes the GNAT implementation of all
of the specialized needs annexes.

@item 
@ref{13,,Implementation of Specific Ada Features}, discusses issues related
to GNAT's implementation of machine code insertions, tasking, and several
other features.

@item 
@ref{14,,Implementation of Ada 2012 Features}, describes the status of the
GNAT implementation of the Ada 2012 language standard.

@item 
@ref{15,,Obsolescent Features} documents implementation dependent features,
including pragmas and attributes, which are considered obsolescent, since
there are other preferred ways of achieving the same results. These
obsolescent forms are retained for backwards compatibility.

@item 
@ref{16,,Compatibility and Porting Guide} presents some guidelines for
developing portable Ada code, describes the compatibility issues that
may arise between GNAT and other Ada compilation systems (including those
for Ada 83), and shows how GNAT can expedite porting applications
developed in other Ada environments.

@item 
@ref{1,,GNU Free Documentation License} contains the license for this document.
@end itemize

@geindex Ada 95 Language Reference Manual

@geindex Ada 2005 Language Reference Manual

This reference manual assumes a basic familiarity with the Ada 95 language, as
described in the
@cite{International Standard ANSI/ISO/IEC-8652:1995}.
It does not require knowledge of the new features introduced by Ada 2005 or
Ada 2012.
All three reference manuals are included in the GNAT documentation
package.

@node Conventions,Related Information,What This Reference Manual Contains,About This Guide
@anchor{gnat_rm/about_this_guide conventions}@anchor{17}
@section Conventions


@geindex Conventions
@geindex typographical

@geindex Typographical conventions

Following are examples of the typographical and graphic conventions used
in this guide:


@itemize *

@item 
@cite{Functions}, @cite{utility program names}, @cite{standard names},
and @cite{classes}.

@item 
@cite{Option flags}

@item 
@code{File names}

@item 
@cite{Variables}

@item 
@emph{Emphasis}

@item 
[optional information or parameters]

@item 
Examples are described by text

@example
and then shown this way.
@end example

@item 
Commands that are entered by the user are shown as preceded by a prompt string
comprising the @code{$} character followed by a space.
@end itemize

@node Related Information,,Conventions,About This Guide
@anchor{gnat_rm/about_this_guide related-information}@anchor{18}
@section Related Information


See the following documents for further information on GNAT:


@itemize *

@item 
@cite{GNAT User's Guide for Native Platforms},
which provides information on how to use the
GNAT development environment.

@item 
@cite{Ada 95 Reference Manual}, the Ada 95 programming language standard.

@item 
@cite{Ada 95 Annotated Reference Manual}, which is an annotated version
of the Ada 95 standard.  The annotations describe
detailed aspects of the design decision, and in particular contain useful
sections on Ada 83 compatibility.

@item 
@cite{Ada 2005 Reference Manual}, the Ada 2005 programming language standard.

@item 
@cite{Ada 2005 Annotated Reference Manual}, which is an annotated version
of the Ada 2005 standard.  The annotations describe
detailed aspects of the design decision.

@item 
@cite{Ada 2012 Reference Manual}, the Ada 2012 programming language standard.

@item 
@cite{DEC Ada@comma{} Technical Overview and Comparison on DIGITAL Platforms},
which contains specific information on compatibility between GNAT and
DEC Ada 83 systems.

@item 
@cite{DEC Ada@comma{} Language Reference Manual}, part number AA-PYZAB-TK, which
describes in detail the pragmas and attributes provided by the DEC Ada 83
compiler system.
@end itemize

@node Implementation Defined Pragmas,Implementation Defined Aspects,About This Guide,Top
@anchor{gnat_rm/implementation_defined_pragmas implementation-defined-pragmas}@anchor{7}@anchor{gnat_rm/implementation_defined_pragmas doc}@anchor{19}@anchor{gnat_rm/implementation_defined_pragmas id1}@anchor{1a}
@chapter Implementation Defined Pragmas


Ada defines a set of pragmas that can be used to supply additional
information to the compiler.  These language defined pragmas are
implemented in GNAT and work as described in the Ada Reference Manual.

In addition, Ada allows implementations to define additional pragmas
whose meaning is defined by the implementation.  GNAT provides a number
of these implementation-defined pragmas, which can be used to extend
and enhance the functionality of the compiler.  This section of the GNAT
Reference Manual describes these additional pragmas.

Note that any program using these pragmas might not be portable to other
compilers (although GNAT implements this set of pragmas on all
platforms).  Therefore if portability to other compilers is an important
consideration, the use of these pragmas should be minimized.

@menu
* Pragma Abort_Defer:: 
* Pragma Abstract_State:: 
* Pragma Ada_83:: 
* Pragma Ada_95:: 
* Pragma Ada_05:: 
* Pragma Ada_2005:: 
* Pragma Ada_12:: 
* Pragma Ada_2012:: 
* Pragma Allow_Integer_Address:: 
* Pragma Annotate:: 
* Pragma Assert:: 
* Pragma Assert_And_Cut:: 
* Pragma Assertion_Policy:: 
* Pragma Assume:: 
* Pragma Assume_No_Invalid_Values:: 
* Pragma Async_Readers:: 
* Pragma Async_Writers:: 
* Pragma Attribute_Definition:: 
* Pragma C_Pass_By_Copy:: 
* Pragma Check:: 
* Pragma Check_Float_Overflow:: 
* Pragma Check_Name:: 
* Pragma Check_Policy:: 
* Pragma CIL_Constructor:: 
* Pragma Comment:: 
* Pragma Common_Object:: 
* Pragma Compile_Time_Error:: 
* Pragma Compile_Time_Warning:: 
* Pragma Compiler_Unit:: 
* Pragma Compiler_Unit_Warning:: 
* Pragma Complete_Representation:: 
* Pragma Complex_Representation:: 
* Pragma Component_Alignment:: 
* Pragma Contract_Cases:: 
* Pragma Convention_Identifier:: 
* Pragma CPP_Class:: 
* Pragma CPP_Constructor:: 
* Pragma CPP_Virtual:: 
* Pragma CPP_Vtable:: 
* Pragma CPU:: 
* Pragma Debug:: 
* Pragma Debug_Policy:: 
* Pragma Default_Scalar_Storage_Order:: 
* Pragma Default_Storage_Pool:: 
* Pragma Depends:: 
* Pragma Detect_Blocking:: 
* Pragma Disable_Atomic_Synchronization:: 
* Pragma Dispatching_Domain:: 
* Pragma Effective_Reads:: 
* Pragma Effective_Writes:: 
* Pragma Elaboration_Checks:: 
* Pragma Eliminate:: 
* Pragma Enable_Atomic_Synchronization:: 
* Pragma Export_Function:: 
* Pragma Export_Object:: 
* Pragma Export_Procedure:: 
* Pragma Export_Value:: 
* Pragma Export_Valued_Procedure:: 
* Pragma Extend_System:: 
* Pragma Extensions_Allowed:: 
* Pragma External:: 
* Pragma External_Name_Casing:: 
* Pragma Fast_Math:: 
* Pragma Favor_Top_Level:: 
* Pragma Finalize_Storage_Only:: 
* Pragma Float_Representation:: 
* Pragma Global:: 
* Pragma Ident:: 
* Pragma Ignore_Pragma:: 
* Pragma Implementation_Defined:: 
* Pragma Implemented:: 
* Pragma Implicit_Packing:: 
* Pragma Import_Function:: 
* Pragma Import_Object:: 
* Pragma Import_Procedure:: 
* Pragma Import_Valued_Procedure:: 
* Pragma Independent:: 
* Pragma Independent_Components:: 
* Pragma Initial_Condition:: 
* Pragma Initialize_Scalars:: 
* Pragma Initializes:: 
* Pragma Inline_Always:: 
* Pragma Inline_Generic:: 
* Pragma Interface:: 
* Pragma Interface_Name:: 
* Pragma Interrupt_Handler:: 
* Pragma Interrupt_State:: 
* Pragma Invariant:: 
* Pragma Java_Constructor:: 
* Pragma Java_Interface:: 
* Pragma Keep_Names:: 
* Pragma License:: 
* Pragma Link_With:: 
* Pragma Linker_Alias:: 
* Pragma Linker_Constructor:: 
* Pragma Linker_Destructor:: 
* Pragma Linker_Section:: 
* Pragma Lock_Free:: 
* Pragma Loop_Invariant:: 
* Pragma Loop_Optimize:: 
* Pragma Loop_Variant:: 
* Pragma Machine_Attribute:: 
* Pragma Main:: 
* Pragma Main_Storage:: 
* Pragma No_Body:: 
* Pragma No_Elaboration_Code_All:: 
* Pragma No_Inline:: 
* Pragma No_Return:: 
* Pragma No_Run_Time:: 
* Pragma No_Strict_Aliasing:: 
* Pragma No_Tagged_Streams:: 
* Pragma Normalize_Scalars:: 
* Pragma Obsolescent:: 
* Pragma Optimize_Alignment:: 
* Pragma Ordered:: 
* Pragma Overflow_Mode:: 
* Pragma Overriding_Renamings:: 
* Pragma Partition_Elaboration_Policy:: 
* Pragma Part_Of:: 
* Pragma Passive:: 
* Pragma Persistent_BSS:: 
* Pragma Polling:: 
* Pragma Post:: 
* Pragma Postcondition:: 
* Pragma Post_Class:: 
* Pragma Pre:: 
* Pragma Precondition:: 
* Pragma Predicate:: 
* Pragma Preelaborable_Initialization:: 
* Pragma Prefix_Exception_Messages:: 
* Pragma Pre_Class:: 
* Pragma Priority_Specific_Dispatching:: 
* Pragma Profile:: 
* Pragma Profile_Warnings:: 
* Pragma Propagate_Exceptions:: 
* Pragma Provide_Shift_Operators:: 
* Pragma Psect_Object:: 
* Pragma Pure_Function:: 
* Pragma Rational:: 
* Pragma Ravenscar:: 
* Pragma Refined_Depends:: 
* Pragma Refined_Global:: 
* Pragma Refined_Post:: 
* Pragma Refined_State:: 
* Pragma Relative_Deadline:: 
* Pragma Remote_Access_Type:: 
* Pragma Restricted_Run_Time:: 
* Pragma Restriction_Warnings:: 
* Pragma Reviewable:: 
* Pragma Share_Generic:: 
* Pragma Shared:: 
* Pragma Short_Circuit_And_Or:: 
* Pragma Short_Descriptors:: 
* Pragma Simple_Storage_Pool_Type:: 
* Pragma Source_File_Name:: 
* Pragma Source_File_Name_Project:: 
* Pragma Source_Reference:: 
* Pragma SPARK_Mode:: 
* Pragma Static_Elaboration_Desired:: 
* Pragma Stream_Convert:: 
* Pragma Style_Checks:: 
* Pragma Subtitle:: 
* Pragma Suppress:: 
* Pragma Suppress_All:: 
* Pragma Suppress_Debug_Info:: 
* Pragma Suppress_Exception_Locations:: 
* Pragma Suppress_Initialization:: 
* Pragma Task_Name:: 
* Pragma Task_Storage:: 
* Pragma Test_Case:: 
* Pragma Thread_Local_Storage:: 
* Pragma Time_Slice:: 
* Pragma Title:: 
* Pragma Type_Invariant:: 
* Pragma Type_Invariant_Class:: 
* Pragma Unchecked_Union:: 
* Pragma Unevaluated_Use_Of_Old:: 
* Pragma Unimplemented_Unit:: 
* Pragma Universal_Aliasing:: 
* Pragma Universal_Data:: 
* Pragma Unmodified:: 
* Pragma Unreferenced:: 
* Pragma Unreferenced_Objects:: 
* Pragma Unreserve_All_Interrupts:: 
* Pragma Unsuppress:: 
* Pragma Use_VADS_Size:: 
* Pragma Validity_Checks:: 
* Pragma Volatile:: 
* Pragma Warning_As_Error:: 
* Pragma Warnings:: 
* Pragma Weak_External:: 
* Pragma Wide_Character_Encoding:: 

@end menu

@node Pragma Abort_Defer,Pragma Abstract_State,,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-abort-defer}@anchor{1b}
@section Pragma Abort_Defer


@geindex Deferring aborts

Syntax:

@example
pragma Abort_Defer;
@end example

This pragma must appear at the start of the statement sequence of a
handled sequence of statements (right after the @cite{begin}).  It has
the effect of deferring aborts for the sequence of statements (but not
for the declarations or handlers, if any, associated with this statement
sequence).

@node Pragma Abstract_State,Pragma Ada_83,Pragma Abort_Defer,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-abstract-state}@anchor{1c}
@section Pragma Abstract_State


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.4.

@node Pragma Ada_83,Pragma Ada_95,Pragma Abstract_State,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-83}@anchor{1d}
@section Pragma Ada_83


Syntax:

@example
pragma Ada_83;
@end example

A configuration pragma that establishes Ada 83 mode for the unit to
which it applies, regardless of the mode set by the command line
switches.  In Ada 83 mode, GNAT attempts to be as compatible with
the syntax and semantics of Ada 83, as defined in the original Ada
83 Reference Manual as possible.  In particular, the keywords added by Ada 95
and Ada 2005 are not recognized, optional package bodies are allowed,
and generics may name types with unknown discriminants without using
the @cite{(<>)} notation.  In addition, some but not all of the additional
restrictions of Ada 83 are enforced.

Ada 83 mode is intended for two purposes.  Firstly, it allows existing
Ada 83 code to be compiled and adapted to GNAT with less effort.
Secondly, it aids in keeping code backwards compatible with Ada 83.
However, there is no guarantee that code that is processed correctly
by GNAT in Ada 83 mode will in fact compile and execute with an Ada
83 compiler, since GNAT does not enforce all the additional checks
required by Ada 83.

@node Pragma Ada_95,Pragma Ada_05,Pragma Ada_83,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-95}@anchor{1e}
@section Pragma Ada_95


Syntax:

@example
pragma Ada_95;
@end example

A configuration pragma that establishes Ada 95 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the @cite{Ada} and @cite{System}
packages and their children, so you need not specify it in these
contexts.  This pragma is useful when writing a reusable component that
itself uses Ada 95 features, but which is intended to be usable from
either Ada 83 or Ada 95 programs.

@node Pragma Ada_05,Pragma Ada_2005,Pragma Ada_95,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-05}@anchor{1f}
@section Pragma Ada_05


Syntax:

@example
pragma Ada_05;
pragma Ada_05 (local_NAME);
@end example

A configuration pragma that establishes Ada 2005 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This pragma is useful when writing a reusable component that
itself uses Ada 2005 features, but which is intended to be usable from
either Ada 83 or Ada 95 programs.

The one argument form (which is not a configuration pragma)
is used for managing the transition from
Ada 95 to Ada 2005 in the run-time library. If an entity is marked
as Ada_2005 only, then referencing the entity in Ada_83 or Ada_95
mode will generate a warning. In addition, in Ada_83 or Ada_95
mode, a preference rule is established which does not choose
such an entity unless it is unambiguously specified. This avoids
extra subprograms marked this way from generating ambiguities in
otherwise legal pre-Ada_2005 programs. The one argument form is
intended for exclusive use in the GNAT run-time library.

@node Pragma Ada_2005,Pragma Ada_12,Pragma Ada_05,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-2005}@anchor{20}
@section Pragma Ada_2005


Syntax:

@example
pragma Ada_2005;
@end example

This configuration pragma is a synonym for pragma Ada_05 and has the
same syntax and effect.

@node Pragma Ada_12,Pragma Ada_2012,Pragma Ada_2005,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-12}@anchor{21}
@section Pragma Ada_12


Syntax:

@example
pragma Ada_12;
pragma Ada_12 (local_NAME);
@end example

A configuration pragma that establishes Ada 2012 mode for the unit to which
it applies, regardless of the mode set by the command line switches.
This mode is set automatically for the @cite{Ada} and @cite{System}
packages and their children, so you need not specify it in these
contexts.  This pragma is useful when writing a reusable component that
itself uses Ada 2012 features, but which is intended to be usable from
Ada 83, Ada 95, or Ada 2005 programs.

The one argument form, which is not a configuration pragma,
is used for managing the transition from Ada
2005 to Ada 2012 in the run-time library. If an entity is marked
as Ada_201 only, then referencing the entity in any pre-Ada_2012
mode will generate a warning. In addition, in any pre-Ada_2012
mode, a preference rule is established which does not choose
such an entity unless it is unambiguously specified. This avoids
extra subprograms marked this way from generating ambiguities in
otherwise legal pre-Ada_2012 programs. The one argument form is
intended for exclusive use in the GNAT run-time library.

@node Pragma Ada_2012,Pragma Allow_Integer_Address,Pragma Ada_12,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ada-2012}@anchor{22}
@section Pragma Ada_2012


Syntax:

@example
pragma Ada_2012;
@end example

This configuration pragma is a synonym for pragma Ada_12 and has the
same syntax and effect.

@node Pragma Allow_Integer_Address,Pragma Annotate,Pragma Ada_2012,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-allow-integer-address}@anchor{23}
@section Pragma Allow_Integer_Address


Syntax:

@example
pragma Allow_Integer_Address;
@end example

In almost all versions of GNAT, @cite{System.Address} is a private
type in accordance with the implementation advice in the RM. This
means that integer values,
in particular integer literals, are not allowed as address values.
If the configuration pragma
@cite{Allow_Integer_Address} is given, then integer expressions may
be used anywhere a value of type @cite{System.Address} is required.
The effect is to introduce an implicit unchecked conversion from the
integer value to type @cite{System.Address}. The reverse case of using
an address where an integer type is required is handled analogously.
The following example compiles without errors:

@example
pragma Allow_Integer_Address;
with System; use System;
package AddrAsInt is
   X : Integer;
   Y : Integer;
   for X'Address use 16#1240#;
   for Y use at 16#3230#;
   m : Address := 16#4000#;
   n : constant Address := 4000;
   p : constant Address := Address (X + Y);
   v : Integer := y'Address;
   w : constant Integer := Integer (Y'Address);
   type R is new integer;
   RR : R := 1000;
   Z : Integer;
   for Z'Address use RR;
end AddrAsInt;
@end example

Note that pragma @cite{Allow_Integer_Address} is ignored if @cite{System.Address}
is not a private type. In implementations of @cite{GNAT} where
System.Address is a visible integer type,
this pragma serves no purpose but is ignored
rather than rejected to allow common sets of sources to be used
in the two situations.

@node Pragma Annotate,Pragma Assert,Pragma Allow_Integer_Address,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-annotate}@anchor{24}
@section Pragma Annotate


Syntax:

@example
pragma Annotate (IDENTIFIER [, IDENTIFIER @{, ARG@}] [, entity => local_NAME]);

ARG ::= NAME | EXPRESSION
@end example

This pragma is used to annotate programs.  @cite{identifier} identifies
the type of annotation.  GNAT verifies that it is an identifier, but does
not otherwise analyze it. The second optional identifier is also left
unanalyzed, and by convention is used to control the action of the tool to
which the annotation is addressed.  The remaining @cite{arg} arguments
can be either string literals or more generally expressions.
String literals are assumed to be either of type
@cite{Standard.String} or else @cite{Wide_String} or @cite{Wide_Wide_String}
depending on the character literals they contain.
All other kinds of arguments are analyzed as expressions, and must be
unambiguous. The last argument if present must have the identifier
@cite{Entity} and GNAT verifies that a local name is given.

The analyzed pragma is retained in the tree, but not otherwise processed
by any part of the GNAT compiler, except to generate corresponding note
lines in the generated ALI file. For the format of these note lines, see
the compiler source file lib-writ.ads. This pragma is intended for use by
external tools, including ASIS. The use of pragma Annotate does not
affect the compilation process in any way. This pragma may be used as
a configuration pragma.

@node Pragma Assert,Pragma Assert_And_Cut,Pragma Annotate,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assert}@anchor{25}
@section Pragma Assert


Syntax:

@example
pragma Assert (
  boolean_EXPRESSION
  [, string_EXPRESSION]);
@end example

The effect of this pragma depends on whether the corresponding command
line switch is set to activate assertions.  The pragma expands into code
equivalent to the following:

@example
if assertions-enabled then
   if not boolean_EXPRESSION then
      System.Assertions.Raise_Assert_Failure
        (string_EXPRESSION);
   end if;
end if;
@end example

The string argument, if given, is the message that will be associated
with the exception occurrence if the exception is raised.  If no second
argument is given, the default message is @cite{file}:@cite{nnn},
where @cite{file} is the name of the source file containing the assert,
and @cite{nnn} is the line number of the assert.  A pragma is not a
statement, so if a statement sequence contains nothing but a pragma
assert, then a null statement is required in addition, as in:

@example
...
if J > 3 then
   pragma Assert (K > 3, "Bad value for K");
   null;
end if;
@end example

Note that, as with the @cite{if} statement to which it is equivalent, the
type of the expression is either @cite{Standard.Boolean}, or any type derived
from this standard type.

Assert checks can be either checked or ignored. By default they are ignored.
They will be checked if either the command line switch @emph{-gnata} is
used, or if an @cite{Assertion_Policy} or @cite{Check_Policy} pragma is used
to enable @cite{Assert_Checks}.

If assertions are ignored, then there
is no run-time effect (and in particular, any side effects from the
expression will not occur at run time).  (The expression is still
analyzed at compile time, and may cause types to be frozen if they are
mentioned here for the first time).

If assertions are checked, then the given expression is tested, and if
it is @cite{False} then @cite{System.Assertions.Raise_Assert_Failure} is called
which results in the raising of @cite{Assert_Failure} with the given message.

You should generally avoid side effects in the expression arguments of
this pragma, because these side effects will turn on and off with the
setting of the assertions mode, resulting in assertions that have an
effect on the program.  However, the expressions are analyzed for
semantic correctness whether or not assertions are enabled, so turning
assertions on and off cannot affect the legality of a program.

Note that the implementation defined policy @cite{DISABLE}, given in a
pragma @cite{Assertion_Policy}, can be used to suppress this semantic analysis.

Note: this is a standard language-defined pragma in versions
of Ada from 2005 on. In GNAT, it is implemented in all versions
of Ada, and the DISABLE policy is an implementation-defined
addition.

@node Pragma Assert_And_Cut,Pragma Assertion_Policy,Pragma Assert,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assert-and-cut}@anchor{26}
@section Pragma Assert_And_Cut


Syntax:

@example
pragma Assert_And_Cut (
  boolean_EXPRESSION
  [, string_EXPRESSION]);
@end example

The effect of this pragma is identical to that of pragma @cite{Assert},
except that in an @cite{Assertion_Policy} pragma, the identifier
@cite{Assert_And_Cut} is used to control whether it is ignored or checked
(or disabled).

The intention is that this be used within a subprogram when the
given test expresion sums up all the work done so far in the
subprogram, so that the rest of the subprogram can be verified
(informally or formally) using only the entry preconditions,
and the expression in this pragma. This allows dividing up
a subprogram into sections for the purposes of testing or
formal verification. The pragma also serves as useful
documentation.

@node Pragma Assertion_Policy,Pragma Assume,Pragma Assert_And_Cut,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assertion-policy}@anchor{27}
@section Pragma Assertion_Policy


Syntax:

@example
pragma Assertion_Policy (CHECK | DISABLE | IGNORE);

pragma Assertion_Policy (
    ASSERTION_KIND => POLICY_IDENTIFIER
 @{, ASSERTION_KIND => POLICY_IDENTIFIER@});

ASSERTION_KIND ::= RM_ASSERTION_KIND | ID_ASSERTION_KIND

RM_ASSERTION_KIND ::= Assert               |
                      Static_Predicate     |
                      Dynamic_Predicate    |
                      Pre                  |
                      Pre'Class            |
                      Post                 |
                      Post'Class           |
                      Type_Invariant       |
                      Type_Invariant'Class

ID_ASSERTION_KIND ::= Assertions           |
                      Assert_And_Cut       |
                      Assume               |
                      Contract_Cases       |
                      Debug                |
                      Invariant            |
                      Invariant'Class      |
                      Loop_Invariant       |
                      Loop_Variant         |
                      Postcondition        |
                      Precondition         |
                      Predicate            |
                      Refined_Post         |
                      Statement_Assertions

POLICY_IDENTIFIER ::= Check | Disable | Ignore
@end example

This is a standard Ada 2012 pragma that is available as an
implementation-defined pragma in earlier versions of Ada.
The assertion kinds @cite{RM_ASSERTION_KIND} are those defined in
the Ada standard. The assertion kinds @cite{ID_ASSERTION_KIND}
are implementation defined additions recognized by the GNAT compiler.

The pragma applies in both cases to pragmas and aspects with matching
names, e.g. @cite{Pre} applies to the Pre aspect, and @cite{Precondition}
applies to both the @cite{Precondition} pragma
and the aspect @cite{Precondition}. Note that the identifiers for
pragmas Pre_Class and Post_Class are Pre'Class and Post'Class (not
Pre_Class and Post_Class), since these pragmas are intended to be
identical to the corresponding aspects).

If the policy is @cite{CHECK}, then assertions are enabled, i.e.
the corresponding pragma or aspect is activated.
If the policy is @cite{IGNORE}, then assertions are ignored, i.e.
the corresponding pragma or aspect is deactivated.
This pragma overrides the effect of the @emph{-gnata} switch on the
command line.

The implementation defined policy @cite{DISABLE} is like
@cite{IGNORE} except that it completely disables semantic
checking of the corresponding pragma or aspect. This is
useful when the pragma or aspect argument references subprograms
in a with'ed package which is replaced by a dummy package
for the final build.

The implementation defined assertion kind @cite{Assertions} applies to all
assertion kinds. The form with no assertion kind given implies this
choice, so it applies to all assertion kinds (RM defined, and
implementation defined).

The implementation defined assertion kind @cite{Statement_Assertions}
applies to @cite{Assert}, @cite{Assert_And_Cut},
@cite{Assume}, @cite{Loop_Invariant}, and @cite{Loop_Variant}.

@node Pragma Assume,Pragma Assume_No_Invalid_Values,Pragma Assertion_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assume}@anchor{28}
@section Pragma Assume


Syntax:

@example
pragma Assume (
  boolean_EXPRESSION
  [, string_EXPRESSION]);
@end example

The effect of this pragma is identical to that of pragma @cite{Assert},
except that in an @cite{Assertion_Policy} pragma, the identifier
@cite{Assume} is used to control whether it is ignored or checked
(or disabled).

The intention is that this be used for assumptions about the
external environment. So you cannot expect to verify formally
or informally that the condition is met, this must be
established by examining things outside the program itself.
For example, we may have code that depends on the size of
@cite{Long_Long_Integer} being at least 64. So we could write:

@example
pragma Assume (Long_Long_Integer'Size >= 64);
@end example

This assumption cannot be proved from the program itself,
but it acts as a useful run-time check that the assumption
is met, and documents the need to ensure that it is met by
reference to information outside the program.

@node Pragma Assume_No_Invalid_Values,Pragma Async_Readers,Pragma Assume,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-assume-no-invalid-values}@anchor{29}
@section Pragma Assume_No_Invalid_Values


@geindex Invalid representations

@geindex Invalid values

Syntax:

@example
pragma Assume_No_Invalid_Values (On | Off);
@end example

This is a configuration pragma that controls the assumptions made by the
compiler about the occurrence of invalid representations (invalid values)
in the code.

The default behavior (corresponding to an Off argument for this pragma), is
to assume that values may in general be invalid unless the compiler can
prove they are valid. Consider the following example:

@example
V1 : Integer range 1 .. 10;
V2 : Integer range 11 .. 20;
...
for J in V2 .. V1 loop
   ...
end loop;
@end example

if V1 and V2 have valid values, then the loop is known at compile
time not to execute since the lower bound must be greater than the
upper bound. However in default mode, no such assumption is made,
and the loop may execute. If @cite{Assume_No_Invalid_Values (On)}
is given, the compiler will assume that any occurrence of a variable
other than in an explicit @cite{'Valid} test always has a valid
value, and the loop above will be optimized away.

The use of @cite{Assume_No_Invalid_Values (On)} is appropriate if
you know your code is free of uninitialized variables and other
possible sources of invalid representations, and may result in
more efficient code. A program that accesses an invalid representation
with this pragma in effect is erroneous, so no guarantees can be made
about its behavior.

It is peculiar though permissible to use this pragma in conjunction
with validity checking (-gnatVa). In such cases, accessing invalid
values will generally give an exception, though formally the program
is erroneous so there are no guarantees that this will always be the
case, and it is recommended that these two options not be used together.

@node Pragma Async_Readers,Pragma Async_Writers,Pragma Assume_No_Invalid_Values,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-async-readers}@anchor{2a}
@section Pragma Async_Readers


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.2.

@node Pragma Async_Writers,Pragma Attribute_Definition,Pragma Async_Readers,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-async-writers}@anchor{2b}
@section Pragma Async_Writers


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.2.

@node Pragma Attribute_Definition,Pragma C_Pass_By_Copy,Pragma Async_Writers,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-attribute-definition}@anchor{2c}
@section Pragma Attribute_Definition


Syntax:

@example
pragma Attribute_Definition
  ([Attribute  =>] ATTRIBUTE_DESIGNATOR,
   [Entity     =>] LOCAL_NAME,
   [Expression =>] EXPRESSION | NAME);
@end example

If @cite{Attribute} is a known attribute name, this pragma is equivalent to
the attribute definition clause:

@example
for Entity'Attribute use Expression;
@end example

If @cite{Attribute} is not a recognized attribute name, the pragma is
ignored, and a warning is emitted. This allows source
code to be written that takes advantage of some new attribute, while remaining
compilable with earlier compilers.

@node Pragma C_Pass_By_Copy,Pragma Check,Pragma Attribute_Definition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-c-pass-by-copy}@anchor{2d}
@section Pragma C_Pass_By_Copy


@geindex Passing by copy

Syntax:

@example
pragma C_Pass_By_Copy
  ([Max_Size =>] static_integer_EXPRESSION);
@end example

Normally the default mechanism for passing C convention records to C
convention subprograms is to pass them by reference, as suggested by RM
B.3(69).  Use the configuration pragma @cite{C_Pass_By_Copy} to change
this default, by requiring that record formal parameters be passed by
copy if all of the following conditions are met:


@itemize *

@item 
The size of the record type does not exceed the value specified for
@cite{Max_Size}.

@item 
The record type has @cite{Convention C}.

@item 
The formal parameter has this record type, and the subprogram has a
foreign (non-Ada) convention.
@end itemize

If these conditions are met the argument is passed by copy; i.e., in a
manner consistent with what C expects if the corresponding formal in the
C prototype is a struct (rather than a pointer to a struct).

You can also pass records by copy by specifying the convention
@cite{C_Pass_By_Copy} for the record type, or by using the extended
@cite{Import} and @cite{Export} pragmas, which allow specification of
passing mechanisms on a parameter by parameter basis.

@node Pragma Check,Pragma Check_Float_Overflow,Pragma C_Pass_By_Copy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check}@anchor{2e}
@section Pragma Check


@geindex Assertions

@geindex Named assertions

Syntax:

@example
pragma Check (
     [Name    =>] CHECK_KIND,
     [Check   =>] Boolean_EXPRESSION
  [, [Message =>] string_EXPRESSION] );

CHECK_KIND ::= IDENTIFIER           |
               Pre'Class            |
               Post'Class           |
               Type_Invariant'Class |
               Invariant'Class
@end example

This pragma is similar to the predefined pragma @cite{Assert} except that an
extra identifier argument is present. In conjunction with pragma
@cite{Check_Policy}, this can be used to define groups of assertions that can
be independently controlled. The identifier @cite{Assertion} is special, it
refers to the normal set of pragma @cite{Assert} statements.

Checks introduced by this pragma are normally deactivated by default. They can
be activated either by the command line option @emph{-gnata}, which turns on
all checks, or individually controlled using pragma @cite{Check_Policy}.

The identifiers @cite{Assertions} and @cite{Statement_Assertions} are not
permitted as check kinds, since this would cause confusion with the use
of these identifiers in @cite{Assertion_Policy} and @cite{Check_Policy}
pragmas, where they are used to refer to sets of assertions.

@node Pragma Check_Float_Overflow,Pragma Check_Name,Pragma Check,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check-float-overflow}@anchor{2f}
@section Pragma Check_Float_Overflow


@geindex Floating-point overflow

Syntax:

@example
pragma Check_Float_Overflow;
@end example

In Ada, the predefined floating-point types (@cite{Short_Float},
@cite{Float}, @cite{Long_Float}, @cite{Long_Long_Float}) are
defined to be @emph{unconstrained}. This means that even though each
has a well-defined base range, an operation that delivers a result
outside this base range is not required to raise an exception.
This implementation permission accommodates the notion
of infinities in IEEE floating-point, and corresponds to the
efficient execution mode on most machines. GNAT will not raise
overflow exceptions on these machines; instead it will generate
infinities and NaN's as defined in the IEEE standard.

Generating infinities, although efficient, is not always desirable.
Often the preferable approach is to check for overflow, even at the
(perhaps considerable) expense of run-time performance.
This can be accomplished by defining your own constrained floating-point subtypes -- i.e., by supplying explicit
range constraints -- and indeed such a subtype
can have the same base range as its base type. For example:

@example
subtype My_Float is Float range Float'Range;
@end example

Here @cite{My_Float} has the same range as
@cite{Float} but is constrained, so operations on
@cite{My_Float} values will be checked for overflow
against this range.

This style will achieve the desired goal, but
it is often more convenient to be able to simply use
the standard predefined floating-point types as long
as overflow checking could be guaranteed.
The @cite{Check_Float_Overflow}
configuration pragma achieves this effect. If a unit is compiled
subject to this configuration pragma, then all operations
on predefined floating-point types including operations on
base types of these floating-point types will be treated as
though those types were constrained, and overflow checks
will be generated. The @cite{Constraint_Error}
exception is raised if the result is out of range.

This mode can also be set by use of the compiler
switch @emph{-gnateF}.

@node Pragma Check_Name,Pragma Check_Policy,Pragma Check_Float_Overflow,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check-name}@anchor{30}
@section Pragma Check_Name


@geindex Defining check names

@geindex Check names
@geindex defining

Syntax:

@example
pragma Check_Name (check_name_IDENTIFIER);
@end example

This is a configuration pragma that defines a new implementation
defined check name (unless IDENTIFIER matches one of the predefined
check names, in which case the pragma has no effect). Check names
are global to a partition, so if two or more configuration pragmas
are present in a partition mentioning the same name, only one new
check name is introduced.

An implementation defined check name introduced with this pragma may
be used in only three contexts: @cite{pragma Suppress},
@cite{pragma Unsuppress},
and as the prefix of a @cite{Check_Name'Enabled} attribute reference. For
any of these three cases, the check name must be visible. A check
name is visible if it is in the configuration pragmas applying to
the current unit, or if it appears at the start of any unit that
is part of the dependency set of the current unit (e.g., units that
are mentioned in @cite{with} clauses).

Check names introduced by this pragma are subject to control by compiler
switches (in particular -gnatp) in the usual manner.

@node Pragma Check_Policy,Pragma CIL_Constructor,Pragma Check_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-check-policy}@anchor{31}
@section Pragma Check_Policy


@geindex Controlling assertions

@geindex Assertions
@geindex control

@geindex Check pragma control

@geindex Named assertions

Syntax:

@example
pragma Check_Policy
 ([Name   =>] CHECK_KIND,
  [Policy =>] POLICY_IDENTIFIER);

pragma Check_Policy (
    CHECK_KIND => POLICY_IDENTIFIER
 @{, CHECK_KIND => POLICY_IDENTIFIER@});

ASSERTION_KIND ::= RM_ASSERTION_KIND | ID_ASSERTION_KIND

CHECK_KIND ::= IDENTIFIER           |
               Pre'Class            |
               Post'Class           |
               Type_Invariant'Class |
               Invariant'Class

The identifiers Name and Policy are not allowed as CHECK_KIND values. This
avoids confusion between the two possible syntax forms for this pragma.

POLICY_IDENTIFIER ::= ON | OFF | CHECK | DISABLE | IGNORE
@end example

This pragma is used to set the checking policy for assertions (specified
by aspects or pragmas), the @cite{Debug} pragma, or additional checks
to be checked using the @cite{Check} pragma. It may appear either as
a configuration pragma, or within a declarative part of package. In the
latter case, it applies from the point where it appears to the end of
the declarative region (like pragma @cite{Suppress}).

The @cite{Check_Policy} pragma is similar to the
predefined @cite{Assertion_Policy} pragma,
and if the check kind corresponds to one of the assertion kinds that
are allowed by @cite{Assertion_Policy}, then the effect is identical.

If the first argument is Debug, then the policy applies to Debug pragmas,
disabling their effect if the policy is @cite{OFF}, @cite{DISABLE}, or
@cite{IGNORE}, and allowing them to execute with normal semantics if
the policy is @cite{ON} or @cite{CHECK}. In addition if the policy is
@cite{DISABLE}, then the procedure call in @cite{Debug} pragmas will
be totally ignored and not analyzed semantically.

Finally the first argument may be some other identifier than the above
possibilities, in which case it controls a set of named assertions
that can be checked using pragma @cite{Check}. For example, if the pragma:

@example
pragma Check_Policy (Critical_Error, OFF);
@end example

is given, then subsequent @cite{Check} pragmas whose first argument is also
@cite{Critical_Error} will be disabled.

The check policy is @cite{OFF} to turn off corresponding checks, and @cite{ON}
to turn on corresponding checks. The default for a set of checks for which no
@cite{Check_Policy} is given is @cite{OFF} unless the compiler switch
@emph{-gnata} is given, which turns on all checks by default.

The check policy settings @cite{CHECK} and @cite{IGNORE} are recognized
as synonyms for @cite{ON} and @cite{OFF}. These synonyms are provided for
compatibility with the standard @cite{Assertion_Policy} pragma. The check
policy setting @cite{DISABLE} causes the second argument of a corresponding
@cite{Check} pragma to be completely ignored and not analyzed.

@node Pragma CIL_Constructor,Pragma Comment,Pragma Check_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cil-constructor}@anchor{32}
@section Pragma CIL_Constructor


Syntax:

@example
pragma CIL_Constructor ([Entity =>] function_LOCAL_NAME);
@end example

This pragma is used to assert that the specified Ada function should be
mapped to the .NET constructor for some Ada tagged record type.

See section 4.1 of the
@cite{GNAT User's Guide: Supplement for the .NET Platform.}
for related information.

@node Pragma Comment,Pragma Common_Object,Pragma CIL_Constructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-comment}@anchor{33}
@section Pragma Comment


Syntax:

@example
pragma Comment (static_string_EXPRESSION);
@end example

This is almost identical in effect to pragma @cite{Ident}.  It allows the
placement of a comment into the object file and hence into the
executable file if the operating system permits such usage.  The
difference is that @cite{Comment}, unlike @cite{Ident}, has
no limitations on placement of the pragma (it can be placed
anywhere in the main source unit), and if more than one pragma
is used, all comments are retained.

@node Pragma Common_Object,Pragma Compile_Time_Error,Pragma Comment,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-common-object}@anchor{34}
@section Pragma Common_Object


Syntax:

@example
pragma Common_Object (
     [Internal =>] LOCAL_NAME
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL] );

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma enables the shared use of variables stored in overlaid
linker areas corresponding to the use of @cite{COMMON}
in Fortran.  The single
object @cite{LOCAL_NAME} is assigned to the area designated by
the @cite{External} argument.
You may define a record to correspond to a series
of fields.  The @cite{Size} argument
is syntax checked in GNAT, but otherwise ignored.

@cite{Common_Object} is not supported on all platforms.  If no
support is available, then the code generator will issue a message
indicating that the necessary attribute for implementation of this
pragma is not available.

@node Pragma Compile_Time_Error,Pragma Compile_Time_Warning,Pragma Common_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-compile-time-error}@anchor{35}
@section Pragma Compile_Time_Error


Syntax:

@example
pragma Compile_Time_Error
         (boolean_EXPRESSION, static_string_EXPRESSION);
@end example

This pragma can be used to generate additional compile time
error messages. It
is particularly useful in generics, where errors can be issued for
specific problematic instantiations. The first parameter is a boolean
expression. The pragma is effective only if the value of this expression
is known at compile time, and has the value True. The set of expressions
whose values are known at compile time includes all static boolean
expressions, and also other values which the compiler can determine
at compile time (e.g., the size of a record type set by an explicit
size representation clause, or the value of a variable which was
initialized to a constant and is known not to have been modified).
If these conditions are met, an error message is generated using
the value given as the second argument. This string value may contain
embedded ASCII.LF characters to break the message into multiple lines.

@node Pragma Compile_Time_Warning,Pragma Compiler_Unit,Pragma Compile_Time_Error,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-compile-time-warning}@anchor{36}
@section Pragma Compile_Time_Warning


Syntax:

@example
pragma Compile_Time_Warning
         (boolean_EXPRESSION, static_string_EXPRESSION);
@end example

Same as pragma Compile_Time_Error, except a warning is issued instead
of an error message. Note that if this pragma is used in a package that
is with'ed by a client, the client will get the warning even though it
is issued by a with'ed package (normally warnings in with'ed units are
suppressed, but this is a special exception to that rule).

One typical use is within a generic where compile time known characteristics
of formal parameters are tested, and warnings given appropriately. Another use
with a first parameter of True is to warn a client about use of a package,
for example that it is not fully implemented.

@node Pragma Compiler_Unit,Pragma Compiler_Unit_Warning,Pragma Compile_Time_Warning,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-compiler-unit}@anchor{37}
@section Pragma Compiler_Unit


Syntax:

@example
pragma Compiler_Unit;
@end example

This pragma is obsolete. It is equivalent to Compiler_Unit_Warning. It is
retained so that old versions of the GNAT run-time that use this pragma can
be compiled with newer versions of the compiler.

@node Pragma Compiler_Unit_Warning,Pragma Complete_Representation,Pragma Compiler_Unit,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-compiler-unit-warning}@anchor{38}
@section Pragma Compiler_Unit_Warning


Syntax:

@example
pragma Compiler_Unit_Warning;
@end example

This pragma is intended only for internal use in the GNAT run-time library.
It indicates that the unit is used as part of the compiler build. The effect
is to generate warnings for the use of constructs (for example, conditional
expressions) that would cause trouble when bootstrapping using an older
version of GNAT. For the exact list of restrictions, see the compiler sources
and references to Check_Compiler_Unit.

@node Pragma Complete_Representation,Pragma Complex_Representation,Pragma Compiler_Unit_Warning,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-complete-representation}@anchor{39}
@section Pragma Complete_Representation


Syntax:

@example
pragma Complete_Representation;
@end example

This pragma must appear immediately within a record representation
clause. Typical placements are before the first component clause
or after the last component clause. The effect is to give an error
message if any component is missing a component clause. This pragma
may be used to ensure that a record representation clause is
complete, and that this invariant is maintained if fields are
added to the record in the future.

@node Pragma Complex_Representation,Pragma Component_Alignment,Pragma Complete_Representation,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-complex-representation}@anchor{3a}
@section Pragma Complex_Representation


Syntax:

@example
pragma Complex_Representation
        ([Entity =>] LOCAL_NAME);
@end example

The @cite{Entity} argument must be the name of a record type which has
two fields of the same floating-point type.  The effect of this pragma is
to force gcc to use the special internal complex representation form for
this record, which may be more efficient.  Note that this may result in
the code for this type not conforming to standard ABI (application
binary interface) requirements for the handling of record types.  For
example, in some environments, there is a requirement for passing
records by pointer, and the use of this pragma may result in passing
this type in floating-point registers.

@node Pragma Component_Alignment,Pragma Contract_Cases,Pragma Complex_Representation,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-component-alignment}@anchor{3b}
@section Pragma Component_Alignment


@geindex Alignments of components

@geindex Pragma Component_Alignment

Syntax:

@example
pragma Component_Alignment (
     [Form =>] ALIGNMENT_CHOICE
  [, [Name =>] type_LOCAL_NAME]);

ALIGNMENT_CHOICE ::=
  Component_Size
| Component_Size_4
| Storage_Unit
| Default
@end example

Specifies the alignment of components in array or record types.
The meaning of the @cite{Form} argument is as follows:

@quotation

@geindex Component_Size (in pragma Component_Alignment)
@end quotation


@table @asis

@item @emph{Component_Size}

Aligns scalar components and subcomponents of the array or record type
on boundaries appropriate to their inherent size (naturally
aligned).  For example, 1-byte components are aligned on byte boundaries,
2-byte integer components are aligned on 2-byte boundaries, 4-byte
integer components are aligned on 4-byte boundaries and so on.  These
alignment rules correspond to the normal rules for C compilers on all
machines except the VAX.

@geindex Component_Size_4 (in pragma Component_Alignment)

@item @emph{Component_Size_4}

Naturally aligns components with a size of four or fewer
bytes.  Components that are larger than 4 bytes are placed on the next
4-byte boundary.

@geindex Storage_Unit (in pragma Component_Alignment)

@item @emph{Storage_Unit}

Specifies that array or record components are byte aligned, i.e.,
aligned on boundaries determined by the value of the constant
@cite{System.Storage_Unit}.

@geindex Default (in pragma Component_Alignment)

@item @emph{Default}

Specifies that array or record components are aligned on default
boundaries, appropriate to the underlying hardware or operating system or
both. The @cite{Default} choice is the same as @cite{Component_Size} (natural
alignment).
@end table

If the @cite{Name} parameter is present, @cite{type_LOCAL_NAME} must
refer to a local record or array type, and the specified alignment
choice applies to the specified type.  The use of
@cite{Component_Alignment} together with a pragma @cite{Pack} causes the
@cite{Component_Alignment} pragma to be ignored.  The use of
@cite{Component_Alignment} together with a record representation clause
is only effective for fields not specified by the representation clause.

If the @cite{Name} parameter is absent, the pragma can be used as either
a configuration pragma, in which case it applies to one or more units in
accordance with the normal rules for configuration pragmas, or it can be
used within a declarative part, in which case it applies to types that
are declared within this declarative part, or within any nested scope
within this declarative part.  In either case it specifies the alignment
to be applied to any record or array type which has otherwise standard
representation.

If the alignment for a record or array type is not specified (using
pragma @cite{Pack}, pragma @cite{Component_Alignment}, or a record rep
clause), the GNAT uses the default alignment as described previously.

@node Pragma Contract_Cases,Pragma Convention_Identifier,Pragma Component_Alignment,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-contract-cases}@anchor{3c}
@section Pragma Contract_Cases


@geindex Contract cases

Syntax:

@example
pragma Contract_Cases (
   Condition => Consequence
 @{,Condition => Consequence@});
@end example

The @cite{Contract_Cases} pragma allows defining fine-grain specifications
that can complement or replace the contract given by a precondition and a
postcondition. Additionally, the @cite{Contract_Cases} pragma can be used
by testing and formal verification tools. The compiler checks its validity and,
depending on the assertion policy at the point of declaration of the pragma,
it may insert a check in the executable. For code generation, the contract
cases

@example
pragma Contract_Cases (
  Cond1 => Pred1,
  Cond2 => Pred2);
@end example

are equivalent to

@example
C1 : constant Boolean := Cond1;  --  evaluated at subprogram entry
C2 : constant Boolean := Cond2;  --  evaluated at subprogram entry
pragma Precondition ((C1 and not C2) or (C2 and not C1));
pragma Postcondition (if C1 then Pred1);
pragma Postcondition (if C2 then Pred2);
@end example

The precondition ensures that one and only one of the conditions is
satisfied on entry to the subprogram.
The postcondition ensures that for the condition that was True on entry,
the corrresponding consequence is True on exit. Other consequence expressions
are not evaluated.

A precondition @cite{P} and postcondition @cite{Q} can also be
expressed as contract cases:

@example
pragma Contract_Cases (P => Q);
@end example

The placement and visibility rules for @cite{Contract_Cases} pragmas are
identical to those described for preconditions and postconditions.

The compiler checks that boolean expressions given in conditions and
consequences are valid, where the rules for conditions are the same as
the rule for an expression in @cite{Precondition} and the rules for
consequences are the same as the rule for an expression in
@cite{Postcondition}. In particular, attributes @cite{'Old} and
@cite{'Result} can only be used within consequence expressions.
The condition for the last contract case may be @cite{others}, to denote
any case not captured by the previous cases. The
following is an example of use within a package spec:

@example
package Math_Functions is
   ...
   function Sqrt (Arg : Float) return Float;
   pragma Contract_Cases ((Arg in 0 .. 99) => Sqrt'Result < 10,
                          Arg >= 100       => Sqrt'Result >= 10,
                          others           => Sqrt'Result = 0);
   ...
end Math_Functions;
@end example

The meaning of contract cases is that only one case should apply at each
call, as determined by the corresponding condition evaluating to True,
and that the consequence for this case should hold when the subprogram
returns.

@node Pragma Convention_Identifier,Pragma CPP_Class,Pragma Contract_Cases,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-convention-identifier}@anchor{3d}
@section Pragma Convention_Identifier


@geindex Conventions
@geindex synonyms

Syntax:

@example
pragma Convention_Identifier (
         [Name =>]       IDENTIFIER,
         [Convention =>] convention_IDENTIFIER);
@end example

This pragma provides a mechanism for supplying synonyms for existing
convention identifiers. The @cite{Name} identifier can subsequently
be used as a synonym for the given convention in other pragmas (including
for example pragma @cite{Import} or another @cite{Convention_Identifier}
pragma). As an example of the use of this, suppose you had legacy code
which used Fortran77 as the identifier for Fortran. Then the pragma:

@example
pragma Convention_Identifier (Fortran77, Fortran);
@end example

would allow the use of the convention identifier @cite{Fortran77} in
subsequent code, avoiding the need to modify the sources. As another
example, you could use this to parameterize convention requirements
according to systems. Suppose you needed to use @cite{Stdcall} on
windows systems, and @cite{C} on some other system, then you could
define a convention identifier @cite{Library} and use a single
@cite{Convention_Identifier} pragma to specify which convention
would be used system-wide.

@node Pragma CPP_Class,Pragma CPP_Constructor,Pragma Convention_Identifier,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-class}@anchor{3e}
@section Pragma CPP_Class


@geindex Interfacing with C++

Syntax:

@example
pragma CPP_Class ([Entity =>] LOCAL_NAME);
@end example

The argument denotes an entity in the current declarative region that is
declared as a record type. It indicates that the type corresponds to an
externally declared C++ class type, and is to be laid out the same way
that C++ would lay out the type. If the C++ class has virtual primitives
then the record must be declared as a tagged record type.

Types for which @cite{CPP_Class} is specified do not have assignment or
equality operators defined (such operations can be imported or declared
as subprograms as required). Initialization is allowed only by constructor
functions (see pragma @cite{CPP_Constructor}). Such types are implicitly
limited if not explicitly declared as limited or derived from a limited
type, and an error is issued in that case.

See @ref{3f,,Interfacing to C++} for related information.

Note: Pragma @cite{CPP_Class} is currently obsolete. It is supported
for backward compatibility but its functionality is available
using pragma @cite{Import} with @cite{Convention} = @cite{CPP}.

@node Pragma CPP_Constructor,Pragma CPP_Virtual,Pragma CPP_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-constructor}@anchor{40}
@section Pragma CPP_Constructor


@geindex Interfacing with C++

Syntax:

@example
pragma CPP_Constructor ([Entity =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION ]
  [, [Link_Name     =>] static_string_EXPRESSION ]);
@end example

This pragma identifies an imported function (imported in the usual way
with pragma @cite{Import}) as corresponding to a C++ constructor. If
@cite{External_Name} and @cite{Link_Name} are not specified then the
@cite{Entity} argument is a name that must have been previously mentioned
in a pragma @cite{Import} with @cite{Convention} = @cite{CPP}. Such name
must be of one of the following forms:


@itemize *

@item 
@strong{function} @cite{Fname} @strong{return} T`

@item 
@strong{function} @cite{Fname} @strong{return} T'Class

@item 
@strong{function} @cite{Fname} (...) @strong{return} T`

@item 
@strong{function} @cite{Fname} (...) @strong{return} T'Class
@end itemize

where @cite{T} is a limited record type imported from C++ with pragma
@cite{Import} and @cite{Convention} = @cite{CPP}.

The first two forms import the default constructor, used when an object
of type @cite{T} is created on the Ada side with no explicit constructor.
The latter two forms cover all the non-default constructors of the type.
See the GNAT User's Guide for details.

If no constructors are imported, it is impossible to create any objects
on the Ada side and the type is implicitly declared abstract.

Pragma @cite{CPP_Constructor} is intended primarily for automatic generation
using an automatic binding generator tool (such as the @cite{-fdump-ada-spec}
GCC switch).
See @ref{3f,,Interfacing to C++} for more related information.

Note: The use of functions returning class-wide types for constructors is
currently obsolete. They are supported for backward compatibility. The
use of functions returning the type T leave the Ada sources more clear
because the imported C++ constructors always return an object of type T;
that is, they never return an object whose type is a descendant of type T.

@node Pragma CPP_Virtual,Pragma CPP_Vtable,Pragma CPP_Constructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-virtual}@anchor{41}
@section Pragma CPP_Virtual


@geindex Interfacing to C++

This pragma is now obsolete and, other than generating a warning if warnings
on obsolescent features are enabled, is completely ignored.
It is retained for compatibility
purposes. It used to be required to ensure compoatibility with C++, but
is no longer required for that purpose because GNAT generates
the same object layout as the G++ compiler by default.

See @ref{3f,,Interfacing to C++} for related information.

@node Pragma CPP_Vtable,Pragma CPU,Pragma CPP_Virtual,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpp-vtable}@anchor{42}
@section Pragma CPP_Vtable


@geindex Interfacing with C++

This pragma is now obsolete and, other than generating a warning if warnings
on obsolescent features are enabled, is completely ignored.
It used to be required to ensure compatibility with C++, but
is no longer required for that purpose because GNAT generates
the same object layout as the G++ compiler by default.

See @ref{3f,,Interfacing to C++} for related information.

@node Pragma CPU,Pragma Debug,Pragma CPP_Vtable,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-cpu}@anchor{43}
@section Pragma CPU


Syntax:

@example
pragma CPU (EXPRESSION);
@end example

This pragma is standard in Ada 2012, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Debug,Pragma Debug_Policy,Pragma CPU,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-debug}@anchor{44}
@section Pragma Debug


Syntax:

@example
pragma Debug ([CONDITION, ]PROCEDURE_CALL_WITHOUT_SEMICOLON);

PROCEDURE_CALL_WITHOUT_SEMICOLON ::=
  PROCEDURE_NAME
| PROCEDURE_PREFIX ACTUAL_PARAMETER_PART
@end example

The procedure call argument has the syntactic form of an expression, meeting
the syntactic requirements for pragmas.

If debug pragmas are not enabled or if the condition is present and evaluates
to False, this pragma has no effect. If debug pragmas are enabled, the
semantics of the pragma is exactly equivalent to the procedure call statement
corresponding to the argument with a terminating semicolon. Pragmas are
permitted in sequences of declarations, so you can use pragma @cite{Debug} to
intersperse calls to debug procedures in the middle of declarations. Debug
pragmas can be enabled either by use of the command line switch @emph{-gnata}
or by use of the pragma @cite{Check_Policy} with a first argument of
@cite{Debug}.

@node Pragma Debug_Policy,Pragma Default_Scalar_Storage_Order,Pragma Debug,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-debug-policy}@anchor{45}
@section Pragma Debug_Policy


Syntax:

@example
pragma Debug_Policy (CHECK | DISABLE | IGNORE | ON | OFF);
@end example

This pragma is equivalent to a corresponding @cite{Check_Policy} pragma
with a first argument of @cite{Debug}. It is retained for historical
compatibility reasons.

@node Pragma Default_Scalar_Storage_Order,Pragma Default_Storage_Pool,Pragma Debug_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-default-scalar-storage-order}@anchor{46}
@section Pragma Default_Scalar_Storage_Order


@geindex Default_Scalar_Storage_Order

@geindex Scalar_Storage_Order

Syntax:

@example
pragma Default_Scalar_Storage_Order (High_Order_First | Low_Order_First);
@end example

Normally if no explicit @cite{Scalar_Storage_Order} is given for a record
type or array type, then the scalar storage order defaults to the ordinary
default for the target. But this default may be overridden using this pragma.
The pragma may appear as a configuration pragma, or locally within a package
spec or declarative part. In the latter case, it applies to all subsequent
types declared within that package spec or declarative part.

The following example shows the use of this pragma:

@example
pragma Default_Scalar_Storage_Order (High_Order_First);
with System; use System;
package DSSO1 is
   type H1 is record
      a : Integer;
   end record;

   type L2 is record
      a : Integer;
   end record;
   for L2'Scalar_Storage_Order use Low_Order_First;

   type L2a is new L2;

   package Inner is
      type H3 is record
         a : Integer;
      end record;

      pragma Default_Scalar_Storage_Order (Low_Order_First);

      type L4 is record
         a : Integer;
      end record;
   end Inner;

   type H4a is new Inner.L4;

   type H5 is record
      a : Integer;
   end record;
end DSSO1;
@end example

In this example record types L.. have @cite{Low_Order_First} scalar
storage order, and record types H.. have @cite{High_Order_First}.
Note that in the case of @cite{H4a}, the order is not inherited
from the parent type. Only an explicitly set @cite{Scalar_Storage_Order}
gets inherited on type derivation.

If this pragma is used as a configuration pragma which appears within a
configuration pragma file (as opposed to appearing explicitly at the start
of a single unit), then the binder will require that all units in a partition
be compiled in a similar manner, other than run-time units, which are not
affected by this pragma. Note that the use of this form is discouraged because
it may significantly degrade the run-time performance of the software, instead
the default scalar storage order ought to be changed only on a local basis.

@node Pragma Default_Storage_Pool,Pragma Depends,Pragma Default_Scalar_Storage_Order,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-default-storage-pool}@anchor{47}
@section Pragma Default_Storage_Pool


@geindex Default_Storage_Pool

Syntax:

@example
pragma Default_Storage_Pool (storage_pool_NAME | null);
@end example

This pragma is standard in Ada 2012, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Depends,Pragma Detect_Blocking,Pragma Default_Storage_Pool,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-depends}@anchor{48}
@section Pragma Depends


For the description of this pragma, see SPARK 2014 Reference Manual,
section 6.1.5.

@node Pragma Detect_Blocking,Pragma Disable_Atomic_Synchronization,Pragma Depends,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-detect-blocking}@anchor{49}
@section Pragma Detect_Blocking


Syntax:

@example
pragma Detect_Blocking;
@end example

This is a standard pragma in Ada 2005, that is available in all earlier
versions of Ada as an implementation-defined pragma.

This is a configuration pragma that forces the detection of potentially
blocking operations within a protected operation, and to raise Program_Error
if that happens.

@node Pragma Disable_Atomic_Synchronization,Pragma Dispatching_Domain,Pragma Detect_Blocking,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-disable-atomic-synchronization}@anchor{4a}
@section Pragma Disable_Atomic_Synchronization


@geindex Atomic Synchronization

Syntax:

@example
pragma Disable_Atomic_Synchronization [(Entity)];
@end example

Ada requires that accesses (reads or writes) of an atomic variable be
regarded as synchronization points in the case of multiple tasks.
Particularly in the case of multi-processors this may require special
handling, e.g. the generation of memory barriers. This capability may
be turned off using this pragma in cases where it is known not to be
required.

The placement and scope rules for this pragma are the same as those
for @cite{pragma Suppress}. In particular it can be used as a
configuration  pragma, or in a declaration sequence where it applies
till the end of the scope. If an @cite{Entity} argument is present,
the action applies only to that entity.

@node Pragma Dispatching_Domain,Pragma Effective_Reads,Pragma Disable_Atomic_Synchronization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-dispatching-domain}@anchor{4b}
@section Pragma Dispatching_Domain


Syntax:

@example
pragma Dispatching_Domain (EXPRESSION);
@end example

This pragma is standard in Ada 2012, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Effective_Reads,Pragma Effective_Writes,Pragma Dispatching_Domain,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-effective-reads}@anchor{4c}
@section Pragma Effective_Reads


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.2.

@node Pragma Effective_Writes,Pragma Elaboration_Checks,Pragma Effective_Reads,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-effective-writes}@anchor{4d}
@section Pragma Effective_Writes


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.2.

@node Pragma Elaboration_Checks,Pragma Eliminate,Pragma Effective_Writes,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-elaboration-checks}@anchor{4e}
@section Pragma Elaboration_Checks


@geindex Elaboration control

Syntax:

@example
pragma Elaboration_Checks (Dynamic | Static);
@end example

This is a configuration pragma that provides control over the
elaboration model used by the compilation affected by the
pragma.  If the parameter is @cite{Dynamic},
then the dynamic elaboration
model described in the Ada Reference Manual is used, as though
the @emph{-gnatE} switch had been specified on the command
line.  If the parameter is @cite{Static}, then the default GNAT static
model is used.  This configuration pragma overrides the setting
of the command line.  For full details on the elaboration models
used by the GNAT compiler, see the chapter on elaboration order handling
in the @emph{GNAT User's Guide}.

@node Pragma Eliminate,Pragma Enable_Atomic_Synchronization,Pragma Elaboration_Checks,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-eliminate}@anchor{4f}
@section Pragma Eliminate


@geindex Elimination of unused subprograms

Syntax:

@example
pragma Eliminate ([Entity          =>] DEFINING_DESIGNATOR,
                  [Source_Location =>] STRING_LITERAL);
@end example

The string literal given for the source location is a string which
specifies the line number of the occurrence of the entity, using
the syntax for SOURCE_TRACE given below:

@example
SOURCE_TRACE     ::= SOURCE_REFERENCE [LBRACKET SOURCE_TRACE RBRACKET]

LBRACKET         ::= [
RBRACKET         ::= ]

SOURCE_REFERENCE ::= FILE_NAME : LINE_NUMBER

LINE_NUMBER      ::= DIGIT @{DIGIT@}
@end example

Spaces around the colon in a @cite{Source_Reference} are optional.

The @cite{DEFINING_DESIGNATOR} matches the defining designator used in an
explicit subprogram declaration, where the @cite{entity} name in this
designator appears on the source line specified by the source location.

The source trace that is given as the @cite{Source_Location} shall obey the
following rules. The @cite{FILE_NAME} is the short name (with no directory
information) of an Ada source file, given using exactly the required syntax
for the underlying file system (e.g. case is important if the underlying
operating system is case sensitive). @cite{LINE_NUMBER} gives the line
number of the occurrence of the @cite{entity}
as a decimal literal without an exponent or point. If an @cite{entity} is not
declared in a generic instantiation (this includes generic subprogram
instances), the source trace includes only one source reference. If an entity
is declared inside a generic instantiation, its source trace (when parsing
from left to right) starts with the source location of the declaration of the
entity in the generic unit and ends with the source location of the
instantiation (it is given in square brackets). This approach is recursively
used in case of nested instantiations: the rightmost (nested most deeply in
square brackets) element of the source trace is the location of the outermost
instantiation, the next to left element is the location of the next (first
nested) instantiation in the code of the corresponding generic unit, and so
on, and the leftmost element (that is out of any square brackets) is the
location of the declaration of the entity to eliminate in a generic unit.

Note that the @cite{Source_Location} argument specifies which of a set of
similarly named entities is being eliminated, dealing both with overloading,
and also appearance of the same entity name in different scopes.

This pragma indicates that the given entity is not used in the program to be
compiled and built. The effect of the pragma is to allow the compiler to
eliminate the code or data associated with the named entity. Any reference to
an eliminated entity causes a compile-time or link-time error.

The intention of pragma @cite{Eliminate} is to allow a program to be compiled
in a system-independent manner, with unused entities eliminated, without
needing to modify the source text. Normally the required set of
@cite{Eliminate} pragmas is constructed automatically using the gnatelim tool.

Any source file change that removes, splits, or
adds lines may make the set of Eliminate pragmas invalid because their
@cite{Source_Location} argument values may get out of date.

Pragma @cite{Eliminate} may be used where the referenced entity is a dispatching
operation. In this case all the subprograms to which the given operation can
dispatch are considered to be unused (are never called as a result of a direct
or a dispatching call).

@node Pragma Enable_Atomic_Synchronization,Pragma Export_Function,Pragma Eliminate,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-enable-atomic-synchronization}@anchor{50}
@section Pragma Enable_Atomic_Synchronization


@geindex Atomic Synchronization

Syntax:

@example
pragma Enable_Atomic_Synchronization [(Entity)];
@end example

Ada requires that accesses (reads or writes) of an atomic variable be
regarded as synchronization points in the case of multiple tasks.
Particularly in the case of multi-processors this may require special
handling, e.g. the generation of memory barriers. This synchronization
is performed by default, but can be turned off using
@cite{pragma Disable_Atomic_Synchronization}. The
@cite{Enable_Atomic_Synchronization} pragma can be used to turn
it back on.

The placement and scope rules for this pragma are the same as those
for @cite{pragma Unsuppress}. In particular it can be used as a
configuration  pragma, or in a declaration sequence where it applies
till the end of the scope. If an @cite{Entity} argument is present,
the action applies only to that entity.

@node Pragma Export_Function,Pragma Export_Object,Pragma Enable_Atomic_Synchronization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-function}@anchor{51}
@section Pragma Export_Function


@geindex Argument passing mechanisms

Syntax:

@example
pragma Export_Function (
     [Internal         =>] LOCAL_NAME
  [, [External         =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types  =>] PARAMETER_TYPES]
  [, [Result_Type      =>] result_SUBTYPE_MARK]
  [, [Mechanism        =>] MECHANISM]
  [, [Result_Mechanism =>] MECHANISM_NAME]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
| ""

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

Use this pragma to make a function externally callable and optionally
provide information on mechanisms to be used for passing parameter and
result values.  We recommend, for the purposes of improving portability,
this pragma always be used in conjunction with a separate pragma
@cite{Export}, which must precede the pragma @cite{Export_Function}.
GNAT does not require a separate pragma @cite{Export}, but if none is
present, @cite{Convention Ada} is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a @cite{Export} or @cite{Convention}
pragma that specifies the desired foreign convention.
Pragma @cite{Export_Function}
(and @cite{Export}, if present) must appear in the same declarative
region as the function to which they apply.

@cite{internal_name} must uniquely designate the function to which the
pragma applies.  If more than one function name exists of this name in
the declarative part you must use the @cite{Parameter_Types} and
@cite{Result_Type} parameters is mandatory to achieve the required
unique designation.  @cite{subtype_mark`s in these parameters must exactly match the subtypes in the corresponding function specification@comma{} using positional notation to match parameters with subtype marks. The form with an `'Access} attribute can be used to match an
anonymous access parameter.

@geindex Suppressing external name

Special treatment is given if the EXTERNAL is an explicit null
string or a static string expressions that evaluates to the null
string. In this case, no external name is generated. This form
still allows the specification of parameter mechanisms.

@node Pragma Export_Object,Pragma Export_Procedure,Pragma Export_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-object}@anchor{52}
@section Pragma Export_Object


Syntax:

@example
pragma Export_Object
      [Internal =>] LOCAL_NAME
   [, [External =>] EXTERNAL_SYMBOL]
   [, [Size     =>] EXTERNAL_SYMBOL]

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma designates an object as exported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal @cite{Export} pragma applied to an object.  You may use a
separate Export pragma (and you probably should from the point of view
of portability), but it is not required.  @cite{Size} is syntax checked,
but otherwise ignored by GNAT.

@node Pragma Export_Procedure,Pragma Export_Value,Pragma Export_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-procedure}@anchor{53}
@section Pragma Export_Procedure


Syntax:

@example
pragma Export_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
| ""

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @cite{Export_Function} except that it
applies to a procedure rather than a function and the parameters
@cite{Result_Type} and @cite{Result_Mechanism} are not permitted.
GNAT does not require a separate pragma @cite{Export}, but if none is
present, @cite{Convention Ada} is assumed, which is usually
not what is wanted, so it is usually appropriate to use this
pragma in conjunction with a @cite{Export} or @cite{Convention}
pragma that specifies the desired foreign convention.

@geindex Suppressing external name

Special treatment is given if the EXTERNAL is an explicit null
string or a static string expressions that evaluates to the null
string. In this case, no external name is generated. This form
still allows the specification of parameter mechanisms.

@node Pragma Export_Value,Pragma Export_Valued_Procedure,Pragma Export_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-value}@anchor{54}
@section Pragma Export_Value


Syntax:

@example
pragma Export_Value (
  [Value     =>] static_integer_EXPRESSION,
  [Link_Name =>] static_string_EXPRESSION);
@end example

This pragma serves to export a static integer value for external use.
The first argument specifies the value to be exported. The Link_Name
argument specifies the symbolic name to be associated with the integer
value. This pragma is useful for defining a named static value in Ada
that can be referenced in assembly language units to be linked with
the application. This pragma is currently supported only for the
AAMP target and is ignored for other targets.

@node Pragma Export_Valued_Procedure,Pragma Extend_System,Pragma Export_Value,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-export-valued-procedure}@anchor{55}
@section Pragma Export_Valued_Procedure


Syntax:

@example
pragma Export_Valued_Procedure (
     [Internal        =>] LOCAL_NAME
  [, [External        =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types =>] PARAMETER_TYPES]
  [, [Mechanism       =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
| ""

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @cite{Export_Procedure} except that the
first parameter of @cite{LOCAL_NAME}, which must be present, must be of
mode @cite{OUT}, and externally the subprogram is treated as a function
with this parameter as the result of the function.  GNAT provides for
this capability to allow the use of @cite{OUT} and @cite{IN OUT}
parameters in interfacing to external functions (which are not permitted
in Ada functions).
GNAT does not require a separate pragma @cite{Export}, but if none is
present, @cite{Convention Ada} is assumed, which is almost certainly
not what is wanted since the whole point of this pragma is to interface
with foreign language functions, so it is usually appropriate to use this
pragma in conjunction with a @cite{Export} or @cite{Convention}
pragma that specifies the desired foreign convention.

@geindex Suppressing external name

Special treatment is given if the EXTERNAL is an explicit null
string or a static string expressions that evaluates to the null
string. In this case, no external name is generated. This form
still allows the specification of parameter mechanisms.

@node Pragma Extend_System,Pragma Extensions_Allowed,Pragma Export_Valued_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-extend-system}@anchor{56}
@section Pragma Extend_System


@geindex System
@geindex extending

@geindex DEC Ada 83

Syntax:

@example
pragma Extend_System ([Name =>] IDENTIFIER);
@end example

This pragma is used to provide backwards compatibility with other
implementations that extend the facilities of package @cite{System}.  In
GNAT, @cite{System} contains only the definitions that are present in
the Ada RM.  However, other implementations, notably the DEC Ada 83
implementation, provide many extensions to package @cite{System}.

For each such implementation accommodated by this pragma, GNAT provides a
package @cite{Aux_`xxx`}, e.g., @cite{Aux_DEC} for the DEC Ada 83
implementation, which provides the required additional definitions.  You
can use this package in two ways.  You can @cite{with} it in the normal
way and access entities either by selection or using a @cite{use}
clause.  In this case no special processing is required.

However, if existing code contains references such as
@cite{System.`xxx`} where @cite{xxx} is an entity in the extended
definitions provided in package @cite{System}, you may use this pragma
to extend visibility in @cite{System} in a non-standard way that
provides greater compatibility with the existing code.  Pragma
@cite{Extend_System} is a configuration pragma whose single argument is
the name of the package containing the extended definition
(e.g., @cite{Aux_DEC} for the DEC Ada case).  A unit compiled under
control of this pragma will be processed using special visibility
processing that looks in package @cite{System.Aux_`xxx`} where
@cite{Aux_`xxx`} is the pragma argument for any entity referenced in
package @cite{System}, but not found in package @cite{System}.

You can use this pragma either to access a predefined @cite{System}
extension supplied with the compiler, for example @cite{Aux_DEC} or
you can construct your own extension unit following the above
definition.  Note that such a package is a child of @cite{System}
and thus is considered part of the implementation.
To compile it you will have to use the @emph{-gnatg} switch
for compiling System units, as explained in the
GNAT User's Guide.

@node Pragma Extensions_Allowed,Pragma External,Pragma Extend_System,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-extensions-allowed}@anchor{57}
@section Pragma Extensions_Allowed


@geindex Ada Extensions

@geindex GNAT Extensions

Syntax:

@example
pragma Extensions_Allowed (On | Off);
@end example

This configuration pragma enables or disables the implementation
extension mode (the use of Off as a parameter cancels the effect
of the @emph{-gnatX} command switch).

In extension mode, the latest version of the Ada language is
implemented (currently Ada 2012), and in addition a small number
of GNAT specific extensions are recognized as follows:


@table @asis

@item @emph{Constrained attribute for generic objects}

The @cite{Constrained} attribute is permitted for objects of
generic types. The result indicates if the corresponding actual
is constrained.
@end table

@node Pragma External,Pragma External_Name_Casing,Pragma Extensions_Allowed,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-external}@anchor{58}
@section Pragma External


Syntax:

@example
pragma External (
  [   Convention    =>] convention_IDENTIFIER,
  [   Entity        =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION ]
  [, [Link_Name     =>] static_string_EXPRESSION ]);
@end example

This pragma is identical in syntax and semantics to pragma
@cite{Export} as defined in the Ada Reference Manual.  It is
provided for compatibility with some Ada 83 compilers that
used this pragma for exactly the same purposes as pragma
@cite{Export} before the latter was standardized.

@node Pragma External_Name_Casing,Pragma Fast_Math,Pragma External,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-external-name-casing}@anchor{59}
@section Pragma External_Name_Casing


@geindex Dec Ada 83 casing compatibility

@geindex External Names
@geindex casing

@geindex Casing of External names

Syntax:

@example
pragma External_Name_Casing (
  Uppercase | Lowercase
  [, Uppercase | Lowercase | As_Is]);
@end example

This pragma provides control over the casing of external names associated
with Import and Export pragmas.  There are two cases to consider:


@itemize *

@item 
Implicit external names

Implicit external names are derived from identifiers.  The most common case
arises when a standard Ada Import or Export pragma is used with only two
arguments, as in:

@example
pragma Import (C, C_Routine);
@end example

Since Ada is a case-insensitive language, the spelling of the identifier in
the Ada source program does not provide any information on the desired
casing of the external name, and so a convention is needed.  In GNAT the
default treatment is that such names are converted to all lower case
letters.  This corresponds to the normal C style in many environments.
The first argument of pragma @cite{External_Name_Casing} can be used to
control this treatment.  If @cite{Uppercase} is specified, then the name
will be forced to all uppercase letters.  If @cite{Lowercase} is specified,
then the normal default of all lower case letters will be used.

This same implicit treatment is also used in the case of extended DEC Ada 83
compatible Import and Export pragmas where an external name is explicitly
specified using an identifier rather than a string.

@item 
Explicit external names

Explicit external names are given as string literals.  The most common case
arises when a standard Ada Import or Export pragma is used with three
arguments, as in:

@example
pragma Import (C, C_Routine, "C_routine");
@end example

In this case, the string literal normally provides the exact casing required
for the external name.  The second argument of pragma
@cite{External_Name_Casing} may be used to modify this behavior.
If @cite{Uppercase} is specified, then the name
will be forced to all uppercase letters.  If @cite{Lowercase} is specified,
then the name will be forced to all lowercase letters.  A specification of
@cite{As_Is} provides the normal default behavior in which the casing is
taken from the string provided.
@end itemize

This pragma may appear anywhere that a pragma is valid.  In particular, it
can be used as a configuration pragma in the @code{gnat.adc} file, in which
case it applies to all subsequent compilations, or it can be used as a program
unit pragma, in which case it only applies to the current unit, or it can
be used more locally to control individual Import/Export pragmas.

It was primarily intended for use with OpenVMS systems, where many
compilers convert all symbols to upper case by default.  For interfacing to
such compilers (e.g., the DEC C compiler), it may be convenient to use
the pragma:

@example
pragma External_Name_Casing (Uppercase, Uppercase);
@end example

to enforce the upper casing of all external symbols.

@node Pragma Fast_Math,Pragma Favor_Top_Level,Pragma External_Name_Casing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-fast-math}@anchor{5a}
@section Pragma Fast_Math


Syntax:

@example
pragma Fast_Math;
@end example

This is a configuration pragma which activates a mode in which speed is
considered more important for floating-point operations than absolutely
accurate adherence to the requirements of the standard. Currently the
following operations are affected:


@table @asis

@item @emph{Complex Multiplication}

The normal simple formula for complex multiplication can result in intermediate
overflows for numbers near the end of the range. The Ada standard requires that
this situation be detected and corrected by scaling, but in Fast_Math mode such
cases will simply result in overflow. Note that to take advantage of this you
must instantiate your own version of @cite{Ada.Numerics.Generic_Complex_Types}
under control of the pragma, rather than use the preinstantiated versions.
@end table

@node Pragma Favor_Top_Level,Pragma Finalize_Storage_Only,Pragma Fast_Math,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-favor-top-level}@anchor{5b}
@section Pragma Favor_Top_Level


Syntax:

@example
pragma Favor_Top_Level (type_NAME);
@end example

The named type must be an access-to-subprogram type. This pragma is an
efficiency hint to the compiler, regarding the use of 'Access or
'Unrestricted_Access on nested (non-library-level) subprograms. The
pragma means that nested subprograms are not used with this type, or
are rare, so that the generated code should be efficient in the
top-level case. When this pragma is used, dynamically generated
trampolines may be used on some targets for nested subprograms.
See also the No_Implicit_Dynamic_Code restriction.

@node Pragma Finalize_Storage_Only,Pragma Float_Representation,Pragma Favor_Top_Level,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-finalize-storage-only}@anchor{5c}
@section Pragma Finalize_Storage_Only


Syntax:

@example
pragma Finalize_Storage_Only (first_subtype_LOCAL_NAME);
@end example

This pragma allows the compiler not to emit a Finalize call for objects
defined at the library level.  This is mostly useful for types where
finalization is only used to deal with storage reclamation since in most
environments it is not necessary to reclaim memory just before terminating
execution, hence the name.

@node Pragma Float_Representation,Pragma Global,Pragma Finalize_Storage_Only,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-float-representation}@anchor{5d}
@section Pragma Float_Representation


Syntax:

@example
pragma Float_Representation (FLOAT_REP[, float_type_LOCAL_NAME]);

FLOAT_REP ::= VAX_Float | IEEE_Float
@end example

In the one argument form, this pragma is a configuration pragma which
allows control over the internal representation chosen for the predefined
floating point types declared in the packages @cite{Standard} and
@cite{System}. This pragma is only provided for compatibility and has no effect.

The two argument form specifies the representation to be used for
the specified floating-point type. The argument must
be @cite{IEEE_Float} to specify the use of IEEE format, as follows:


@itemize *

@item 
For a digits value of 6, 32-bit IEEE short format will be used.

@item 
For a digits value of 15, 64-bit IEEE long format will be used.

@item 
No other value of digits is permitted.
@end itemize

@node Pragma Global,Pragma Ident,Pragma Float_Representation,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-global}@anchor{5e}
@section Pragma Global


For the description of this pragma, see SPARK 2014 Reference Manual,
section 6.1.4.

@node Pragma Ident,Pragma Ignore_Pragma,Pragma Global,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ident}@anchor{5f}
@section Pragma Ident


Syntax:

@example
pragma Ident (static_string_EXPRESSION);
@end example

This pragma is identical in effect to pragma @cite{Comment}. It is provided
for compatibility with other Ada compilers providing this pragma.

@node Pragma Ignore_Pragma,Pragma Implementation_Defined,Pragma Ident,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ignore-pragma}@anchor{60}
@section Pragma Ignore_Pragma


Syntax:

@example
pragma Ignore_Pragma (pragma_IDENTIFIER);
@end example

This is a configuration pragma
that takes a single argument that is a simple identifier. Any subsequent
use of a pragma whose pragma identifier matches this argument will be
silently ignored. This may be useful when legacy code or code intended
for compilation with some other compiler contains pragmas that match the
name, but not the exact implementation, of a @cite{GNAT} pragma. The use of this
pragma allows such pragmas to be ignored, which may be useful in @cite{CodePeer}
mode, or during porting of legacy code.

@node Pragma Implementation_Defined,Pragma Implemented,Pragma Ignore_Pragma,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-implementation-defined}@anchor{61}
@section Pragma Implementation_Defined


Syntax:

@example
pragma Implementation_Defined (local_NAME);
@end example

This pragma marks a previously declared entioty as implementation-defined.
For an overloaded entity, applies to the most recent homonym.

@example
pragma Implementation_Defined;
@end example

The form with no arguments appears anywhere within a scope, most
typically a package spec, and indicates that all entities that are
defined within the package spec are Implementation_Defined.

This pragma is used within the GNAT runtime library to identify
implementation-defined entities introduced in language-defined units,
for the purpose of implementing the No_Implementation_Identifiers
restriction.

@node Pragma Implemented,Pragma Implicit_Packing,Pragma Implementation_Defined,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-implemented}@anchor{62}
@section Pragma Implemented


Syntax:

@example
pragma Implemented (procedure_LOCAL_NAME, implementation_kind);

implementation_kind ::= By_Entry | By_Protected_Procedure | By_Any
@end example

This is an Ada 2012 representation pragma which applies to protected, task
and synchronized interface primitives. The use of pragma Implemented provides
a way to impose a static requirement on the overriding operation by adhering
to one of the three implementation kinds: entry, protected procedure or any of
the above. This pragma is available in all earlier versions of Ada as an
implementation-defined pragma.

@example
type Synch_Iface is synchronized interface;
procedure Prim_Op (Obj : in out Iface) is abstract;
pragma Implemented (Prim_Op, By_Protected_Procedure);

protected type Prot_1 is new Synch_Iface with
   procedure Prim_Op;  --  Legal
end Prot_1;

protected type Prot_2 is new Synch_Iface with
   entry Prim_Op;      --  Illegal
end Prot_2;

task type Task_Typ is new Synch_Iface with
   entry Prim_Op;      --  Illegal
end Task_Typ;
@end example

When applied to the procedure_or_entry_NAME of a requeue statement, pragma
Implemented determines the runtime behavior of the requeue. Implementation kind
By_Entry guarantees that the action of requeueing will proceed from an entry to
another entry. Implementation kind By_Protected_Procedure transforms the
requeue into a dispatching call, thus eliminating the chance of blocking. Kind
By_Any shares the behavior of By_Entry and By_Protected_Procedure depending on
the target's overriding subprogram kind.

@node Pragma Implicit_Packing,Pragma Import_Function,Pragma Implemented,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-implicit-packing}@anchor{63}
@section Pragma Implicit_Packing


@geindex Rational Profile

Syntax:

@example
pragma Implicit_Packing;
@end example

This is a configuration pragma that requests implicit packing for packed
arrays for which a size clause is given but no explicit pragma Pack or
specification of Component_Size is present. It also applies to records
where no record representation clause is present. Consider this example:

@example
type R is array (0 .. 7) of Boolean;
for R'Size use 8;
@end example

In accordance with the recommendation in the RM (RM 13.3(53)), a Size clause
does not change the layout of a composite object. So the Size clause in the
above example is normally rejected, since the default layout of the array uses
8-bit components, and thus the array requires a minimum of 64 bits.

If this declaration is compiled in a region of code covered by an occurrence
of the configuration pragma Implicit_Packing, then the Size clause in this
and similar examples will cause implicit packing and thus be accepted. For
this implicit packing to occur, the type in question must be an array of small
components whose size is known at compile time, and the Size clause must
specify the exact size that corresponds to the number of elements in the array
multiplied by the size in bits of the component type (both single and
multi-dimensioned arrays can be controlled with this pragma).

@geindex Array packing

Similarly, the following example shows the use in the record case

@example
type r is record
   a, b, c, d, e, f, g, h : boolean;
   chr                    : character;
end record;
for r'size use 16;
@end example

Without a pragma Pack, each Boolean field requires 8 bits, so the
minimum size is 72 bits, but with a pragma Pack, 16 bits would be
sufficient. The use of pragma Implicit_Packing allows this record
declaration to compile without an explicit pragma Pack.

@node Pragma Import_Function,Pragma Import_Object,Pragma Implicit_Packing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-function}@anchor{64}
@section Pragma Import_Function


Syntax:

@example
pragma Import_Function (
     [Internal                 =>] LOCAL_NAME,
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Result_Type              =>] SUBTYPE_MARK]
  [, [Mechanism                =>] MECHANISM]
  [, [Result_Mechanism         =>] MECHANISM_NAME]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::=
  Value
| Reference
@end example

This pragma is used in conjunction with a pragma @cite{Import} to
specify additional information for an imported function.  The pragma
@cite{Import} (or equivalent pragma @cite{Interface}) must precede the
@cite{Import_Function} pragma and both must appear in the same
declarative part as the function specification.

The @cite{Internal} argument must uniquely designate
the function to which the
pragma applies.  If more than one function name exists of this name in
the declarative part you must use the @cite{Parameter_Types} and
@cite{Result_Type} parameters to achieve the required unique
designation.  Subtype marks in these parameters must exactly match the
subtypes in the corresponding function specification, using positional
notation to match parameters with subtype marks.
The form with an @cite{'Access} attribute can be used to match an
anonymous access parameter.

You may optionally use the @cite{Mechanism} and @cite{Result_Mechanism}
parameters to specify passing mechanisms for the
parameters and result.  If you specify a single mechanism name, it
applies to all parameters.  Otherwise you may specify a mechanism on a
parameter by parameter basis using either positional or named
notation.  If the mechanism is not specified, the default mechanism
is used.

@node Pragma Import_Object,Pragma Import_Procedure,Pragma Import_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-object}@anchor{65}
@section Pragma Import_Object


Syntax:

@example
pragma Import_Object
     [Internal =>] LOCAL_NAME
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma designates an object as imported, and apart from the
extended rules for external symbols, is identical in effect to the use of
the normal @cite{Import} pragma applied to an object.  Unlike the
subprogram case, you need not use a separate @cite{Import} pragma,
although you may do so (and probably should do so from a portability
point of view).  @cite{size} is syntax checked, but otherwise ignored by
GNAT.

@node Pragma Import_Procedure,Pragma Import_Valued_Procedure,Pragma Import_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-procedure}@anchor{66}
@section Pragma Import_Procedure


Syntax:

@example
pragma Import_Procedure (
     [Internal                 =>] LOCAL_NAME
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Mechanism                =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @cite{Import_Function} except that it
applies to a procedure rather than a function and the parameters
@cite{Result_Type} and @cite{Result_Mechanism} are not permitted.

@node Pragma Import_Valued_Procedure,Pragma Independent,Pragma Import_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-import-valued-procedure}@anchor{67}
@section Pragma Import_Valued_Procedure


Syntax:

@example
pragma Import_Valued_Procedure (
     [Internal                 =>] LOCAL_NAME
  [, [External                 =>] EXTERNAL_SYMBOL]
  [, [Parameter_Types          =>] PARAMETER_TYPES]
  [, [Mechanism                =>] MECHANISM]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION

PARAMETER_TYPES ::=
  null
| TYPE_DESIGNATOR @{, TYPE_DESIGNATOR@}

TYPE_DESIGNATOR ::=
  subtype_NAME
| subtype_Name ' Access

MECHANISM ::=
  MECHANISM_NAME
| (MECHANISM_ASSOCIATION @{, MECHANISM_ASSOCIATION@})

MECHANISM_ASSOCIATION ::=
  [formal_parameter_NAME =>] MECHANISM_NAME

MECHANISM_NAME ::= Value | Reference
@end example

This pragma is identical to @cite{Import_Procedure} except that the
first parameter of @cite{LOCAL_NAME}, which must be present, must be of
mode @cite{OUT}, and externally the subprogram is treated as a function
with this parameter as the result of the function.  The purpose of this
capability is to allow the use of @cite{OUT} and @cite{IN OUT}
parameters in interfacing to external functions (which are not permitted
in Ada functions).  You may optionally use the @cite{Mechanism}
parameters to specify passing mechanisms for the parameters.
If you specify a single mechanism name, it applies to all parameters.
Otherwise you may specify a mechanism on a parameter by parameter
basis using either positional or named notation.  If the mechanism is not
specified, the default mechanism is used.

Note that it is important to use this pragma in conjunction with a separate
pragma Import that specifies the desired convention, since otherwise the
default convention is Ada, which is almost certainly not what is required.

@node Pragma Independent,Pragma Independent_Components,Pragma Import_Valued_Procedure,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-independent}@anchor{68}
@section Pragma Independent


Syntax:

@example
pragma Independent (Local_NAME);
@end example

This pragma is standard in Ada 2012 mode (which also provides an aspect
of the same name). It is also available as an implementation-defined
pragma in all earlier versions. It specifies that the
designated object or all objects of the designated type must be
independently addressable. This means that separate tasks can safely
manipulate such objects. For example, if two components of a record are
independent, then two separate tasks may access these two components.
This may place
constraints on the representation of the object (for instance prohibiting
tight packing).

@node Pragma Independent_Components,Pragma Initial_Condition,Pragma Independent,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-independent-components}@anchor{69}
@section Pragma Independent_Components


Syntax:

@example
pragma Independent_Components (Local_NAME);
@end example

This pragma is standard in Ada 2012 mode (which also provides an aspect
of the same name). It is also available as an implementation-defined
pragma in all earlier versions. It specifies that the components of the
designated object, or the components of each object of the designated
type, must be
independently addressable. This means that separate tasks can safely
manipulate separate components in the composite object. This may place
constraints on the representation of the object (for instance prohibiting
tight packing).

@node Pragma Initial_Condition,Pragma Initialize_Scalars,Pragma Independent_Components,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-initial-condition}@anchor{6a}
@section Pragma Initial_Condition


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.6.

@node Pragma Initialize_Scalars,Pragma Initializes,Pragma Initial_Condition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-initialize-scalars}@anchor{6b}
@section Pragma Initialize_Scalars


@geindex debugging with Initialize_Scalars

Syntax:

@example
pragma Initialize_Scalars;
@end example

This pragma is similar to @cite{Normalize_Scalars} conceptually but has
two important differences.  First, there is no requirement for the pragma
to be used uniformly in all units of a partition, in particular, it is fine
to use this just for some or all of the application units of a partition,
without needing to recompile the run-time library.

In the case where some units are compiled with the pragma, and some without,
then a declaration of a variable where the type is defined in package
Standard or is locally declared will always be subject to initialization,
as will any declaration of a scalar variable.  For composite variables,
whether the variable is initialized may also depend on whether the package
in which the type of the variable is declared is compiled with the pragma.

The other important difference is that you can control the value used
for initializing scalar objects.  At bind time, you can select several
options for initialization. You can
initialize with invalid values (similar to Normalize_Scalars, though for
Initialize_Scalars it is not always possible to determine the invalid
values in complex cases like signed component fields with non-standard
sizes). You can also initialize with high or
low values, or with a specified bit pattern.  See the GNAT
User's Guide for binder options for specifying these cases.

This means that you can compile a program, and then without having to
recompile the program, you can run it with different values being used
for initializing otherwise uninitialized values, to test if your program
behavior depends on the choice.  Of course the behavior should not change,
and if it does, then most likely you have an incorrect reference to an
uninitialized value.

It is even possible to change the value at execution time eliminating even
the need to rebind with a different switch using an environment variable.
See the GNAT User's Guide for details.

Note that pragma @cite{Initialize_Scalars} is particularly useful in
conjunction with the enhanced validity checking that is now provided
in GNAT, which checks for invalid values under more conditions.
Using this feature (see description of the @emph{-gnatV} flag in the
GNAT User's Guide) in conjunction with
pragma @cite{Initialize_Scalars}
provides a powerful new tool to assist in the detection of problems
caused by uninitialized variables.

Note: the use of @cite{Initialize_Scalars} has a fairly extensive
effect on the generated code. This may cause your code to be
substantially larger. It may also cause an increase in the amount
of stack required, so it is probably a good idea to turn on stack
checking (see description of stack checking in the GNAT
User's Guide) when using this pragma.

@node Pragma Initializes,Pragma Inline_Always,Pragma Initialize_Scalars,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-initializes}@anchor{6c}
@section Pragma Initializes


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.1.5.

@node Pragma Inline_Always,Pragma Inline_Generic,Pragma Initializes,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-inline-always}@anchor{6d}
@section Pragma Inline_Always


Syntax:

@example
pragma Inline_Always (NAME [, NAME]);
@end example

Similar to pragma @cite{Inline} except that inlining is not subject to
the use of option @emph{-gnatn} or @emph{-gnatN} and the inlining
happens regardless of whether these options are used.

@node Pragma Inline_Generic,Pragma Interface,Pragma Inline_Always,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-inline-generic}@anchor{6e}
@section Pragma Inline_Generic


Syntax:

@example
pragma Inline_Generic (GNAME @{, GNAME@});

GNAME ::= generic_unit_NAME | generic_instance_NAME
@end example

This pragma is provided for compatibility with Dec Ada 83. It has
no effect in @cite{GNAT} (which always inlines generics), other
than to check that the given names are all names of generic units or
generic instances.

@node Pragma Interface,Pragma Interface_Name,Pragma Inline_Generic,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interface}@anchor{6f}
@section Pragma Interface


Syntax:

@example
pragma Interface (
     [Convention    =>] convention_identifier,
     [Entity        =>] local_NAME
  [, [External_Name =>] static_string_expression]
  [, [Link_Name     =>] static_string_expression]);
@end example

This pragma is identical in syntax and semantics to
the standard Ada pragma @cite{Import}.  It is provided for compatibility
with Ada 83.  The definition is upwards compatible both with pragma
@cite{Interface} as defined in the Ada 83 Reference Manual, and also
with some extended implementations of this pragma in certain Ada 83
implementations.  The only difference between pragma @cite{Interface}
and pragma @cite{Import} is that there is special circuitry to allow
both pragmas to appear for the same subprogram entity (normally it
is illegal to have multiple @cite{Import} pragmas. This is useful in
maintaining Ada 83/Ada 95 compatibility and is compatible with other
Ada 83 compilers.

@node Pragma Interface_Name,Pragma Interrupt_Handler,Pragma Interface,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interface-name}@anchor{70}
@section Pragma Interface_Name


Syntax:

@example
pragma Interface_Name (
     [Entity        =>] LOCAL_NAME
  [, [External_Name =>] static_string_EXPRESSION]
  [, [Link_Name     =>] static_string_EXPRESSION]);
@end example

This pragma provides an alternative way of specifying the interface name
for an interfaced subprogram, and is provided for compatibility with Ada
83 compilers that use the pragma for this purpose.  You must provide at
least one of @cite{External_Name} or @cite{Link_Name}.

@node Pragma Interrupt_Handler,Pragma Interrupt_State,Pragma Interface_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interrupt-handler}@anchor{71}
@section Pragma Interrupt_Handler


Syntax:

@example
pragma Interrupt_Handler (procedure_LOCAL_NAME);
@end example

This program unit pragma is supported for parameterless protected procedures
as described in Annex C of the Ada Reference Manual. On the AAMP target
the pragma can also be specified for nonprotected parameterless procedures
that are declared at the library level (which includes procedures
declared at the top level of a library package). In the case of AAMP,
when this pragma is applied to a nonprotected procedure, the instruction
@cite{IERET} is generated for returns from the procedure, enabling
maskable interrupts, in place of the normal return instruction.

@node Pragma Interrupt_State,Pragma Invariant,Pragma Interrupt_Handler,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-interrupt-state}@anchor{72}
@section Pragma Interrupt_State


Syntax:

@example
pragma Interrupt_State
 ([Name  =>] value,
  [State =>] SYSTEM | RUNTIME | USER);
@end example

Normally certain interrupts are reserved to the implementation.  Any attempt
to attach an interrupt causes Program_Error to be raised, as described in
RM C.3.2(22).  A typical example is the @cite{SIGINT} interrupt used in
many systems for an @code{Ctrl-C} interrupt.  Normally this interrupt is
reserved to the implementation, so that @code{Ctrl-C} can be used to
interrupt execution.  Additionally, signals such as @cite{SIGSEGV},
@cite{SIGABRT}, @cite{SIGFPE} and @cite{SIGILL} are often mapped to specific
Ada exceptions, or used to implement run-time functions such as the
@cite{abort} statement and stack overflow checking.

Pragma @cite{Interrupt_State} provides a general mechanism for overriding
such uses of interrupts.  It subsumes the functionality of pragma
@cite{Unreserve_All_Interrupts}.  Pragma @cite{Interrupt_State} is not
available on Windows or VMS.  On all other platforms than VxWorks,
it applies to signals; on VxWorks, it applies to vectored hardware interrupts
and may be used to mark interrupts required by the board support package
as reserved.

Interrupts can be in one of three states:


@itemize *

@item 
System

The interrupt is reserved (no Ada handler can be installed), and the
Ada run-time may not install a handler. As a result you are guaranteed
standard system default action if this interrupt is raised.

@item 
Runtime

The interrupt is reserved (no Ada handler can be installed). The run time
is allowed to install a handler for internal control purposes, but is
not required to do so.

@item 
User

The interrupt is unreserved.  The user may install a handler to provide
some other action.
@end itemize

These states are the allowed values of the @cite{State} parameter of the
pragma.  The @cite{Name} parameter is a value of the type
@cite{Ada.Interrupts.Interrupt_ID}.  Typically, it is a name declared in
@cite{Ada.Interrupts.Names}.

This is a configuration pragma, and the binder will check that there
are no inconsistencies between different units in a partition in how a
given interrupt is specified. It may appear anywhere a pragma is legal.

The effect is to move the interrupt to the specified state.

By declaring interrupts to be SYSTEM, you guarantee the standard system
action, such as a core dump.

By declaring interrupts to be USER, you guarantee that you can install
a handler.

Note that certain signals on many operating systems cannot be caught and
handled by applications.  In such cases, the pragma is ignored.  See the
operating system documentation, or the value of the array @cite{Reserved}
declared in the spec of package @cite{System.OS_Interface}.

Overriding the default state of signals used by the Ada runtime may interfere
with an application's runtime behavior in the cases of the synchronous signals,
and in the case of the signal used to implement the @cite{abort} statement.

@node Pragma Invariant,Pragma Java_Constructor,Pragma Interrupt_State,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-invariant}@anchor{73}
@section Pragma Invariant


Syntax:

@example
pragma Invariant
  ([Entity =>]    private_type_LOCAL_NAME,
   [Check  =>]    EXPRESSION
   [,[Message =>] String_Expression]);
@end example

This pragma provides exactly the same capabilities as the Type_Invariant aspect
defined in AI05-0146-1, and in the Ada 2012 Reference Manual. The
Type_Invariant aspect is fully implemented in Ada 2012 mode, but since it
requires the use of the aspect syntax, which is not available except in 2012
mode, it is not possible to use the Type_Invariant aspect in earlier versions
of Ada. However the Invariant pragma may be used in any version of Ada. Also
note that the aspect Invariant is a synonym in GNAT for the aspect
Type_Invariant, but there is no pragma Type_Invariant.

The pragma must appear within the visible part of the package specification,
after the type to which its Entity argument appears. As with the Invariant
aspect, the Check expression is not analyzed until the end of the visible
part of the package, so it may contain forward references. The Message
argument, if present, provides the exception message used if the invariant
is violated. If no Message parameter is provided, a default message that
identifies the line on which the pragma appears is used.

It is permissible to have multiple Invariants for the same type entity, in
which case they are and'ed together. It is permissible to use this pragma
in Ada 2012 mode, but you cannot have both an invariant aspect and an
invariant pragma for the same entity.

For further details on the use of this pragma, see the Ada 2012 documentation
of the Type_Invariant aspect.

@node Pragma Java_Constructor,Pragma Java_Interface,Pragma Invariant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-java-constructor}@anchor{74}
@section Pragma Java_Constructor


Syntax:

@example
pragma Java_Constructor ([Entity =>] function_LOCAL_NAME);
@end example

This pragma is used to assert that the specified Ada function should be
mapped to the Java constructor for some Ada tagged record type.

See section 7.3.2 of the
@cite{GNAT User's Guide: Supplement for the JVM Platform.}
for related information.

@node Pragma Java_Interface,Pragma Keep_Names,Pragma Java_Constructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-java-interface}@anchor{75}
@section Pragma Java_Interface


Syntax:

@example
pragma Java_Interface ([Entity =>] abstract_tagged_type_LOCAL_NAME);
@end example

This pragma is used to assert that the specified Ada abstract tagged type
is to be mapped to a Java interface name.

See sections 7.1 and 7.2 of the
@cite{GNAT User's Guide: Supplement for the JVM Platform.}
for related information.

@node Pragma Keep_Names,Pragma License,Pragma Java_Interface,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-keep-names}@anchor{76}
@section Pragma Keep_Names


Syntax:

@example
pragma Keep_Names ([On =>] enumeration_first_subtype_LOCAL_NAME);
@end example

The @cite{LOCAL_NAME} argument
must refer to an enumeration first subtype
in the current declarative part. The effect is to retain the enumeration
literal names for use by @cite{Image} and @cite{Value} even if a global
@cite{Discard_Names} pragma applies. This is useful when you want to
generally suppress enumeration literal names and for example you therefore
use a @cite{Discard_Names} pragma in the @code{gnat.adc} file, but you
want to retain the names for specific enumeration types.

@node Pragma License,Pragma Link_With,Pragma Keep_Names,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-license}@anchor{77}
@section Pragma License


@geindex License checking

Syntax:

@example
pragma License (Unrestricted | GPL | Modified_GPL | Restricted);
@end example

This pragma is provided to allow automated checking for appropriate license
conditions with respect to the standard and modified GPL.  A pragma
@cite{License}, which is a configuration pragma that typically appears at
the start of a source file or in a separate @code{gnat.adc} file, specifies
the licensing conditions of a unit as follows:


@itemize *

@item 
Unrestricted
This is used for a unit that can be freely used with no license restrictions.
Examples of such units are public domain units, and units from the Ada
Reference Manual.

@item 
GPL
This is used for a unit that is licensed under the unmodified GPL, and which
therefore cannot be @cite{with}'ed by a restricted unit.

@item 
Modified_GPL
This is used for a unit licensed under the GNAT modified GPL that includes
a special exception paragraph that specifically permits the inclusion of
the unit in programs without requiring the entire program to be released
under the GPL.

@item 
Restricted
This is used for a unit that is restricted in that it is not permitted to
depend on units that are licensed under the GPL.  Typical examples are
proprietary code that is to be released under more restrictive license
conditions.  Note that restricted units are permitted to @cite{with} units
which are licensed under the modified GPL (this is the whole point of the
modified GPL).
@end itemize

Normally a unit with no @cite{License} pragma is considered to have an
unknown license, and no checking is done.  However, standard GNAT headers
are recognized, and license information is derived from them as follows.

A GNAT license header starts with a line containing 78 hyphens.  The following
comment text is searched for the appearance of any of the following strings.

If the string 'GNU General Public License' is found, then the unit is assumed
to have GPL license, unless the string 'As a special exception' follows, in
which case the license is assumed to be modified GPL.

If one of the strings
'This specification is adapted from the Ada Semantic Interface' or
'This specification is derived from the Ada Reference Manual' is found
then the unit is assumed to be unrestricted.

These default actions means that a program with a restricted license pragma
will automatically get warnings if a GPL unit is inappropriately
@cite{with}'ed.  For example, the program:

@example
with Sem_Ch3;
with GNAT.Sockets;
procedure Secret_Stuff is
  ...
end Secret_Stuff
@end example

if compiled with pragma @cite{License} (@cite{Restricted}) in a
@code{gnat.adc} file will generate the warning:

@example
1.  with Sem_Ch3;
        |
   >>> license of withed unit "Sem_Ch3" is incompatible

2.  with GNAT.Sockets;
3.  procedure Secret_Stuff is
@end example

Here we get a warning on @cite{Sem_Ch3} since it is part of the GNAT
compiler and is licensed under the
GPL, but no warning for @cite{GNAT.Sockets} which is part of the GNAT
run time, and is therefore licensed under the modified GPL.

@node Pragma Link_With,Pragma Linker_Alias,Pragma License,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-link-with}@anchor{78}
@section Pragma Link_With


Syntax:

@example
pragma Link_With (static_string_EXPRESSION @{,static_string_EXPRESSION@});
@end example

This pragma is provided for compatibility with certain Ada 83 compilers.
It has exactly the same effect as pragma @cite{Linker_Options} except
that spaces occurring within one of the string expressions are treated
as separators. For example, in the following case:

@example
pragma Link_With ("-labc -ldef");
@end example

results in passing the strings @cite{-labc} and @cite{-ldef} as two
separate arguments to the linker. In addition pragma Link_With allows
multiple arguments, with the same effect as successive pragmas.

@node Pragma Linker_Alias,Pragma Linker_Constructor,Pragma Link_With,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-alias}@anchor{79}
@section Pragma Linker_Alias


Syntax:

@example
pragma Linker_Alias (
  [Entity =>] LOCAL_NAME,
  [Target =>] static_string_EXPRESSION);
@end example

@cite{LOCAL_NAME} must refer to an object that is declared at the library
level. This pragma establishes the given entity as a linker alias for the
given target. It is equivalent to @cite{__attribute__((alias))} in GNU C
and causes @cite{LOCAL_NAME} to be emitted as an alias for the symbol
@cite{static_string_EXPRESSION} in the object file, that is to say no space
is reserved for @cite{LOCAL_NAME} by the assembler and it will be resolved
to the same address as @cite{static_string_EXPRESSION} by the linker.

The actual linker name for the target must be used (e.g., the fully
encoded name with qualification in Ada, or the mangled name in C++),
or it must be declared using the C convention with @cite{pragma Import}
or @cite{pragma Export}.

Not all target machines support this pragma. On some of them it is accepted
only if @cite{pragma Weak_External} has been applied to @cite{LOCAL_NAME}.

@example
--  Example of the use of pragma Linker_Alias

package p is
  i : Integer := 1;
  pragma Export (C, i);

  new_name_for_i : Integer;
  pragma Linker_Alias (new_name_for_i, "i");
end p;
@end example

@node Pragma Linker_Constructor,Pragma Linker_Destructor,Pragma Linker_Alias,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-constructor}@anchor{7a}
@section Pragma Linker_Constructor


Syntax:

@example
pragma Linker_Constructor (procedure_LOCAL_NAME);
@end example

@cite{procedure_LOCAL_NAME} must refer to a parameterless procedure that
is declared at the library level. A procedure to which this pragma is
applied will be treated as an initialization routine by the linker.
It is equivalent to @cite{__attribute__((constructor))} in GNU C and
causes @cite{procedure_LOCAL_NAME} to be invoked before the entry point
of the executable is called (or immediately after the shared library is
loaded if the procedure is linked in a shared library), in particular
before the Ada run-time environment is set up.

Because of these specific contexts, the set of operations such a procedure
can perform is very limited and the type of objects it can manipulate is
essentially restricted to the elementary types. In particular, it must only
contain code to which pragma Restrictions (No_Elaboration_Code) applies.

This pragma is used by GNAT to implement auto-initialization of shared Stand
Alone Libraries, which provides a related capability without the restrictions
listed above. Where possible, the use of Stand Alone Libraries is preferable
to the use of this pragma.

@node Pragma Linker_Destructor,Pragma Linker_Section,Pragma Linker_Constructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-destructor}@anchor{7b}
@section Pragma Linker_Destructor


Syntax:

@example
pragma Linker_Destructor (procedure_LOCAL_NAME);
@end example

@cite{procedure_LOCAL_NAME} must refer to a parameterless procedure that
is declared at the library level. A procedure to which this pragma is
applied will be treated as a finalization routine by the linker.
It is equivalent to @cite{__attribute__((destructor))} in GNU C and
causes @cite{procedure_LOCAL_NAME} to be invoked after the entry point
of the executable has exited (or immediately before the shared library
is unloaded if the procedure is linked in a shared library), in particular
after the Ada run-time environment is shut down.

See @cite{pragma Linker_Constructor} for the set of restrictions that apply
because of these specific contexts.

@node Pragma Linker_Section,Pragma Lock_Free,Pragma Linker_Destructor,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-linker-section}@anchor{7c}
@section Pragma Linker_Section


Syntax:

@example
pragma Linker_Section (
  [Entity  =>] LOCAL_NAME,
  [Section =>] static_string_EXPRESSION);
@end example

@cite{LOCAL_NAME} must refer to an object, type, or subprogram that is
declared at the library level. This pragma specifies the name of the
linker section for the given entity. It is equivalent to
@cite{__attribute__((section))} in GNU C and causes @cite{LOCAL_NAME} to
be placed in the @cite{static_string_EXPRESSION} section of the
executable (assuming the linker doesn't rename the section).
GNAT also provides an implementation defined aspect of the same name.

In the case of specifying this aspect for a type, the effect is to
specify the corresponding for all library level objects of the type which
do not have an explicit linker section set. Note that this only applies to
whole objects, not to components of composite objects.

In the case of a subprogram, the linker section applies to all previously
declared matching overloaded subprograms in the current declarative part
which do not already have a linker section assigned. The linker section
aspect is useful in this case for specifying different linker sections
for different elements of such an overloaded set.

Note that an empty string specifies that no linker section is specified.
This is not quite the same as omitting the pragma or aspect, since it
can be used to specify that one element of an overloaded set of subprograms
has the default linker section, or that one object of a type for which a
linker section is specified should has the default linker section.

The compiler normally places library-level entities in standard sections
depending on the class: procedures and functions generally go in the
@cite{.text} section, initialized variables in the @cite{.data} section
and uninitialized variables in the @cite{.bss} section.

Other, special sections may exist on given target machines to map special
hardware, for example I/O ports or flash memory. This pragma is a means to
defer the final layout of the executable to the linker, thus fully working
at the symbolic level with the compiler.

Some file formats do not support arbitrary sections so not all target
machines support this pragma. The use of this pragma may cause a program
execution to be erroneous if it is used to place an entity into an
inappropriate section (e.g., a modified variable into the @cite{.text}
section). See also @cite{pragma Persistent_BSS}.

@example
--  Example of the use of pragma Linker_Section

package IO_Card is
  Port_A : Integer;
  pragma Volatile (Port_A);
  pragma Linker_Section (Port_A, ".bss.port_a");

  Port_B : Integer;
  pragma Volatile (Port_B);
  pragma Linker_Section (Port_B, ".bss.port_b");

  type Port_Type is new Integer with Linker_Section => ".bss";
  PA : Port_Type with Linker_Section => ".bss.PA";
  PB : Port_Type; --  ends up in linker section ".bss"

  procedure Q with Linker_Section => "Qsection";
end IO_Card;
@end example

@node Pragma Lock_Free,Pragma Loop_Invariant,Pragma Linker_Section,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-lock-free}@anchor{7d}
@section Pragma Lock_Free


Syntax:
This pragma may be specified for protected types or objects. It specifies that
the implementation of protected operations must be implemented without locks.
Compilation fails if the compiler cannot generate lock-free code for the
operations.

@node Pragma Loop_Invariant,Pragma Loop_Optimize,Pragma Lock_Free,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-loop-invariant}@anchor{7e}
@section Pragma Loop_Invariant


Syntax:

@example
pragma Loop_Invariant ( boolean_EXPRESSION );
@end example

The effect of this pragma is similar to that of pragma @cite{Assert},
except that in an @cite{Assertion_Policy} pragma, the identifier
@cite{Loop_Invariant} is used to control whether it is ignored or checked
(or disabled).

@cite{Loop_Invariant} can only appear as one of the items in the sequence
of statements of a loop body, or nested inside block statements that
appear in the sequence of statements of a loop body.
The intention is that it be used to
represent a "loop invariant" assertion, i.e. something that is true each
time through the loop, and which can be used to show that the loop is
achieving its purpose.

Multiple @cite{Loop_Invariant} and @cite{Loop_Variant} pragmas that
apply to the same loop should be grouped in the same sequence of
statements.

To aid in writing such invariants, the special attribute @cite{Loop_Entry}
may be used to refer to the value of an expression on entry to the loop. This
attribute can only be used within the expression of a @cite{Loop_Invariant}
pragma. For full details, see documentation of attribute @cite{Loop_Entry}.

@node Pragma Loop_Optimize,Pragma Loop_Variant,Pragma Loop_Invariant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-loop-optimize}@anchor{7f}
@section Pragma Loop_Optimize


Syntax:

@example
pragma Loop_Optimize (OPTIMIZATION_HINT @{, OPTIMIZATION_HINT@});

OPTIMIZATION_HINT ::= Ivdep | No_Unroll | Unroll | No_Vector | Vector
@end example

This pragma must appear immediately within a loop statement.  It allows the
programmer to specify optimization hints for the enclosing loop.  The hints
are not mutually exclusive and can be freely mixed, but not all combinations
will yield a sensible outcome.

There are five supported optimization hints for a loop:


@itemize *

@item 
Ivdep

The programmer asserts that there are no loop-carried dependencies
which would prevent consecutive iterations of the loop from being
executed simultaneously.

@item 
No_Unroll

The loop must not be unrolled.  This is a strong hint: the compiler will not
unroll a loop marked with this hint.

@item 
Unroll

The loop should be unrolled.  This is a weak hint: the compiler will try to
apply unrolling to this loop preferably to other optimizations, notably
vectorization, but there is no guarantee that the loop will be unrolled.

@item 
No_Vector

The loop must not be vectorized.  This is a strong hint: the compiler will not
vectorize a loop marked with this hint.

@item 
Vector

The loop should be vectorized.  This is a weak hint: the compiler will try to
apply vectorization to this loop preferably to other optimizations, notably
unrolling, but there is no guarantee that the loop will be vectorized.
@end itemize

These hints do not remove the need to pass the appropriate switches to the
compiler in order to enable the relevant optimizations, that is to say
@emph{-funroll-loops} for unrolling and @emph{-ftree-vectorize} for
vectorization.

@node Pragma Loop_Variant,Pragma Machine_Attribute,Pragma Loop_Optimize,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-loop-variant}@anchor{80}
@section Pragma Loop_Variant


Syntax:

@example
pragma Loop_Variant ( LOOP_VARIANT_ITEM @{, LOOP_VARIANT_ITEM @} );
LOOP_VARIANT_ITEM ::= CHANGE_DIRECTION => discrete_EXPRESSION
CHANGE_DIRECTION ::= Increases | Decreases
@end example

@cite{Loop_Variant} can only appear as one of the items in the sequence
of statements of a loop body, or nested inside block statements that
appear in the sequence of statements of a loop body.
It allows the specification of quantities which must always
decrease or increase in successive iterations of the loop. In its simplest
form, just one expression is specified, whose value must increase or decrease
on each iteration of the loop.

In a more complex form, multiple arguments can be given which are intepreted
in a nesting lexicographic manner. For example:

@example
pragma Loop_Variant (Increases => X, Decreases => Y);
@end example

specifies that each time through the loop either X increases, or X stays
the same and Y decreases. A @cite{Loop_Variant} pragma ensures that the
loop is making progress. It can be useful in helping to show informally
or prove formally that the loop always terminates.

@cite{Loop_Variant} is an assertion whose effect can be controlled using
an @cite{Assertion_Policy} with a check name of @cite{Loop_Variant}. The
policy can be @cite{Check} to enable the loop variant check, @cite{Ignore}
to ignore the check (in which case the pragma has no effect on the program),
or @cite{Disable} in which case the pragma is not even checked for correct
syntax.

Multiple @cite{Loop_Invariant} and @cite{Loop_Variant} pragmas that
apply to the same loop should be grouped in the same sequence of
statements.

The @cite{Loop_Entry} attribute may be used within the expressions of the
@cite{Loop_Variant} pragma to refer to values on entry to the loop.

@node Pragma Machine_Attribute,Pragma Main,Pragma Loop_Variant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-machine-attribute}@anchor{81}
@section Pragma Machine_Attribute


Syntax:

@example
pragma Machine_Attribute (
     [Entity         =>] LOCAL_NAME,
     [Attribute_Name =>] static_string_EXPRESSION
  [, [Info           =>] static_EXPRESSION] );
@end example

Machine-dependent attributes can be specified for types and/or
declarations.  This pragma is semantically equivalent to
@cite{__attribute__((`attribute_name}))` (if @cite{info} is not
specified) or @cite{__attribute__((`attribute_name`(`info})))
in GNU C, where @code{attribute_name} is recognized by the
compiler middle-end or the @cite{TARGET_ATTRIBUTE_TABLE} machine
specific macro.  A string literal for the optional parameter @cite{info}
is transformed into an identifier, which may make this pragma unusable
for some attributes.
For further information see @cite{GNU Compiler Collection (GCC) Internals}.

@node Pragma Main,Pragma Main_Storage,Pragma Machine_Attribute,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-main}@anchor{82}
@section Pragma Main


Syntax:

@example
pragma Main
 (MAIN_OPTION [, MAIN_OPTION]);

MAIN_OPTION ::=
  [Stack_Size              =>] static_integer_EXPRESSION
| [Task_Stack_Size_Default =>] static_integer_EXPRESSION
| [Time_Slicing_Enabled    =>] static_boolean_EXPRESSION
@end example

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has
no effect in GNAT, other than being syntax checked.

@node Pragma Main_Storage,Pragma No_Body,Pragma Main,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-main-storage}@anchor{83}
@section Pragma Main_Storage


Syntax:

@example
pragma Main_Storage
  (MAIN_STORAGE_OPTION [, MAIN_STORAGE_OPTION]);

MAIN_STORAGE_OPTION ::=
  [WORKING_STORAGE =>] static_SIMPLE_EXPRESSION
| [TOP_GUARD       =>] static_SIMPLE_EXPRESSION
@end example

This pragma is provided for compatibility with OpenVMS VAX Systems.  It has
no effect in GNAT, other than being syntax checked.

@node Pragma No_Body,Pragma No_Elaboration_Code_All,Pragma Main_Storage,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-body}@anchor{84}
@section Pragma No_Body


Syntax:

@example
pragma No_Body;
@end example

There are a number of cases in which a package spec does not require a body,
and in fact a body is not permitted. GNAT will not permit the spec to be
compiled if there is a body around. The pragma No_Body allows you to provide
a body file, even in a case where no body is allowed. The body file must
contain only comments and a single No_Body pragma. This is recognized by
the compiler as indicating that no body is logically present.

This is particularly useful during maintenance when a package is modified in
such a way that a body needed before is no longer needed. The provision of a
dummy body with a No_Body pragma ensures that there is no interference from
earlier versions of the package body.

@node Pragma No_Elaboration_Code_All,Pragma No_Inline,Pragma No_Body,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-elaboration-code-all}@anchor{85}
@section Pragma No_Elaboration_Code_All


Syntax:

@example
pragma No_Elaboration_Code_All [(program_unit_NAME)];
@end example

This is a program unit pragma (there is also an equivalent aspect of the
same name) that establishes the restriction @cite{No_Elaboration_Code} for
the current unit and any extended main source units (body and subunits.
It also has has the effect of enforcing a transitive application of this
aspect, so that if any unit is implicitly or explicitly WITH'ed by the
current unit, it must also have the No_Elaboration_Code_All aspect set.
It may be applied to package or subprogram specs or their generic versions.

@node Pragma No_Inline,Pragma No_Return,Pragma No_Elaboration_Code_All,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-inline}@anchor{86}
@section Pragma No_Inline


Syntax:

@example
pragma No_Inline (NAME @{, NAME@});
@end example

This pragma suppresses inlining for the callable entity or the instances of
the generic subprogram designated by @cite{NAME}, including inlining that
results from the use of pragma @cite{Inline}.  This pragma is always active,
in particular it is not subject to the use of option @emph{-gnatn} or
@emph{-gnatN}.  It is illegal to specify both pragma @cite{No_Inline} and
pragma @cite{Inline_Always} for the same @cite{NAME}.

@node Pragma No_Return,Pragma No_Run_Time,Pragma No_Inline,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-return}@anchor{87}
@section Pragma No_Return


Syntax:

@example
pragma No_Return (procedure_LOCAL_NAME @{, procedure_LOCAL_NAME@});
@end example

Each @cite{procedure_LOCAL_NAME} argument must refer to one or more procedure
declarations in the current declarative part.  A procedure to which this
pragma is applied may not contain any explicit @cite{return} statements.
In addition, if the procedure contains any implicit returns from falling
off the end of a statement sequence, then execution of that implicit
return will cause Program_Error to be raised.

One use of this pragma is to identify procedures whose only purpose is to raise
an exception. Another use of this pragma is to suppress incorrect warnings
about missing returns in functions, where the last statement of a function
statement sequence is a call to such a procedure.

Note that in Ada 2005 mode, this pragma is part of the language. It is
available in all earlier versions of Ada as an implementation-defined
pragma.

@node Pragma No_Run_Time,Pragma No_Strict_Aliasing,Pragma No_Return,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-run-time}@anchor{88}
@section Pragma No_Run_Time


Syntax:

@example
pragma No_Run_Time;
@end example

This is an obsolete configuration pragma that historically was used to
set up a runtime library with no object code. It is now used only for
internal testing. The pragma has been superseded by the reconfigurable
runtime capability of @cite{GNAT}.

@node Pragma No_Strict_Aliasing,Pragma No_Tagged_Streams,Pragma No_Run_Time,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-strict-aliasing}@anchor{89}
@section Pragma No_Strict_Aliasing


Syntax:

@example
pragma No_Strict_Aliasing [([Entity =>] type_LOCAL_NAME)];
@end example

@cite{type_LOCAL_NAME} must refer to an access type
declaration in the current declarative part.  The effect is to inhibit
strict aliasing optimization for the given type.  The form with no
arguments is a configuration pragma which applies to all access types
declared in units to which the pragma applies. For a detailed
description of the strict aliasing optimization, and the situations
in which it must be suppressed, see the section on Optimization and Strict Aliasing
in the @cite{GNAT User's Guide}.

This pragma currently has no effects on access to unconstrained array types.

@node Pragma No_Tagged_Streams,Pragma Normalize_Scalars,Pragma No_Strict_Aliasing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-no-tagged-streams}@anchor{8a}
@section Pragma No_Tagged_Streams


Syntax:

@example
pragma No_Tagged_Streams;
pragma No_Tagged_Streams [([Entity =>] tagged_type_LOCAL_NAME)];
@end example

Normally when a tagged type is introduced using a full type declaration,
part of the processing includes generating stream access routines to be
used by stream attributes referencing the type (or one of its subtypes
or derived types). This can involve the generation of significant amounts
of code which is wasted space if stream routines are not needed for the
type in question.

The @cite{No_Tagged_Streams} pragma causes the generation of these stream
routines to be skipped, and any attempt to use stream operations on
types subject to this pragma will be statically rejected as illegal.

There are two forms of the pragma. The form with no arguments must appear
in a declarative sequence or in the declarations of a package spec. This
pragma affects all subsequent root tagged types declared in the declaration
sequence, and specifies that no stream routines be generated. The form with
an argument (for which there is also a corresponding aspect) specifies a
single root tagged type for which stream routines are not to be generated.

Once the pragma has been given for a particular root tagged type, all subtypes
and derived types of this type inherit the pragma automatically, so the effect
applies to a complete hierarchy (this is necessary to deal with the class-wide
dispatching versions of the stream routines).

@node Pragma Normalize_Scalars,Pragma Obsolescent,Pragma No_Tagged_Streams,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-normalize-scalars}@anchor{8b}
@section Pragma Normalize_Scalars


Syntax:

@example
pragma Normalize_Scalars;
@end example

This is a language defined pragma which is fully implemented in GNAT.  The
effect is to cause all scalar objects that are not otherwise initialized
to be initialized.  The initial values are implementation dependent and
are as follows:


@table @asis

@item @emph{Standard.Character}

Objects whose root type is Standard.Character are initialized to
Character'Last unless the subtype range excludes NUL (in which case
NUL is used). This choice will always generate an invalid value if
one exists.

@item @emph{Standard.Wide_Character}

Objects whose root type is Standard.Wide_Character are initialized to
Wide_Character'Last unless the subtype range excludes NUL (in which case
NUL is used). This choice will always generate an invalid value if
one exists.

@item @emph{Standard.Wide_Wide_Character}

Objects whose root type is Standard.Wide_Wide_Character are initialized to
the invalid value 16#FFFF_FFFF# unless the subtype range excludes NUL (in
which case NUL is used). This choice will always generate an invalid value if
one exists.

@item @emph{Integer types}

Objects of an integer type are treated differently depending on whether
negative values are present in the subtype. If no negative values are
present, then all one bits is used as the initial value except in the
special case where zero is excluded from the subtype, in which case
all zero bits are used. This choice will always generate an invalid
value if one exists.

For subtypes with negative values present, the largest negative number
is used, except in the unusual case where this largest negative number
is in the subtype, and the largest positive number is not, in which case
the largest positive value is used. This choice will always generate
an invalid value if one exists.

@item @emph{Floating-Point Types}

Objects of all floating-point types are initialized to all 1-bits. For
standard IEEE format, this corresponds to a NaN (not a number) which is
indeed an invalid value.

@item @emph{Fixed-Point Types}

Objects of all fixed-point types are treated as described above for integers,
with the rules applying to the underlying integer value used to represent
the fixed-point value.

@item @emph{Modular types}

Objects of a modular type are initialized to all one bits, except in
the special case where zero is excluded from the subtype, in which
case all zero bits are used. This choice will always generate an
invalid value if one exists.

@item @emph{Enumeration types}

Objects of an enumeration type are initialized to all one-bits, i.e., to
the value @cite{2 ** typ'Size - 1} unless the subtype excludes the literal
whose Pos value is zero, in which case a code of zero is used. This choice
will always generate an invalid value if one exists.
@end table

@node Pragma Obsolescent,Pragma Optimize_Alignment,Pragma Normalize_Scalars,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-obsolescent}@anchor{8c}@anchor{gnat_rm/implementation_defined_pragmas id2}@anchor{8d}
@section Pragma Obsolescent


Syntax:

@example
pragma Obsolescent;

pragma Obsolescent (
  [Message =>] static_string_EXPRESSION
[,[Version =>] Ada_05]]);

pragma Obsolescent (
  [Entity  =>] NAME
[,[Message =>] static_string_EXPRESSION
[,[Version =>] Ada_05]] );
@end example

This pragma can occur immediately following a declaration of an entity,
including the case of a record component. If no Entity argument is present,
then this declaration is the one to which the pragma applies. If an Entity
parameter is present, it must either match the name of the entity in this
declaration, or alternatively, the pragma can immediately follow an enumeration
type declaration, where the Entity argument names one of the enumeration
literals.

This pragma is used to indicate that the named entity
is considered obsolescent and should not be used. Typically this is
used when an API must be modified by eventually removing or modifying
existing subprograms or other entities. The pragma can be used at an
intermediate stage when the entity is still present, but will be
removed later.

The effect of this pragma is to output a warning message on a reference to
an entity thus marked that the subprogram is obsolescent if the appropriate
warning option in the compiler is activated. If the Message parameter is
present, then a second warning message is given containing this text. In
addition, a reference to the entity is considered to be a violation of pragma
Restrictions (No_Obsolescent_Features).

This pragma can also be used as a program unit pragma for a package,
in which case the entity name is the name of the package, and the
pragma indicates that the entire package is considered
obsolescent. In this case a client @cite{with}'ing such a package
violates the restriction, and the @cite{with} statement is
flagged with warnings if the warning option is set.

If the Version parameter is present (which must be exactly
the identifier Ada_05, no other argument is allowed), then the
indication of obsolescence applies only when compiling in Ada 2005
mode. This is primarily intended for dealing with the situations
in the predefined library where subprograms or packages
have become defined as obsolescent in Ada 2005
(e.g., in Ada.Characters.Handling), but may be used anywhere.

The following examples show typical uses of this pragma:

@example
package p is
   pragma Obsolescent (p, Message => "use pp instead of p");
end p;

package q is
   procedure q2;
   pragma Obsolescent ("use q2new instead");

   type R is new integer;
   pragma Obsolescent
     (Entity  => R,
      Message => "use RR in Ada 2005",
      Version => Ada_05);

   type M is record
      F1 : Integer;
      F2 : Integer;
      pragma Obsolescent;
      F3 : Integer;
   end record;

   type E is (a, bc, 'd', quack);
   pragma Obsolescent (Entity => bc)
   pragma Obsolescent (Entity => 'd')

   function "+"
     (a, b : character) return character;
   pragma Obsolescent (Entity => "+");
end;
@end example

Note that, as for all pragmas, if you use a pragma argument identifier,
then all subsequent parameters must also use a pragma argument identifier.
So if you specify "Entity =>" for the Entity argument, and a Message
argument is present, it must be preceded by "Message =>".

@node Pragma Optimize_Alignment,Pragma Ordered,Pragma Obsolescent,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-optimize-alignment}@anchor{8e}
@section Pragma Optimize_Alignment


@geindex Alignment
@geindex default settings

Syntax:

@example
pragma Optimize_Alignment (TIME | SPACE | OFF);
@end example

This is a configuration pragma which affects the choice of default alignments
for types and objects where no alignment is explicitly specified. There is a
time/space trade-off in the selection of these values. Large alignments result
in more efficient code, at the expense of larger data space, since sizes have
to be increased to match these alignments. Smaller alignments save space, but
the access code is slower. The normal choice of default alignments for types
and individual alignment promotions for objects (which is what you get if you
do not use this pragma, or if you use an argument of OFF), tries to balance
these two requirements.

Specifying SPACE causes smaller default alignments to be chosen in two cases.
First any packed record is given an alignment of 1. Second, if a size is given
for the type, then the alignment is chosen to avoid increasing this size. For
example, consider:

@example
type R is record
   X : Integer;
   Y : Character;
end record;

for R'Size use 5*8;
@end example

In the default mode, this type gets an alignment of 4, so that access to the
Integer field X are efficient. But this means that objects of the type end up
with a size of 8 bytes. This is a valid choice, since sizes of objects are
allowed to be bigger than the size of the type, but it can waste space if for
example fields of type R appear in an enclosing record. If the above type is
compiled in @cite{Optimize_Alignment (Space)} mode, the alignment is set to 1.

However, there is one case in which SPACE is ignored. If a variable length
record (that is a discriminated record with a component which is an array
whose length depends on a discriminant), has a pragma Pack, then it is not
in general possible to set the alignment of such a record to one, so the
pragma is ignored in this case (with a warning).

Specifying SPACE also disables alignment promotions for standalone objects,
which occur when the compiler increases the alignment of a specific object
without changing the alignment of its type.

Specifying TIME causes larger default alignments to be chosen in the case of
small types with sizes that are not a power of 2. For example, consider:

@example
type R is record
   A : Character;
   B : Character;
   C : Boolean;
end record;

pragma Pack (R);
for R'Size use 17;
@end example

The default alignment for this record is normally 1, but if this type is
compiled in @cite{Optimize_Alignment (Time)} mode, then the alignment is set
to 4, which wastes space for objects of the type, since they are now 4 bytes
long, but results in more efficient access when the whole record is referenced.

As noted above, this is a configuration pragma, and there is a requirement
that all units in a partition be compiled with a consistent setting of the
optimization setting. This would normally be achieved by use of a configuration
pragma file containing the appropriate setting. The exception to this rule is
that units with an explicit configuration pragma in the same file as the source
unit are excluded from the consistency check, as are all predefined units. The
latter are compiled by default in pragma Optimize_Alignment (Off) mode if no
pragma appears at the start of the file.

@node Pragma Ordered,Pragma Overflow_Mode,Pragma Optimize_Alignment,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ordered}@anchor{8f}
@section Pragma Ordered


Syntax:

@example
pragma Ordered (enumeration_first_subtype_LOCAL_NAME);
@end example

Most enumeration types are from a conceptual point of view unordered.
For example, consider:

@example
type Color is (Red, Blue, Green, Yellow);
@end example

By Ada semantics @cite{Blue > Red} and @cite{Green > Blue},
but really these relations make no sense; the enumeration type merely
specifies a set of possible colors, and the order is unimportant.

For unordered enumeration types, it is generally a good idea if
clients avoid comparisons (other than equality or inequality) and
explicit ranges. (A @emph{client} is a unit where the type is referenced,
other than the unit where the type is declared, its body, and its subunits.)
For example, if code buried in some client says:

@example
if Current_Color < Yellow then ...
if Current_Color in Blue .. Green then ...
@end example

then the client code is relying on the order, which is undesirable.
It makes the code hard to read and creates maintenance difficulties if
entries have to be added to the enumeration type. Instead,
the code in the client should list the possibilities, or an
appropriate subtype should be declared in the unit that declares
the original enumeration type. E.g., the following subtype could
be declared along with the type @cite{Color}:

@example
subtype RBG is Color range Red .. Green;
@end example

and then the client could write:

@example
if Current_Color in RBG then ...
if Current_Color = Blue or Current_Color = Green then ...
@end example

However, some enumeration types are legitimately ordered from a conceptual
point of view. For example, if you declare:

@example
type Day is (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
@end example

then the ordering imposed by the language is reasonable, and
clients can depend on it, writing for example:

@example
if D in Mon .. Fri then ...
if D < Wed then ...
@end example

The pragma @emph{Ordered} is provided to mark enumeration types that
are conceptually ordered, alerting the reader that clients may depend
on the ordering. GNAT provides a pragma to mark enumerations as ordered
rather than one to mark them as unordered, since in our experience,
the great majority of enumeration types are conceptually unordered.

The types @cite{Boolean}, @cite{Character}, @cite{Wide_Character},
and @cite{Wide_Wide_Character}
are considered to be ordered types, so each is declared with a
pragma @cite{Ordered} in package @cite{Standard}.

Normally pragma @cite{Ordered} serves only as documentation and a guide for
coding standards, but GNAT provides a warning switch @emph{-gnatw.u} that
requests warnings for inappropriate uses (comparisons and explicit
subranges) for unordered types. If this switch is used, then any
enumeration type not marked with pragma @cite{Ordered} will be considered
as unordered, and will generate warnings for inappropriate uses.

Note that generic types are not considered ordered or unordered (since the
template can be instantiated for both cases), so we never generate warnings
for the case of generic enumerated types.

For additional information please refer to the description of the
@emph{-gnatw.u} switch in the GNAT User's Guide.

@node Pragma Overflow_Mode,Pragma Overriding_Renamings,Pragma Ordered,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-overflow-mode}@anchor{90}
@section Pragma Overflow_Mode


Syntax:

@example
pragma Overflow_Mode
 (  [General    =>] MODE
  [,[Assertions =>] MODE]);

MODE ::= STRICT | MINIMIZED | ELIMINATED
@end example

This pragma sets the current overflow mode to the given setting. For details
of the meaning of these modes, please refer to the
'Overflow Check Handling in GNAT' appendix in the
GNAT User's Guide. If only the @cite{General} parameter is present,
the given mode applies to all expressions. If both parameters are present,
the @cite{General} mode applies to expressions outside assertions, and
the @cite{Eliminated} mode applies to expressions within assertions.

The case of the @cite{MODE} parameter is ignored,
so @cite{MINIMIZED}, @cite{Minimized} and
@cite{minimized} all have the same effect.

The @cite{Overflow_Mode} pragma has the same scoping and placement
rules as pragma @cite{Suppress}, so it can occur either as a
configuration pragma, specifying a default for the whole
program, or in a declarative scope, where it applies to the
remaining declarations and statements in that scope.

The pragma @cite{Suppress (Overflow_Check)} suppresses
overflow checking, but does not affect the overflow mode.

The pragma @cite{Unsuppress (Overflow_Check)} unsuppresses (enables)
overflow checking, but does not affect the overflow mode.

@node Pragma Overriding_Renamings,Pragma Partition_Elaboration_Policy,Pragma Overflow_Mode,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-overriding-renamings}@anchor{91}
@section Pragma Overriding_Renamings


@geindex Rational profile

@geindex Rational compatibility

Syntax:

@example
pragma Overriding_Renamings;
@end example

This is a GNAT configuration pragma to simplify porting
legacy code accepted by the Rational
Ada compiler. In the presence of this pragma, a renaming declaration that
renames an inherited operation declared in the same scope is legal if selected
notation is used as in:

@example
pragma Overriding_Renamings;
...
package R is
  function F (..);
  ...
  function F (..) renames R.F;
end R;
@end example

even though
RM 8.3 (15) stipulates that an overridden operation is not visible within the
declaration of the overriding operation.

@node Pragma Partition_Elaboration_Policy,Pragma Part_Of,Pragma Overriding_Renamings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-partition-elaboration-policy}@anchor{92}
@section Pragma Partition_Elaboration_Policy


Syntax:

@example
pragma Partition_Elaboration_Policy (POLICY_IDENTIFIER);

POLICY_IDENTIFIER ::= Concurrent | Sequential
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Part_Of,Pragma Passive,Pragma Partition_Elaboration_Policy,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-part-of}@anchor{93}
@section Pragma Part_Of


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.2.6.

@node Pragma Passive,Pragma Persistent_BSS,Pragma Part_Of,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-passive}@anchor{94}
@section Pragma Passive


Syntax:

@example
pragma Passive [(Semaphore | No)];
@end example

Syntax checked, but otherwise ignored by GNAT.  This is recognized for
compatibility with DEC Ada 83 implementations, where it is used within a
task definition to request that a task be made passive.  If the argument
@cite{Semaphore} is present, or the argument is omitted, then DEC Ada 83
treats the pragma as an assertion that the containing task is passive
and that optimization of context switch with this task is permitted and
desired.  If the argument @cite{No} is present, the task must not be
optimized.  GNAT does not attempt to optimize any tasks in this manner
(since protected objects are available in place of passive tasks).

For more information on the subject of passive tasks, see the section
'Passive Task Optimization' in the GNAT Users Guide.

@node Pragma Persistent_BSS,Pragma Polling,Pragma Passive,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-persistent-bss}@anchor{95}
@section Pragma Persistent_BSS


Syntax:

@example
pragma Persistent_BSS [(LOCAL_NAME)]
@end example

This pragma allows selected objects to be placed in the @cite{.persistent_bss}
section. On some targets the linker and loader provide for special
treatment of this section, allowing a program to be reloaded without
affecting the contents of this data (hence the name persistent).

There are two forms of usage. If an argument is given, it must be the
local name of a library level object, with no explicit initialization
and whose type is potentially persistent. If no argument is given, then
the pragma is a configuration pragma, and applies to all library level
objects with no explicit initialization of potentially persistent types.

A potentially persistent type is a scalar type, or an untagged,
non-discriminated record, all of whose components have no explicit
initialization and are themselves of a potentially persistent type,
or an array, all of whose constraints are static, and whose component
type is potentially persistent.

If this pragma is used on a target where this feature is not supported,
then the pragma will be ignored. See also @cite{pragma Linker_Section}.

@node Pragma Polling,Pragma Post,Pragma Persistent_BSS,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-polling}@anchor{96}
@section Pragma Polling


Syntax:

@example
pragma Polling (ON | OFF);
@end example

This pragma controls the generation of polling code.  This is normally off.
If @cite{pragma Polling (ON)} is used then periodic calls are generated to
the routine @cite{Ada.Exceptions.Poll}.  This routine is a separate unit in the
runtime library, and can be found in file @code{a-excpol.adb}.

Pragma @cite{Polling} can appear as a configuration pragma (for example it
can be placed in the @code{gnat.adc} file) to enable polling globally, or it
can be used in the statement or declaration sequence to control polling
more locally.

A call to the polling routine is generated at the start of every loop and
at the start of every subprogram call.  This guarantees that the @cite{Poll}
routine is called frequently, and places an upper bound (determined by
the complexity of the code) on the period between two @cite{Poll} calls.

The primary purpose of the polling interface is to enable asynchronous
aborts on targets that cannot otherwise support it (for example Windows
NT), but it may be used for any other purpose requiring periodic polling.
The standard version is null, and can be replaced by a user program.  This
will require re-compilation of the @cite{Ada.Exceptions} package that can
be found in files @code{a-except.ads} and @code{a-except.adb}.

A standard alternative unit (in file @code{4wexcpol.adb} in the standard GNAT
distribution) is used to enable the asynchronous abort capability on
targets that do not normally support the capability.  The version of
@cite{Poll} in this file makes a call to the appropriate runtime routine
to test for an abort condition.

Note that polling can also be enabled by use of the @emph{-gnatP} switch.
See the section on switches for gcc in the @cite{GNAT User's Guide}.

@node Pragma Post,Pragma Postcondition,Pragma Polling,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-post}@anchor{97}
@section Pragma Post


@geindex Post

@geindex Checks
@geindex postconditions

Syntax:

@example
pragma Post (Boolean_Expression);
@end example

The @cite{Post} pragma is intended to be an exact replacement for
the language-defined
@cite{Post} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

@node Pragma Postcondition,Pragma Post_Class,Pragma Post,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-postcondition}@anchor{98}
@section Pragma Postcondition


@geindex Postcondition

@geindex Checks
@geindex postconditions

Syntax:

@example
pragma Postcondition (
   [Check   =>] Boolean_Expression
 [,[Message =>] String_Expression]);
@end example

The @cite{Postcondition} pragma allows specification of automatic
postcondition checks for subprograms. These checks are similar to
assertions, but are automatically inserted just prior to the return
statements of the subprogram with which they are associated (including
implicit returns at the end of procedure bodies and associated
exception handlers).

In addition, the boolean expression which is the condition which
must be true may contain references to function'Result in the case
of a function to refer to the returned value.

@cite{Postcondition} pragmas may appear either immediately following the
(separate) declaration of a subprogram, or at the start of the
declarations of a subprogram body. Only other pragmas may intervene
(that is appear between the subprogram declaration and its
postconditions, or appear before the postcondition in the
declaration sequence in a subprogram body). In the case of a
postcondition appearing after a subprogram declaration, the
formal arguments of the subprogram are visible, and can be
referenced in the postcondition expressions.

The postconditions are collected and automatically tested just
before any return (implicit or explicit) in the subprogram body.
A postcondition is only recognized if postconditions are active
at the time the pragma is encountered. The compiler switch @emph{gnata}
turns on all postconditions by default, and pragma @cite{Check_Policy}
with an identifier of @cite{Postcondition} can also be used to
control whether postconditions are active.

The general approach is that postconditions are placed in the spec
if they represent functional aspects which make sense to the client.
For example we might have:

@example
function Direction return Integer;
pragma Postcondition
 (Direction'Result = +1
    or else
  Direction'Result = -1);
@end example

which serves to document that the result must be +1 or -1, and
will test that this is the case at run time if postcondition
checking is active.

Postconditions within the subprogram body can be used to
check that some internal aspect of the implementation,
not visible to the client, is operating as expected.
For instance if a square root routine keeps an internal
counter of the number of times it is called, then we
might have the following postcondition:

@example
Sqrt_Calls : Natural := 0;

function Sqrt (Arg : Float) return Float is
  pragma Postcondition
    (Sqrt_Calls = Sqrt_Calls'Old + 1);
  ...
end Sqrt
@end example

As this example, shows, the use of the @cite{Old} attribute
is often useful in postconditions to refer to the state on
entry to the subprogram.

Note that postconditions are only checked on normal returns
from the subprogram. If an abnormal return results from
raising an exception, then the postconditions are not checked.

If a postcondition fails, then the exception
@cite{System.Assertions.Assert_Failure} is raised. If
a message argument was supplied, then the given string
will be used as the exception message. If no message
argument was supplied, then the default message has
the form "Postcondition failed at file_name:line". The
exception is raised in the context of the subprogram
body, so it is possible to catch postcondition failures
within the subprogram body itself.

Within a package spec, normal visibility rules
in Ada would prevent forward references within a
postcondition pragma to functions defined later in
the same package. This would introduce undesirable
ordering constraints. To avoid this problem, all
postcondition pragmas are analyzed at the end of
the package spec, allowing forward references.

The following example shows that this even allows
mutually recursive postconditions as in:

@example
package Parity_Functions is
   function Odd  (X : Natural) return Boolean;
   pragma Postcondition
     (Odd'Result =
        (x = 1
          or else
        (x /= 0 and then Even (X - 1))));

   function Even (X : Natural) return Boolean;
   pragma Postcondition
     (Even'Result =
        (x = 0
          or else
        (x /= 1 and then Odd (X - 1))));

end Parity_Functions;
@end example

There are no restrictions on the complexity or form of
conditions used within @cite{Postcondition} pragmas.
The following example shows that it is even possible
to verify performance behavior.

@example
package Sort is

   Performance : constant Float;
   --  Performance constant set by implementation
   --  to match target architecture behavior.

   procedure Treesort (Arg : String);
   --  Sorts characters of argument using N*logN sort
   pragma Postcondition
     (Float (Clock - Clock'Old) <=
        Float (Arg'Length) *
        log (Float (Arg'Length)) *
        Performance);
end Sort;
@end example

Note: postcondition pragmas associated with subprograms that are
marked as Inline_Always, or those marked as Inline with front-end
inlining (-gnatN option set) are accepted and legality-checked
by the compiler, but are ignored at run-time even if postcondition
checking is enabled.

Note that pragma @cite{Postcondition} differs from the language-defined
@cite{Post} aspect (and corresponding @cite{Post} pragma) in allowing
multiple occurrences, allowing occurences in the body even if there
is a separate spec, and allowing a second string parameter, and the
use of the pragma identifier @cite{Check}. Historically, pragma
@cite{Postcondition} was implemented prior to the development of
Ada 2012, and has been retained in its original form for
compatibility purposes.

@node Pragma Post_Class,Pragma Pre,Pragma Postcondition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-post-class}@anchor{99}
@section Pragma Post_Class


@geindex Post

@geindex Checks
@geindex postconditions

Syntax:

@example
pragma Post_Class (Boolean_Expression);
@end example

The @cite{Post_Class} pragma is intended to be an exact replacement for
the language-defined
@cite{Post'Class} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

Note: This pragma is called @cite{Post_Class} rather than
@cite{Post'Class} because the latter would not be strictly
conforming to the allowed syntax for pragmas. The motivation
for provinding pragmas equivalent to the aspects is to allow a program
to be written using the pragmas, and then compiled if necessary
using an Ada compiler that does not recognize the pragmas or
aspects, but is prepared to ignore the pragmas. The assertion
policy that controls this pragma is @cite{Post'Class}, not
@cite{Post_Class}.

@node Pragma Pre,Pragma Precondition,Pragma Post_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-pre}@anchor{9a}
@section Pragma Pre


@geindex Pre

@geindex Checks
@geindex preconditions

Syntax:

@example
pragma Pre (Boolean_Expression);
@end example

The @cite{Pre} pragma is intended to be an exact replacement for
the language-defined
@cite{Pre} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

@node Pragma Precondition,Pragma Predicate,Pragma Pre,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-precondition}@anchor{9b}
@section Pragma Precondition


@geindex Preconditions

@geindex Checks
@geindex preconditions

Syntax:

@example
pragma Precondition (
   [Check   =>] Boolean_Expression
 [,[Message =>] String_Expression]);
@end example

The @cite{Precondition} pragma is similar to @cite{Postcondition}
except that the corresponding checks take place immediately upon
entry to the subprogram, and if a precondition fails, the exception
is raised in the context of the caller, and the attribute 'Result
cannot be used within the precondition expression.

Otherwise, the placement and visibility rules are identical to those
described for postconditions. The following is an example of use
within a package spec:

@example
package Math_Functions is
   ...
   function Sqrt (Arg : Float) return Float;
   pragma Precondition (Arg >= 0.0)
   ...
end Math_Functions;
@end example

@cite{Precondition} pragmas may appear either immediately following the
(separate) declaration of a subprogram, or at the start of the
declarations of a subprogram body. Only other pragmas may intervene
(that is appear between the subprogram declaration and its
postconditions, or appear before the postcondition in the
declaration sequence in a subprogram body).

Note: precondition pragmas associated with subprograms that are
marked as Inline_Always, or those marked as Inline with front-end
inlining (-gnatN option set) are accepted and legality-checked
by the compiler, but are ignored at run-time even if precondition
checking is enabled.

Note that pragma @cite{Precondition} differs from the language-defined
@cite{Pre} aspect (and corresponding @cite{Pre} pragma) in allowing
multiple occurrences, allowing occurences in the body even if there
is a separate spec, and allowing a second string parameter, and the
use of the pragma identifier @cite{Check}. Historically, pragma
@cite{Precondition} was implemented prior to the development of
Ada 2012, and has been retained in its original form for
compatibility purposes.

@node Pragma Predicate,Pragma Preelaborable_Initialization,Pragma Precondition,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-predicate}@anchor{9c}
@section Pragma Predicate


Syntax:

@example
pragma Predicate
  ([Entity =>] type_LOCAL_NAME,
   [Check  =>] EXPRESSION);
@end example

This pragma (available in all versions of Ada in GNAT) encompasses both
the @cite{Static_Predicate} and @cite{Dynamic_Predicate} aspects in
Ada 2012. A predicate is regarded as static if it has an allowed form
for @cite{Static_Predicate} and is otherwise treated as a
@cite{Dynamic_Predicate}. Otherwise, predicates specified by this
pragma behave exactly as described in the Ada 2012 reference manual.
For example, if we have

@example
type R is range 1 .. 10;
subtype S is R;
pragma Predicate (Entity => S, Check => S not in 4 .. 6);
subtype Q is R
pragma Predicate (Entity => Q, Check => F(Q) or G(Q));
@end example

the effect is identical to the following Ada 2012 code:

@example
type R is range 1 .. 10;
subtype S is R with
  Static_Predicate => S not in 4 .. 6;
subtype Q is R with
  Dynamic_Predicate => F(Q) or G(Q);
@end example

Note that there is are no pragmas @cite{Dynamic_Predicate}
or @cite{Static_Predicate}. That is
because these pragmas would affect legality and semantics of
the program and thus do not have a neutral effect if ignored.
The motivation behind providing pragmas equivalent to
corresponding aspects is to allow a program to be written
using the pragmas, and then compiled with a compiler that
will ignore the pragmas. That doesn't work in the case of
static and dynamic predicates, since if the corresponding
pragmas are ignored, then the behavior of the program is
fundamentally changed (for example a membership test
@cite{A in B} would not take into account a predicate
defined for subtype B). When following this approach, the
use of predicates should be avoided.

@node Pragma Preelaborable_Initialization,Pragma Prefix_Exception_Messages,Pragma Predicate,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-preelaborable-initialization}@anchor{9d}
@section Pragma Preelaborable_Initialization


Syntax:

@example
pragma Preelaborable_Initialization (DIRECT_NAME);
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Prefix_Exception_Messages,Pragma Pre_Class,Pragma Preelaborable_Initialization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-prefix-exception-messages}@anchor{9e}
@section Pragma Prefix_Exception_Messages


@geindex Prefix_Exception_Messages

@geindex exception

@geindex Exception_Message

Syntax:

@example
pragma Prefix_Exception_Messages;
@end example

This is an implementation-defined configuration pragma that affects the
behavior of raise statements with a message given as a static string
constant (typically a string literal). In such cases, the string will
be automatically prefixed by the name of the enclosing entity (giving
the package and subprogram containing the raise statement). This helps
to identify where messages are coming from, and this mode is automatic
for the run-time library.

The pragma has no effect if the message is computed with an expression other
than a static string constant, since the assumption in this case is that
the program computes exactly the string it wants. If you still want the
prefixing in this case, you can always call
@cite{GNAT.Source_Info.Enclosing_Entity} and prepend the string manually.

@node Pragma Pre_Class,Pragma Priority_Specific_Dispatching,Pragma Prefix_Exception_Messages,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-pre-class}@anchor{9f}
@section Pragma Pre_Class


@geindex Pre_Class

@geindex Checks
@geindex preconditions

Syntax:

@example
pragma Pre_Class (Boolean_Expression);
@end example

The @cite{Pre_Class} pragma is intended to be an exact replacement for
the language-defined
@cite{Pre'Class} aspect, and shares its restrictions and semantics.
It must appear either immediately following the corresponding
subprogram declaration (only other pragmas may intervene), or
if there is no separate subprogram declaration, then it can
appear at the start of the declarations in a subprogram body
(preceded only by other pragmas).

Note: This pragma is called @cite{Pre_Class} rather than
@cite{Pre'Class} because the latter would not be strictly
conforming to the allowed syntax for pragmas. The motivation
for providing pragmas equivalent to the aspects is to allow a program
to be written using the pragmas, and then compiled if necessary
using an Ada compiler that does not recognize the pragmas or
aspects, but is prepared to ignore the pragmas. The assertion
policy that controls this pragma is @cite{Pre'Class}, not
@cite{Pre_Class}.

@node Pragma Priority_Specific_Dispatching,Pragma Profile,Pragma Pre_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-priority-specific-dispatching}@anchor{a0}
@section Pragma Priority_Specific_Dispatching


Syntax:

@example
pragma Priority_Specific_Dispatching (
   POLICY_IDENTIFIER,
   first_priority_EXPRESSION,
   last_priority_EXPRESSION)

POLICY_IDENTIFIER ::=
   EDF_Across_Priorities            |
   FIFO_Within_Priorities           |
   Non_Preemptive_Within_Priorities |
   Round_Robin_Within_Priorities
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Profile,Pragma Profile_Warnings,Pragma Priority_Specific_Dispatching,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-profile}@anchor{a1}
@section Pragma Profile


Syntax:

@example
pragma Profile (Ravenscar | Restricted | Rational);
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma. This is a
configuration pragma that establishes a set of configuration pragmas
that depend on the argument. @cite{Ravenscar} is standard in Ada 2005.
The other two possibilities (@cite{Restricted} or @cite{Rational})
are implementation-defined. The set of configuration pragmas
is defined in the following sections.


@itemize *

@item 
Pragma Profile (Ravenscar)

The @cite{Ravenscar} profile is standard in Ada 2005,
but is available in all earlier
versions of Ada as an implementation-defined pragma. This profile
establishes the following set of configuration pragmas:


@itemize *

@item 
@code{Task_Dispatching_Policy (FIFO_Within_Priorities)}

[RM D.2.2] Tasks are dispatched following a preemptive
priority-ordered scheduling policy.

@item 
@code{Locking_Policy (Ceiling_Locking)}

[RM D.3] While tasks and interrupts execute a protected action, they inherit
the ceiling priority of the corresponding protected object.

@item 
@code{Detect_Blocking}

This pragma forces the detection of potentially blocking operations within a
protected operation, and to raise Program_Error if that happens.
@end itemize

plus the following set of restrictions:


@itemize *

@item 
@code{Max_Entry_Queue_Length => 1}

No task can be queued on a protected entry.

@item 
@code{Max_Protected_Entries => 1}

@item 
@code{Max_Task_Entries => 0}

No rendezvous statements are allowed.

@item 
@code{No_Abort_Statements}

@item 
@code{No_Dynamic_Attachment}

@item 
@code{No_Dynamic_Priorities}

@item 
@code{No_Implicit_Heap_Allocations}

@item 
@code{No_Local_Protected_Objects}

@item 
@code{No_Local_Timing_Events}

@item 
@code{No_Protected_Type_Allocators}

@item 
@code{No_Relative_Delay}

@item 
@code{No_Requeue_Statements}

@item 
@code{No_Select_Statements}

@item 
@code{No_Specific_Termination_Handlers}

@item 
@code{No_Task_Allocators}

@item 
@code{No_Task_Hierarchy}

@item 
@code{No_Task_Termination}

@item 
@code{Simple_Barriers}
@end itemize

The Ravenscar profile also includes the following restrictions that specify
that there are no semantic dependences on the corresponding predefined
packages:


@itemize *

@item 
@code{No_Dependence => Ada.Asynchronous_Task_Control}

@item 
@code{No_Dependence => Ada.Calendar}

@item 
@code{No_Dependence => Ada.Execution_Time.Group_Budget}

@item 
@code{No_Dependence => Ada.Execution_Time.Timers}

@item 
@code{No_Dependence => Ada.Task_Attributes}

@item 
@code{No_Dependence => System.Multiprocessors.Dispatching_Domains}
@end itemize

This set of configuration pragmas and restrictions correspond to the
definition of the 'Ravenscar Profile' for limited tasking, devised and
published by the @cite{International Real-Time Ada Workshop@comma{} 1997}.
A description is also available at
@indicateurl{http://www-users.cs.york.ac.uk/~burns/ravenscar.ps}.

The original definition of the profile was revised at subsequent IRTAW
meetings. It has been included in the ISO
@cite{Guide for the Use of the Ada Programming Language in High Integrity Systems},
and was made part of the Ada 2005 standard.
The formal definition given by
the Ada Rapporteur Group (ARG) can be found in two Ada Issues (AI-249 and
AI-305) available at
@indicateurl{http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00249.txt} and
@indicateurl{http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ais/ai-00305.txt}.

The above set is a superset of the restrictions provided by pragma
@code{Profile (Restricted)}, it includes six additional restrictions
(@code{Simple_Barriers}, @code{No_Select_Statements},
@code{No_Calendar}, @code{No_Implicit_Heap_Allocations},
@code{No_Relative_Delay} and @code{No_Task_Termination}).  This means
that pragma @code{Profile (Ravenscar)}, like the pragma
@code{Profile (Restricted)},
automatically causes the use of a simplified,
more efficient version of the tasking run-time library.

@item 
Pragma Profile (Restricted)

This profile corresponds to the GNAT restricted run time. It
establishes the following set of restrictions:


@itemize *

@item 
@code{No_Abort_Statements}

@item 
@code{No_Entry_Queue}

@item 
@code{No_Task_Hierarchy}

@item 
@code{No_Task_Allocators}

@item 
@code{No_Dynamic_Priorities}

@item 
@code{No_Terminate_Alternatives}

@item 
@code{No_Dynamic_Attachment}

@item 
@code{No_Protected_Type_Allocators}

@item 
@code{No_Local_Protected_Objects}

@item 
@code{No_Requeue_Statements}

@item 
@code{No_Task_Attributes_Package}

@item 
@code{Max_Asynchronous_Select_Nesting =  0}

@item 
@code{Max_Task_Entries =  0}

@item 
@code{Max_Protected_Entries = 1}

@item 
@code{Max_Select_Alternatives = 0}
@end itemize

This set of restrictions causes the automatic selection of a simplified
version of the run time that provides improved performance for the
limited set of tasking functionality permitted by this set of restrictions.

@item 
Pragma Profile (Rational)

The Rational profile is intended to facilitate porting legacy code that
compiles with the Rational APEX compiler, even when the code includes non-
conforming Ada constructs.  The profile enables the following three pragmas:


@itemize *

@item 
@code{pragma Implicit_Packing}

@item 
@code{pragma Overriding_Renamings}

@item 
@code{pragma Use_VADS_Size}
@end itemize
@end itemize

@node Pragma Profile_Warnings,Pragma Propagate_Exceptions,Pragma Profile,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-profile-warnings}@anchor{a2}
@section Pragma Profile_Warnings


Syntax:

@example
pragma Profile_Warnings (Ravenscar | Restricted | Rational);
@end example

This is an implementation-defined pragma that is similar in
effect to @cite{pragma Profile} except that instead of
generating @cite{Restrictions} pragmas, it generates
@cite{Restriction_Warnings} pragmas. The result is that
violations of the profile generate warning messages instead
of error messages.

@node Pragma Propagate_Exceptions,Pragma Provide_Shift_Operators,Pragma Profile_Warnings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-propagate-exceptions}@anchor{a3}
@section Pragma Propagate_Exceptions


@geindex Interfacing to C++

Syntax:

@example
pragma Propagate_Exceptions;
@end example

This pragma is now obsolete and, other than generating a warning if warnings
on obsolescent features are enabled, is ignored.
It is retained for compatibility
purposes. It used to be used in connection with optimization of
a now-obsolete mechanism for implementation of exceptions.

@node Pragma Provide_Shift_Operators,Pragma Psect_Object,Pragma Propagate_Exceptions,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-provide-shift-operators}@anchor{a4}
@section Pragma Provide_Shift_Operators


@geindex Shift operators

Syntax:

@example
pragma Provide_Shift_Operators (integer_first_subtype_LOCAL_NAME);
@end example

This pragma can be applied to a first subtype local name that specifies
either an unsigned or signed type. It has the effect of providing the
five shift operators (Shift_Left, Shift_Right, Shift_Right_Arithmetic,
Rotate_Left and Rotate_Right) for the given type. It is similar to
including the function declarations for these five operators, together
with the pragma Import (Intrinsic, ...) statements.

@node Pragma Psect_Object,Pragma Pure_Function,Pragma Provide_Shift_Operators,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-psect-object}@anchor{a5}
@section Pragma Psect_Object


Syntax:

@example
pragma Psect_Object (
     [Internal =>] LOCAL_NAME,
  [, [External =>] EXTERNAL_SYMBOL]
  [, [Size     =>] EXTERNAL_SYMBOL]);

EXTERNAL_SYMBOL ::=
  IDENTIFIER
| static_string_EXPRESSION
@end example

This pragma is identical in effect to pragma @cite{Common_Object}.

@node Pragma Pure_Function,Pragma Rational,Pragma Psect_Object,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-pure-function}@anchor{a6}
@section Pragma Pure_Function


Syntax:

@example
pragma Pure_Function ([Entity =>] function_LOCAL_NAME);
@end example

This pragma appears in the same declarative part as a function
declaration (or a set of function declarations if more than one
overloaded declaration exists, in which case the pragma applies
to all entities).  It specifies that the function @cite{Entity} is
to be considered pure for the purposes of code generation.  This means
that the compiler can assume that there are no side effects, and
in particular that two calls with identical arguments produce the
same result.  It also means that the function can be used in an
address clause.

Note that, quite deliberately, there are no static checks to try
to ensure that this promise is met, so @cite{Pure_Function} can be used
with functions that are conceptually pure, even if they do modify
global variables.  For example, a square root function that is
instrumented to count the number of times it is called is still
conceptually pure, and can still be optimized, even though it
modifies a global variable (the count).  Memo functions are another
example (where a table of previous calls is kept and consulted to
avoid re-computation).

Note also that the normal rules excluding optimization of subprograms
in pure units (when parameter types are descended from System.Address,
or when the full view of a parameter type is limited), do not apply
for the Pure_Function case. If you explicitly specify Pure_Function,
the compiler may optimize away calls with identical arguments, and
if that results in unexpected behavior, the proper action is not to
use the pragma for subprograms that are not (conceptually) pure.

Note: Most functions in a @cite{Pure} package are automatically pure, and
there is no need to use pragma @cite{Pure_Function} for such functions.  One
exception is any function that has at least one formal of type
@cite{System.Address} or a type derived from it.  Such functions are not
considered pure by default, since the compiler assumes that the
@cite{Address} parameter may be functioning as a pointer and that the
referenced data may change even if the address value does not.
Similarly, imported functions are not considered to be pure by default,
since there is no way of checking that they are in fact pure.  The use
of pragma @cite{Pure_Function} for such a function will override these default
assumption, and cause the compiler to treat a designated subprogram as pure
in these cases.

Note: If pragma @cite{Pure_Function} is applied to a renamed function, it
applies to the underlying renamed function.  This can be used to
disambiguate cases of overloading where some but not all functions
in a set of overloaded functions are to be designated as pure.

If pragma @cite{Pure_Function} is applied to a library level function, the
function is also considered pure from an optimization point of view, but the
unit is not a Pure unit in the categorization sense. So for example, a function
thus marked is free to @cite{with} non-pure units.

@node Pragma Rational,Pragma Ravenscar,Pragma Pure_Function,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-rational}@anchor{a7}
@section Pragma Rational


Syntax:

@example
pragma Rational;
@end example

This pragma is considered obsolescent, but is retained for
compatibility purposes. It is equivalent to:

@example
pragma Profile (Rational);
@end example

@node Pragma Ravenscar,Pragma Refined_Depends,Pragma Rational,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-ravenscar}@anchor{a8}
@section Pragma Ravenscar


Syntax:

@example
pragma Ravenscar;
@end example

This pragma is considered obsolescent, but is retained for
compatibility purposes. It is equivalent to:

@example
pragma Profile (Ravenscar);
@end example

which is the preferred method of setting the @cite{Ravenscar} profile.

@node Pragma Refined_Depends,Pragma Refined_Global,Pragma Ravenscar,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-depends}@anchor{a9}
@section Pragma Refined_Depends


For the description of this pragma, see SPARK 2014 Reference Manual,
section 6.1.5.

@node Pragma Refined_Global,Pragma Refined_Post,Pragma Refined_Depends,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-global}@anchor{aa}
@section Pragma Refined_Global


For the description of this pragma, see SPARK 2014 Reference Manual,
section 6.1.4.

@node Pragma Refined_Post,Pragma Refined_State,Pragma Refined_Global,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-post}@anchor{ab}
@section Pragma Refined_Post


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.2.7.

@node Pragma Refined_State,Pragma Relative_Deadline,Pragma Refined_Post,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-refined-state}@anchor{ac}
@section Pragma Refined_State


For the description of this pragma, see SPARK 2014 Reference Manual,
section 7.2.2.

@node Pragma Relative_Deadline,Pragma Remote_Access_Type,Pragma Refined_State,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-relative-deadline}@anchor{ad}
@section Pragma Relative_Deadline


Syntax:

@example
pragma Relative_Deadline (time_span_EXPRESSION);
@end example

This pragma is standard in Ada 2005, but is available in all earlier
versions of Ada as an implementation-defined pragma.
See Ada 2012 Reference Manual for details.

@node Pragma Remote_Access_Type,Pragma Restricted_Run_Time,Pragma Relative_Deadline,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-remote-access-type}@anchor{ae}
@section Pragma Remote_Access_Type


Syntax:

@example
pragma Remote_Access_Type ([Entity =>] formal_access_type_LOCAL_NAME);
@end example

This pragma appears in the formal part of a generic declaration.
It specifies an exception to the RM rule from E.2.2(17/2), which forbids
the use of a remote access to class-wide type as actual for a formal
access type.

When this pragma applies to a formal access type @cite{Entity}, that
type is treated as a remote access to class-wide type in the generic.
It must be a formal general access type, and its designated type must
be the class-wide type of a formal tagged limited private type from the
same generic declaration.

In the generic unit, the formal type is subject to all restrictions
pertaining to remote access to class-wide types. At instantiation, the
actual type must be a remote access to class-wide type.

@node Pragma Restricted_Run_Time,Pragma Restriction_Warnings,Pragma Remote_Access_Type,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-restricted-run-time}@anchor{af}
@section Pragma Restricted_Run_Time


Syntax:

@example
pragma Restricted_Run_Time;
@end example

This pragma is considered obsolescent, but is retained for
compatibility purposes. It is equivalent to:

@example
pragma Profile (Restricted);
@end example

which is the preferred method of setting the restricted run time
profile.

@node Pragma Restriction_Warnings,Pragma Reviewable,Pragma Restricted_Run_Time,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-restriction-warnings}@anchor{b0}
@section Pragma Restriction_Warnings


Syntax:

@example
pragma Restriction_Warnings
  (restriction_IDENTIFIER @{, restriction_IDENTIFIER@});
@end example

This pragma allows a series of restriction identifiers to be
specified (the list of allowed identifiers is the same as for
pragma @cite{Restrictions}). For each of these identifiers
the compiler checks for violations of the restriction, but
generates a warning message rather than an error message
if the restriction is violated.

One use of this is in situations where you want to know
about violations of a restriction, but you want to ignore some of
these violations. Consider this example, where you want to set
Ada_95 mode and enable style checks, but you want to know about
any other use of implementation pragmas:

@example
pragma Restriction_Warnings (No_Implementation_Pragmas);
pragma Warnings (Off, "violation of No_Implementation_Pragmas");
pragma Ada_95;
pragma Style_Checks ("2bfhkM160");
pragma Warnings (On, "violation of No_Implementation_Pragmas");
@end example

By including the above lines in a configuration pragmas file,
the Ada_95 and Style_Checks pragmas are accepted without
generating a warning, but any other use of implementation
defined pragmas will cause a warning to be generated.

@node Pragma Reviewable,Pragma Share_Generic,Pragma Restriction_Warnings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-reviewable}@anchor{b1}
@section Pragma Reviewable


Syntax:

@example
pragma Reviewable;
@end example

This pragma is an RM-defined standard pragma, but has no effect on the
program being compiled, or on the code generated for the program.

To obtain the required output specified in RM H.3.1, the compiler must be
run with various special switches as follows:


@itemize *

@item 
@emph{Where compiler-generated run-time checks remain}

The switch @emph{-gnatGL}
may be used to list the expanded code in pseudo-Ada form.
Runtime checks show up in the listing either as explicit
checks or operators marked with @{@} to indicate a check is present.

@item 
@emph{An identification of known exceptions at compile time}

If the program is compiled with @emph{-gnatwa},
the compiler warning messages will indicate all cases where the compiler
detects that an exception is certain to occur at run time.

@item 
@emph{Possible reads of uninitialized variables}

The compiler warns of many such cases, but its output is incomplete.
@end itemize


A supplemental static analysis tool
may be used to obtain a comprehensive list of all
possible points at which uninitialized data may be read.


@itemize *

@item 
@emph{Where run-time support routines are implicitly invoked}

In the output from @emph{-gnatGL},
run-time calls are explicitly listed as calls to the relevant
run-time routine.

@item 
@emph{Object code listing}

This may be obtained either by using the @emph{-S} switch,
or the objdump utility.

@item 
@emph{Constructs known to be erroneous at compile time}

These are identified by warnings issued by the compiler (use @emph{-gnatwa}).

@item 
@emph{Stack usage information}

Static stack usage data (maximum per-subprogram) can be obtained via the
@emph{-fstack-usage} switch to the compiler.
Dynamic stack usage data (per task) can be obtained via the @emph{-u} switch
to gnatbind
@end itemize



@itemize *

@item 
@emph{Object code listing of entire partition}

This can be obtained by compiling the partition with @emph{-S},
or by applying objdump
to all the object files that are part of the partition.

@item 
@emph{A description of the run-time model}

The full sources of the run-time are available, and the documentation of
these routines describes how these run-time routines interface to the
underlying operating system facilities.

@item 
@emph{Control and data-flow information}
@end itemize


A supplemental static analysis tool
may be used to obtain complete control and data-flow information, as well as
comprehensive messages identifying possible problems based on this
information.

@node Pragma Share_Generic,Pragma Shared,Pragma Reviewable,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-share-generic}@anchor{b2}
@section Pragma Share_Generic


Syntax:

@example
pragma Share_Generic (GNAME @{, GNAME@});

GNAME ::= generic_unit_NAME | generic_instance_NAME
@end example

This pragma is provided for compatibility with Dec Ada 83. It has
no effect in @cite{GNAT} (which does not implement shared generics), other
than to check that the given names are all names of generic units or
generic instances.

@node Pragma Shared,Pragma Short_Circuit_And_Or,Pragma Share_Generic,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-shared}@anchor{b3}
@section Pragma Shared


This pragma is provided for compatibility with Ada 83. The syntax and
semantics are identical to pragma Atomic.

@node Pragma Short_Circuit_And_Or,Pragma Short_Descriptors,Pragma Shared,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-short-circuit-and-or}@anchor{b4}
@section Pragma Short_Circuit_And_Or


Syntax:

@example
pragma Short_Circuit_And_Or;
@end example

This configuration pragma causes any occurrence of the AND operator applied to
operands of type Standard.Boolean to be short-circuited (i.e. the AND operator
is treated as if it were AND THEN). Or is similarly treated as OR ELSE. This
may be useful in the context of certification protocols requiring the use of
short-circuited logical operators. If this configuration pragma occurs locally
within the file being compiled, it applies only to the file being compiled.
There is no requirement that all units in a partition use this option.

@node Pragma Short_Descriptors,Pragma Simple_Storage_Pool_Type,Pragma Short_Circuit_And_Or,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-short-descriptors}@anchor{b5}
@section Pragma Short_Descriptors


Syntax:

@example
pragma Short_Descriptors
@end example

This pragma is provided for compatibility with other Ada implementations. It
is recognized but ignored by all current versions of GNAT.

@node Pragma Simple_Storage_Pool_Type,Pragma Source_File_Name,Pragma Short_Descriptors,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-simple-storage-pool-type}@anchor{b6}
@section Pragma Simple_Storage_Pool_Type


@geindex Storage pool
@geindex simple

@geindex Simple storage pool

Syntax:

@example
pragma Simple_Storage_Pool_Type (type_LOCAL_NAME);
@end example

A type can be established as a 'simple storage pool type' by applying
the representation pragma @cite{Simple_Storage_Pool_Type} to the type.
A type named in the pragma must be a library-level immutably limited record
type or limited tagged type declared immediately within a package declaration.
The type can also be a limited private type whose full type is allowed as
a simple storage pool type.

For a simple storage pool type @cite{SSP}, nonabstract primitive subprograms
@cite{Allocate}, @cite{Deallocate}, and @cite{Storage_Size} can be declared that
are subtype conformant with the following subprogram declarations:

@example
procedure Allocate
  (Pool                     : in out SSP;
   Storage_Address          : out System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count);

procedure Deallocate
  (Pool : in out SSP;
   Storage_Address          : System.Address;
   Size_In_Storage_Elements : System.Storage_Elements.Storage_Count;
   Alignment                : System.Storage_Elements.Storage_Count);

function Storage_Size (Pool : SSP)
  return System.Storage_Elements.Storage_Count;
@end example

Procedure @cite{Allocate} must be declared, whereas @cite{Deallocate} and
@cite{Storage_Size} are optional. If @cite{Deallocate} is not declared, then
applying an unchecked deallocation has no effect other than to set its actual
parameter to null. If @cite{Storage_Size} is not declared, then the
@cite{Storage_Size} attribute applied to an access type associated with
a pool object of type SSP returns zero. Additional operations can be declared
for a simple storage pool type (such as for supporting a mark/release
storage-management discipline).

An object of a simple storage pool type can be associated with an access
type by specifying the attribute
@ref{b7,,Simple_Storage_Pool}. For example:

@example
My_Pool : My_Simple_Storage_Pool_Type;

type Acc is access My_Data_Type;

for Acc'Simple_Storage_Pool use My_Pool;
@end example

See attribute @ref{b7,,Simple_Storage_Pool}
for further details.

@node Pragma Source_File_Name,Pragma Source_File_Name_Project,Pragma Simple_Storage_Pool_Type,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-source-file-name}@anchor{b8}@anchor{gnat_rm/implementation_defined_pragmas id3}@anchor{b9}
@section Pragma Source_File_Name


Syntax:

@example
pragma Source_File_Name (
  [Unit_Name   =>] unit_NAME,
  Spec_File_Name =>  STRING_LITERAL,
  [Index => INTEGER_LITERAL]);

pragma Source_File_Name (
  [Unit_Name   =>] unit_NAME,
  Body_File_Name =>  STRING_LITERAL,
  [Index => INTEGER_LITERAL]);
@end example

Use this to override the normal naming convention.  It is a configuration
pragma, and so has the usual applicability of configuration pragmas
(i.e., it applies to either an entire partition, or to all units in a
compilation, or to a single unit, depending on how it is used.
@cite{unit_name} is mapped to @cite{file_name_literal}.  The identifier for
the second argument is required, and indicates whether this is the file
name for the spec or for the body.

The optional Index argument should be used when a file contains multiple
units, and when you do not want to use @cite{gnatchop} to separate then
into multiple files (which is the recommended procedure to limit the
number of recompilations that are needed when some sources change).
For instance, if the source file @code{source.ada} contains

@example
package B is
...
end B;

with B;
procedure A is
begin
   ..
end A;
@end example

you could use the following configuration pragmas:

@example
pragma Source_File_Name
  (B, Spec_File_Name => "source.ada", Index => 1);
pragma Source_File_Name
  (A, Body_File_Name => "source.ada", Index => 2);
@end example

Note that the @cite{gnatname} utility can also be used to generate those
configuration pragmas.

Another form of the @cite{Source_File_Name} pragma allows
the specification of patterns defining alternative file naming schemes
to apply to all files.

@example
pragma Source_File_Name
  (  [Spec_File_Name  =>] STRING_LITERAL
   [,[Casing          =>] CASING_SPEC]
   [,[Dot_Replacement =>] STRING_LITERAL]);

pragma Source_File_Name
  (  [Body_File_Name  =>] STRING_LITERAL
   [,[Casing          =>] CASING_SPEC]
   [,[Dot_Replacement =>] STRING_LITERAL]);

pragma Source_File_Name
  (  [Subunit_File_Name =>] STRING_LITERAL
   [,[Casing            =>] CASING_SPEC]
   [,[Dot_Replacement   =>] STRING_LITERAL]);

CASING_SPEC ::= Lowercase | Uppercase | Mixedcase
@end example

The first argument is a pattern that contains a single asterisk indicating
the point at which the unit name is to be inserted in the pattern string
to form the file name.  The second argument is optional.  If present it
specifies the casing of the unit name in the resulting file name string.
The default is lower case.  Finally the third argument allows for systematic
replacement of any dots in the unit name by the specified string literal.

Note that Source_File_Name pragmas should not be used if you are using
project files. The reason for this rule is that the project manager is not
aware of these pragmas, and so other tools that use the projet file would not
be aware of the intended naming conventions. If you are using project files,
file naming is controlled by Source_File_Name_Project pragmas, which are
usually supplied automatically by the project manager. A pragma
Source_File_Name cannot appear after a @ref{ba,,Pragma Source_File_Name_Project}.

For more details on the use of the @cite{Source_File_Name} pragma, see the
sections on @cite{Using Other File Names} and @cite{Alternative File Naming Schemes' in the :title:`GNAT User's Guide}.

@node Pragma Source_File_Name_Project,Pragma Source_Reference,Pragma Source_File_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id4}@anchor{bb}@anchor{gnat_rm/implementation_defined_pragmas pragma-source-file-name-project}@anchor{ba}
@section Pragma Source_File_Name_Project


This pragma has the same syntax and semantics as pragma Source_File_Name.
It is only allowed as a stand alone configuration pragma.
It cannot appear after a @ref{b8,,Pragma Source_File_Name}, and
most importantly, once pragma Source_File_Name_Project appears,
no further Source_File_Name pragmas are allowed.

The intention is that Source_File_Name_Project pragmas are always
generated by the Project Manager in a manner consistent with the naming
specified in a project file, and when naming is controlled in this manner,
it is not permissible to attempt to modify this naming scheme using
Source_File_Name or Source_File_Name_Project pragmas (which would not be
known to the project manager).

@node Pragma Source_Reference,Pragma SPARK_Mode,Pragma Source_File_Name_Project,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-source-reference}@anchor{bc}
@section Pragma Source_Reference


Syntax:

@example
pragma Source_Reference (INTEGER_LITERAL, STRING_LITERAL);
@end example

This pragma must appear as the first line of a source file.
@cite{integer_literal} is the logical line number of the line following
the pragma line (for use in error messages and debugging
information).  @cite{string_literal} is a static string constant that
specifies the file name to be used in error messages and debugging
information.  This is most notably used for the output of @cite{gnatchop}
with the @emph{-r} switch, to make sure that the original unchopped
source file is the one referred to.

The second argument must be a string literal, it cannot be a static
string expression other than a string literal.  This is because its value
is needed for error messages issued by all phases of the compiler.

@node Pragma SPARK_Mode,Pragma Static_Elaboration_Desired,Pragma Source_Reference,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-spark-mode}@anchor{bd}
@section Pragma SPARK_Mode


Syntax:

@example
pragma SPARK_Mode [(On | Off)] ;
@end example

In general a program can have some parts that are in SPARK 2014 (and
follow all the rules in the SPARK Reference Manual), and some parts
that are full Ada 2012.

The SPARK_Mode pragma is used to identify which parts are in SPARK
2014 (by default programs are in full Ada). The SPARK_Mode pragma can
be used in the following places:


@itemize *

@item 
As a configuration pragma, in which case it sets the default mode for
all units compiled with this pragma.

@item 
Immediately following a library-level subprogram spec

@item 
Immediately within a library-level package body

@item 
Immediately following the @cite{private} keyword of a library-level
package spec

@item 
Immediately following the @cite{begin} keyword of a library-level
package body

@item 
Immediately within a library-level subprogram body
@end itemize

Normally a subprogram or package spec/body inherits the current mode
that is active at the point it is declared. But this can be overridden
by pragma within the spec or body as above.

The basic consistency rule is that you can't turn SPARK_Mode back
@cite{On}, once you have explicitly (with a pragma) turned if
@cite{Off}. So the following rules apply:

If a subprogram spec has SPARK_Mode @cite{Off}, then the body must
also have SPARK_Mode @cite{Off}.

For a package, we have four parts:


@itemize *

@item 
the package public declarations

@item 
the package private part

@item 
the body of the package

@item 
the elaboration code after @cite{begin}
@end itemize

For a package, the rule is that if you explicitly turn SPARK_Mode
@cite{Off} for any part, then all the following parts must have
SPARK_Mode @cite{Off}. Note that this may require repeating a pragma
SPARK_Mode (@cite{Off}) in the body. For example, if we have a
configuration pragma SPARK_Mode (@cite{On}) that turns the mode on by
default everywhere, and one particular package spec has pragma
SPARK_Mode (@cite{Off}), then that pragma will need to be repeated in
the package body.

@node Pragma Static_Elaboration_Desired,Pragma Stream_Convert,Pragma SPARK_Mode,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-static-elaboration-desired}@anchor{be}
@section Pragma Static_Elaboration_Desired


Syntax:

@example
pragma Static_Elaboration_Desired;
@end example

This pragma is used to indicate that the compiler should attempt to initialize
statically the objects declared in the library unit to which the pragma applies,
when these objects are initialized (explicitly or implicitly) by an aggregate.
In the absence of this pragma, aggregates in object declarations are expanded
into assignments and loops, even when the aggregate components are static
constants. When the aggregate is present the compiler builds a static expression
that requires no run-time code, so that the initialized object can be placed in
read-only data space. If the components are not static, or the aggregate has
more that 100 components, the compiler emits a warning that the pragma cannot
be obeyed. (See also the restriction No_Implicit_Loops, which supports static
construction of larger aggregates with static components that include an others
choice.)

@node Pragma Stream_Convert,Pragma Style_Checks,Pragma Static_Elaboration_Desired,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-stream-convert}@anchor{bf}
@section Pragma Stream_Convert


Syntax:

@example
pragma Stream_Convert (
  [Entity =>] type_LOCAL_NAME,
  [Read   =>] function_NAME,
  [Write  =>] function_NAME);
@end example

This pragma provides an efficient way of providing user-defined stream
attributes.  Not only is it simpler to use than specifying the attributes
directly, but more importantly, it allows the specification to be made in such
a way that the predefined unit Ada.Streams is not loaded unless it is actually
needed (i.e. unless the stream attributes are actually used); the use of
the Stream_Convert pragma adds no overhead at all, unless the stream
attributes are actually used on the designated type.

The first argument specifies the type for which stream functions are
provided.  The second parameter provides a function used to read values
of this type.  It must name a function whose argument type may be any
subtype, and whose returned type must be the type given as the first
argument to the pragma.

The meaning of the @cite{Read} parameter is that if a stream attribute directly
or indirectly specifies reading of the type given as the first parameter,
then a value of the type given as the argument to the Read function is
read from the stream, and then the Read function is used to convert this
to the required target type.

Similarly the @cite{Write} parameter specifies how to treat write attributes
that directly or indirectly apply to the type given as the first parameter.
It must have an input parameter of the type specified by the first parameter,
and the return type must be the same as the input type of the Read function.
The effect is to first call the Write function to convert to the given stream
type, and then write the result type to the stream.

The Read and Write functions must not be overloaded subprograms.  If necessary
renamings can be supplied to meet this requirement.
The usage of this attribute is best illustrated by a simple example, taken
from the GNAT implementation of package Ada.Strings.Unbounded:

@example
function To_Unbounded (S : String) return Unbounded_String
  renames To_Unbounded_String;

pragma Stream_Convert
  (Unbounded_String, To_Unbounded, To_String);
@end example

The specifications of the referenced functions, as given in the Ada
Reference Manual are:

@example
function To_Unbounded_String (Source : String)
  return Unbounded_String;

function To_String (Source : Unbounded_String)
  return String;
@end example

The effect is that if the value of an unbounded string is written to a stream,
then the representation of the item in the stream is in the same format that
would be used for @cite{Standard.String'Output}, and this same representation
is expected when a value of this type is read from the stream. Note that the
value written always includes the bounds, even for Unbounded_String'Write,
since Unbounded_String is not an array type.

Note that the @cite{Stream_Convert} pragma is not effective in the case of
a derived type of a non-limited tagged type. If such a type is specified then
the pragma is silently ignored, and the default implementation of the stream
attributes is used instead.

@node Pragma Style_Checks,Pragma Subtitle,Pragma Stream_Convert,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-style-checks}@anchor{c0}
@section Pragma Style_Checks


Syntax:

@example
pragma Style_Checks (string_LITERAL | ALL_CHECKS |
                     On | Off [, LOCAL_NAME]);
@end example

This pragma is used in conjunction with compiler switches to control the
built in style checking provided by GNAT.  The compiler switches, if set,
provide an initial setting for the switches, and this pragma may be used
to modify these settings, or the settings may be provided entirely by
the use of the pragma.  This pragma can be used anywhere that a pragma
is legal, including use as a configuration pragma (including use in
the @code{gnat.adc} file).

The form with a string literal specifies which style options are to be
activated.  These are additive, so they apply in addition to any previously
set style check options.  The codes for the options are the same as those
used in the @emph{-gnaty} switch to @emph{gcc} or @emph{gnatmake}.
For example the following two methods can be used to enable
layout checking:


@itemize *

@item 
@example
pragma Style_Checks ("l");
@end example

@item 
@example
gcc -c -gnatyl ...
@end example
@end itemize

The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the @cite{gnaty} switch with no options.
See the @cite{GNAT User's Guide} for details.)

Note: the behavior is slightly different in GNAT mode (@emph{-gnatg} used).
In this case, ALL_CHECKS implies the standard set of GNAT mode style check
options (i.e. equivalent to @emph{-gnatyg}).

The forms with @cite{Off} and @cite{On}
can be used to temporarily disable style checks
as shown in the following example:

@example
pragma Style_Checks ("k"); -- requires keywords in lower case
pragma Style_Checks (Off); -- turn off style checks
NULL;                      -- this will not generate an error message
pragma Style_Checks (On);  -- turn style checks back on
NULL;                      -- this will generate an error message
@end example

Finally the two argument form is allowed only if the first argument is
@cite{On} or @cite{Off}.  The effect is to turn of semantic style checks
for the specified entity, as shown in the following example:

@example
pragma Style_Checks ("r"); -- require consistency of identifier casing
Arg : Integer;
Rf1 : Integer := ARG;      -- incorrect, wrong case
pragma Style_Checks (Off, Arg);
Rf2 : Integer := ARG;      -- OK, no error
@end example

@node Pragma Subtitle,Pragma Suppress,Pragma Style_Checks,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-subtitle}@anchor{c1}
@section Pragma Subtitle


Syntax:

@example
pragma Subtitle ([Subtitle =>] STRING_LITERAL);
@end example

This pragma is recognized for compatibility with other Ada compilers
but is ignored by GNAT.

@node Pragma Suppress,Pragma Suppress_All,Pragma Subtitle,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress}@anchor{c2}
@section Pragma Suppress


Syntax:

@example
pragma Suppress (Identifier [, [On =>] Name]);
@end example

This is a standard pragma, and supports all the check names required in
the RM. It is included here because GNAT recognizes some additional check
names that are implementation defined (as permitted by the RM):


@itemize *

@item 
@cite{Alignment_Check} can be used to suppress alignment checks
on addresses used in address clauses. Such checks can also be suppressed
by suppressing range checks, but the specific use of @cite{Alignment_Check}
allows suppression of alignment checks without suppressing other range checks.
Note that @cite{Alignment_Check} is suppressed by default on machines (such as
the x86) with non-strict alignment.

@item 
@cite{Atomic_Synchronization} can be used to suppress the special memory
synchronization instructions that are normally generated for access to
@cite{Atomic} variables to ensure correct synchronization between tasks
that use such variables for synchronization purposes.

@item 
@cite{Duplicated_Tag_Check} Can be used to suppress the check that is generated
for a duplicated tag value when a tagged type is declared.

@item 
@cite{Predicate_Check} can be used to control whether predicate checks are
active. It is applicable only to predicates for which the policy is
@cite{Check}. Unlike @cite{Assertion_Policy}, which determines if a given
predicate is ignored or checked for the whole program, the use of
@cite{Suppress} and @cite{Unsuppress} with this check name allows a given
predicate to be turned on and off at specific points in the program.

@item 
@cite{Validity_Check} can be used specifically to control validity checks.
If @cite{Suppress} is used to suppress validity checks, then no validity
checks are performed, including those specified by the appropriate compiler
switch or the @cite{Validity_Checks} pragma.

@item 
Additional check names previously introduced by use of the @cite{Check_Name}
pragma are also allowed.
@end itemize

Note that pragma Suppress gives the compiler permission to omit
checks, but does not require the compiler to omit checks. The compiler
will generate checks if they are essentially free, even when they are
suppressed. In particular, if the compiler can prove that a certain
check will necessarily fail, it will generate code to do an
unconditional 'raise', even if checks are suppressed. The compiler
warns in this case.

Of course, run-time checks are omitted whenever the compiler can prove
that they will not fail, whether or not checks are suppressed.

@node Pragma Suppress_All,Pragma Suppress_Debug_Info,Pragma Suppress,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-all}@anchor{c3}
@section Pragma Suppress_All


Syntax:

@example
pragma Suppress_All;
@end example

This pragma can appear anywhere within a unit.
The effect is to apply @cite{Suppress (All_Checks)} to the unit
in which it appears.  This pragma is implemented for compatibility with DEC
Ada 83 usage where it appears at the end of a unit, and for compatibility
with Rational Ada, where it appears as a program unit pragma.
The use of the standard Ada pragma @cite{Suppress (All_Checks)}
as a normal configuration pragma is the preferred usage in GNAT.

@node Pragma Suppress_Debug_Info,Pragma Suppress_Exception_Locations,Pragma Suppress_All,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-debug-info}@anchor{c4}
@section Pragma Suppress_Debug_Info


Syntax:

@example
pragma Suppress_Debug_Info ([Entity =>] LOCAL_NAME);
@end example

This pragma can be used to suppress generation of debug information
for the specified entity. It is intended primarily for use in debugging
the debugger, and navigating around debugger problems.

@node Pragma Suppress_Exception_Locations,Pragma Suppress_Initialization,Pragma Suppress_Debug_Info,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-exception-locations}@anchor{c5}
@section Pragma Suppress_Exception_Locations


Syntax:

@example
pragma Suppress_Exception_Locations;
@end example

In normal mode, a raise statement for an exception by default generates
an exception message giving the file name and line number for the location
of the raise. This is useful for debugging and logging purposes, but this
entails extra space for the strings for the messages. The configuration
pragma @cite{Suppress_Exception_Locations} can be used to suppress the
generation of these strings, with the result that space is saved, but the
exception message for such raises is null. This configuration pragma may
appear in a global configuration pragma file, or in a specific unit as
usual. It is not required that this pragma be used consistently within
a partition, so it is fine to have some units within a partition compiled
with this pragma and others compiled in normal mode without it.

@node Pragma Suppress_Initialization,Pragma Task_Name,Pragma Suppress_Exception_Locations,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-suppress-initialization}@anchor{c6}
@section Pragma Suppress_Initialization


@geindex Suppressing initialization

@geindex Initialization
@geindex suppression of

Syntax:

@example
pragma Suppress_Initialization ([Entity =>] variable_or_subtype_Name);
@end example

Here variable_or_subtype_Name is the name introduced by a type declaration
or subtype declaration or the name of a variable introduced by an
object declaration.

In the case of a type or subtype
this pragma suppresses any implicit or explicit initialization
for all variables of the given type or subtype,
including initialization resulting from the use of pragmas
Normalize_Scalars or Initialize_Scalars.

This is considered a representation item, so it cannot be given after
the type is frozen. It applies to all subsequent object declarations,
and also any allocator that creates objects of the type.

If the pragma is given for the first subtype, then it is considered
to apply to the base type and all its subtypes. If the pragma is given
for other than a first subtype, then it applies only to the given subtype.
The pragma may not be given after the type is frozen.

Note that this includes eliminating initialization of discriminants
for discriminated types, and tags for tagged types. In these cases,
you will have to use some non-portable mechanism (e.g. address
overlays or unchecked conversion) to achieve required initialization
of these fields before accessing any object of the corresponding type.

For the variable case, implicit initialization for the named variable
is suppressed, just as though its subtype had been given in a pragma
Suppress_Initialization, as described above.

@node Pragma Task_Name,Pragma Task_Storage,Pragma Suppress_Initialization,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-task-name}@anchor{c7}
@section Pragma Task_Name


Syntax

@example
pragma Task_Name (string_EXPRESSION);
@end example

This pragma appears within a task definition (like pragma
@cite{Priority}) and applies to the task in which it appears.  The
argument must be of type String, and provides a name to be used for
the task instance when the task is created.  Note that this expression
is not required to be static, and in particular, it can contain
references to task discriminants.  This facility can be used to
provide different names for different tasks as they are created,
as illustrated in the example below.

The task name is recorded internally in the run-time structures
and is accessible to tools like the debugger.  In addition the
routine @cite{Ada.Task_Identification.Image} will return this
string, with a unique task address appended.

@example
--  Example of the use of pragma Task_Name

with Ada.Task_Identification;
use Ada.Task_Identification;
with Text_IO; use Text_IO;
procedure t3 is

   type Astring is access String;

   task type Task_Typ (Name : access String) is
      pragma Task_Name (Name.all);
   end Task_Typ;

   task body Task_Typ is
      Nam : constant String := Image (Current_Task);
   begin
      Put_Line ("-->" & Nam (1 .. 14) & "<--");
   end Task_Typ;

   type Ptr_Task is access Task_Typ;
   Task_Var : Ptr_Task;

begin
   Task_Var :=
     new Task_Typ (new String'("This is task 1"));
   Task_Var :=
     new Task_Typ (new String'("This is task 2"));
end;
@end example

@node Pragma Task_Storage,Pragma Test_Case,Pragma Task_Name,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-task-storage}@anchor{c8}
@section Pragma Task_Storage


Syntax:

@example
pragma Task_Storage (
  [Task_Type =>] LOCAL_NAME,
  [Top_Guard =>] static_integer_EXPRESSION);
@end example

This pragma specifies the length of the guard area for tasks.  The guard
area is an additional storage area allocated to a task.  A value of zero
means that either no guard area is created or a minimal guard area is
created, depending on the target.  This pragma can appear anywhere a
@cite{Storage_Size} attribute definition clause is allowed for a task
type.

@node Pragma Test_Case,Pragma Thread_Local_Storage,Pragma Task_Storage,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-test-case}@anchor{c9}
@section Pragma Test_Case


@geindex Test cases

Syntax:

@example
pragma Test_Case (
   [Name     =>] static_string_Expression
  ,[Mode     =>] (Nominal | Robustness)
 [, Requires =>  Boolean_Expression]
 [, Ensures  =>  Boolean_Expression]);
@end example

The @cite{Test_Case} pragma allows defining fine-grain specifications
for use by testing tools.
The compiler checks the validity of the @cite{Test_Case} pragma, but its
presence does not lead to any modification of the code generated by the
compiler.

@cite{Test_Case} pragmas may only appear immediately following the
(separate) declaration of a subprogram in a package declaration, inside
a package spec unit. Only other pragmas may intervene (that is appear
between the subprogram declaration and a test case).

The compiler checks that boolean expressions given in @cite{Requires} and
@cite{Ensures} are valid, where the rules for @cite{Requires} are the
same as the rule for an expression in @cite{Precondition} and the rules
for @cite{Ensures} are the same as the rule for an expression in
@cite{Postcondition}. In particular, attributes @cite{'Old} and
@cite{'Result} can only be used within the @cite{Ensures}
expression. The following is an example of use within a package spec:

@example
package Math_Functions is
   ...
   function Sqrt (Arg : Float) return Float;
   pragma Test_Case (Name     => "Test 1",
                     Mode     => Nominal,
                     Requires => Arg < 10000,
                     Ensures  => Sqrt'Result < 10);
   ...
end Math_Functions;
@end example

The meaning of a test case is that there is at least one context where
@cite{Requires} holds such that, if the associated subprogram is executed in
that context, then @cite{Ensures} holds when the subprogram returns.
Mode @cite{Nominal} indicates that the input context should also satisfy the
precondition of the subprogram, and the output context should also satisfy its
postcondition. Mode @cite{Robustness} indicates that the precondition and
postcondition of the subprogram should be ignored for this test case.

@node Pragma Thread_Local_Storage,Pragma Time_Slice,Pragma Test_Case,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-thread-local-storage}@anchor{ca}
@section Pragma Thread_Local_Storage


@geindex Task specific storage

@geindex TLS (Thread Local Storage)

@geindex Task_Attributes

Syntax:

@example
pragma Thread_Local_Storage ([Entity =>] LOCAL_NAME);
@end example

This pragma specifies that the specified entity, which must be
a variable declared in a library level package, is to be marked as
"Thread Local Storage" (@cite{TLS}). On systems supporting this (which
include Solaris, GNU/Linux and VxWorks 6), this causes each thread
(and hence each Ada task) to see a distinct copy of the variable.

The variable may not have default initialization, and if there is
an explicit initialization, it must be either @cite{null} for an
access variable, or a static expression for a scalar variable.
This provides a low level mechanism similar to that provided by
the @cite{Ada.Task_Attributes} package, but much more efficient
and is also useful in writing interface code that will interact
with foreign threads.

If this pragma is used on a system where @cite{TLS} is not supported,
then an error message will be generated and the program will be rejected.

@node Pragma Time_Slice,Pragma Title,Pragma Thread_Local_Storage,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-time-slice}@anchor{cb}
@section Pragma Time_Slice


Syntax:

@example
pragma Time_Slice (static_duration_EXPRESSION);
@end example

For implementations of GNAT on operating systems where it is possible
to supply a time slice value, this pragma may be used for this purpose.
It is ignored if it is used in a system that does not allow this control,
or if it appears in other than the main program unit.

@node Pragma Title,Pragma Type_Invariant,Pragma Time_Slice,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-title}@anchor{cc}
@section Pragma Title


Syntax:

@example
pragma Title (TITLING_OPTION [, TITLING OPTION]);

TITLING_OPTION ::=
  [Title    =>] STRING_LITERAL,
| [Subtitle =>] STRING_LITERAL
@end example

Syntax checked but otherwise ignored by GNAT.  This is a listing control
pragma used in DEC Ada 83 implementations to provide a title and/or
subtitle for the program listing.  The program listing generated by GNAT
does not have titles or subtitles.

Unlike other pragmas, the full flexibility of named notation is allowed
for this pragma, i.e., the parameters may be given in any order if named
notation is used, and named and positional notation can be mixed
following the normal rules for procedure calls in Ada.

@node Pragma Type_Invariant,Pragma Type_Invariant_Class,Pragma Title,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-type-invariant}@anchor{cd}
@section Pragma Type_Invariant


Syntax:

@example
pragma Type_Invariant
  ([Entity =>] type_LOCAL_NAME,
   [Check  =>] EXPRESSION);
@end example

The @cite{Type_Invariant} pragma is intended to be an exact
replacement for the language-defined @cite{Type_Invariant}
aspect, and shares its restrictions and semantics. It differs
from the language defined @cite{Invariant} pragma in that it
does not permit a string parameter, and it is
controlled by the assertion identifier @cite{Type_Invariant}
rather than @cite{Invariant}.

@node Pragma Type_Invariant_Class,Pragma Unchecked_Union,Pragma Type_Invariant,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-type-invariant-class}@anchor{ce}
@section Pragma Type_Invariant_Class


Syntax:

@example
pragma Type_Invariant_Class
  ([Entity =>] type_LOCAL_NAME,
   [Check  =>] EXPRESSION);
@end example

The @cite{Type_Invariant_Class} pragma is intended to be an exact
replacement for the language-defined @cite{Type_Invariant'Class}
aspect, and shares its restrictions and semantics.

Note: This pragma is called @cite{Type_Invariant_Class} rather than
@cite{Type_Invariant'Class} because the latter would not be strictly
conforming to the allowed syntax for pragmas. The motivation
for providing pragmas equivalent to the aspects is to allow a program
to be written using the pragmas, and then compiled if necessary
using an Ada compiler that does not recognize the pragmas or
aspects, but is prepared to ignore the pragmas. The assertion
policy that controls this pragma is @cite{Type_Invariant'Class},
not @cite{Type_Invariant_Class}.

@node Pragma Unchecked_Union,Pragma Unevaluated_Use_Of_Old,Pragma Type_Invariant_Class,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unchecked-union}@anchor{cf}
@section Pragma Unchecked_Union


@geindex Unions in C

Syntax:

@example
pragma Unchecked_Union (first_subtype_LOCAL_NAME);
@end example

This pragma is used to specify a representation of a record type that is
equivalent to a C union. It was introduced as a GNAT implementation defined
pragma in the GNAT Ada 95 mode. Ada 2005 includes an extended version of this
pragma, making it language defined, and GNAT fully implements this extended
version in all language modes (Ada 83, Ada 95, and Ada 2005). For full
details, consult the Ada 2012 Reference Manual, section B.3.3.

@node Pragma Unevaluated_Use_Of_Old,Pragma Unimplemented_Unit,Pragma Unchecked_Union,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unevaluated-use-of-old}@anchor{d0}
@section Pragma Unevaluated_Use_Of_Old


@geindex Attribute Old

@geindex Attribute Loop_Entry

@geindex Unevaluated_Use_Of_Old

Syntax:

@example
pragma Unevaluated_Use_Of_Old (Error | Warn | Allow);
@end example

This pragma controls the processing of attributes Old and Loop_Entry.
If either of these attributes is used in a potentially unevaluated
expression  (e.g. the then or else parts of an if expression), then
normally this usage is considered illegal if the prefix of the attribute
is other than an entity name. The language requires this
behavior for Old, and GNAT copies the same rule for Loop_Entry.

The reason for this rule is that otherwise, we can have a situation
where we save the Old value, and this results in an exception, even
though we might not evaluate the attribute. Consider this example:

@example
package UnevalOld is
   K : Character;
   procedure U (A : String; C : Boolean)  -- ERROR
     with Post => (if C then A(1)'Old = K else True);
end;
@end example

If procedure U is called with a string with a lower bound of 2, and
C false, then an exception would be raised trying to evaluate A(1)
on entry even though the value would not be actually used.

Although the rule guarantees against this possibility, it is sometimes
too restrictive. For example if we know that the string has a lower
bound of 1, then we will never raise an exception.
The pragma @cite{Unevaluated_Use_Of_Old} can be
used to modify this behavior. If the argument is @cite{Error} then an
error is given (this is the default RM behavior). If the argument is
@cite{Warn} then the usage is allowed as legal but with a warning
that an exception might be raised. If the argument is @cite{Allow}
then the usage is allowed as legal without generating a warning.

This pragma may appear as a configuration pragma, or in a declarative
part or package specification. In the latter case it applies to
uses up to the end of the corresponding statement sequence or
sequence of package declarations.

@node Pragma Unimplemented_Unit,Pragma Universal_Aliasing,Pragma Unevaluated_Use_Of_Old,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unimplemented-unit}@anchor{d1}
@section Pragma Unimplemented_Unit


Syntax:

@example
pragma Unimplemented_Unit;
@end example

If this pragma occurs in a unit that is processed by the compiler, GNAT
aborts with the message @code{xxx not implemented}, where
@cite{xxx} is the name of the current compilation unit.  This pragma is
intended to allow the compiler to handle unimplemented library units in
a clean manner.

The abort only happens if code is being generated.  Thus you can use
specs of unimplemented packages in syntax or semantic checking mode.

@node Pragma Universal_Aliasing,Pragma Universal_Data,Pragma Unimplemented_Unit,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-universal-aliasing}@anchor{d2}
@section Pragma Universal_Aliasing


Syntax:

@example
pragma Universal_Aliasing [([Entity =>] type_LOCAL_NAME)];
@end example

@cite{type_LOCAL_NAME} must refer to a type declaration in the current
declarative part.  The effect is to inhibit strict type-based aliasing
optimization for the given type.  In other words, the effect is as though
access types designating this type were subject to pragma No_Strict_Aliasing.
For a detailed description of the strict aliasing optimization, and the
situations in which it must be suppressed, see the section on
@cite{Optimization and Strict Aliasing} in the @cite{GNAT User's Guide}.

@node Pragma Universal_Data,Pragma Unmodified,Pragma Universal_Aliasing,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-universal-data}@anchor{d3}
@section Pragma Universal_Data


Syntax:

@example
pragma Universal_Data [(library_unit_Name)];
@end example

This pragma is supported only for the AAMP target and is ignored for
other targets. The pragma specifies that all library-level objects
(Counter 0 data) associated with the library unit are to be accessed
and updated using universal addressing (24-bit addresses for AAMP5)
rather than the default of 16-bit Data Environment (DENV) addressing.
Use of this pragma will generally result in less efficient code for
references to global data associated with the library unit, but
allows such data to be located anywhere in memory. This pragma is
a library unit pragma, but can also be used as a configuration pragma
(including use in the @code{gnat.adc} file). The functionality
of this pragma is also available by applying the -univ switch on the
compilations of units where universal addressing of the data is desired.

@node Pragma Unmodified,Pragma Unreferenced,Pragma Universal_Data,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unmodified}@anchor{d4}
@section Pragma Unmodified


@geindex Warnings
@geindex unmodified

Syntax:

@example
pragma Unmodified (LOCAL_NAME @{, LOCAL_NAME@});
@end example

This pragma signals that the assignable entities (variables,
@cite{out} parameters, @cite{in out} parameters) whose names are listed are
deliberately not assigned in the current source unit. This
suppresses warnings about the
entities being referenced but not assigned, and in addition a warning will be
generated if one of these entities is in fact assigned in the
same unit as the pragma (or in the corresponding body, or one
of its subunits).

This is particularly useful for clearly signaling that a particular
parameter is not modified, even though the spec suggests that it might
be.

For the variable case, warnings are never given for unreferenced variables
whose name contains one of the substrings
@cite{DISCARD@comma{} DUMMY@comma{} IGNORE@comma{} JUNK@comma{} UNUSED} in any casing. Such names
are typically to be used in cases where such warnings are expected.
Thus it is never necessary to use @cite{pragma Unmodified} for such
variables, though it is harmless to do so.

@node Pragma Unreferenced,Pragma Unreferenced_Objects,Pragma Unmodified,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unreferenced}@anchor{d5}
@section Pragma Unreferenced


@geindex Warnings
@geindex unreferenced

Syntax:

@example
pragma Unreferenced (LOCAL_NAME @{, LOCAL_NAME@});
pragma Unreferenced (library_unit_NAME @{, library_unit_NAME@});
@end example

This pragma signals that the entities whose names are listed are
deliberately not referenced in the current source unit after the
occurrence of the pragma. This
suppresses warnings about the
entities being unreferenced, and in addition a warning will be
generated if one of these entities is in fact subsequently referenced in the
same unit as the pragma (or in the corresponding body, or one
of its subunits).

This is particularly useful for clearly signaling that a particular
parameter is not referenced in some particular subprogram implementation
and that this is deliberate. It can also be useful in the case of
objects declared only for their initialization or finalization side
effects.

If @cite{LOCAL_NAME} identifies more than one matching homonym in the
current scope, then the entity most recently declared is the one to which
the pragma applies. Note that in the case of accept formals, the pragma
Unreferenced may appear immediately after the keyword @cite{do} which
allows the indication of whether or not accept formals are referenced
or not to be given individually for each accept statement.

The left hand side of an assignment does not count as a reference for the
purpose of this pragma. Thus it is fine to assign to an entity for which
pragma Unreferenced is given.

Note that if a warning is desired for all calls to a given subprogram,
regardless of whether they occur in the same unit as the subprogram
declaration, then this pragma should not be used (calls from another
unit would not be flagged); pragma Obsolescent can be used instead
for this purpose, see @ref{8c,,Pragma Obsolescent}.

The second form of pragma @cite{Unreferenced} is used within a context
clause. In this case the arguments must be unit names of units previously
mentioned in @cite{with} clauses (similar to the usage of pragma
@cite{Elaborate_All}. The effect is to suppress warnings about unreferenced
units and unreferenced entities within these units.

For the variable case, warnings are never given for unreferenced variables
whose name contains one of the substrings
@cite{DISCARD@comma{} DUMMY@comma{} IGNORE@comma{} JUNK@comma{} UNUSED} in any casing. Such names
are typically to be used in cases where such warnings are expected.
Thus it is never necessary to use @cite{pragma Unreferenced} for such
variables, though it is harmless to do so.

@node Pragma Unreferenced_Objects,Pragma Unreserve_All_Interrupts,Pragma Unreferenced,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unreferenced-objects}@anchor{d6}
@section Pragma Unreferenced_Objects


@geindex Warnings
@geindex unreferenced

Syntax:

@example
pragma Unreferenced_Objects (local_subtype_NAME @{, local_subtype_NAME@});
@end example

This pragma signals that for the types or subtypes whose names are
listed, objects which are declared with one of these types or subtypes may
not be referenced, and if no references appear, no warnings are given.

This is particularly useful for objects which are declared solely for their
initialization and finalization effect. Such variables are sometimes referred
to as RAII variables (Resource Acquisition Is Initialization). Using this
pragma on the relevant type (most typically a limited controlled type), the
compiler will automatically suppress unwanted warnings about these variables
not being referenced.

@node Pragma Unreserve_All_Interrupts,Pragma Unsuppress,Pragma Unreferenced_Objects,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unreserve-all-interrupts}@anchor{d7}
@section Pragma Unreserve_All_Interrupts


Syntax:

@example
pragma Unreserve_All_Interrupts;
@end example

Normally certain interrupts are reserved to the implementation.  Any attempt
to attach an interrupt causes Program_Error to be raised, as described in
RM C.3.2(22).  A typical example is the @cite{SIGINT} interrupt used in
many systems for a @code{Ctrl-C} interrupt.  Normally this interrupt is
reserved to the implementation, so that @code{Ctrl-C} can be used to
interrupt execution.

If the pragma @cite{Unreserve_All_Interrupts} appears anywhere in any unit in
a program, then all such interrupts are unreserved.  This allows the
program to handle these interrupts, but disables their standard
functions.  For example, if this pragma is used, then pressing
@code{Ctrl-C} will not automatically interrupt execution.  However,
a program can then handle the @cite{SIGINT} interrupt as it chooses.

For a full list of the interrupts handled in a specific implementation,
see the source code for the spec of @cite{Ada.Interrupts.Names} in
file @code{a-intnam.ads}.  This is a target dependent file that contains the
list of interrupts recognized for a given target.  The documentation in
this file also specifies what interrupts are affected by the use of
the @cite{Unreserve_All_Interrupts} pragma.

For a more general facility for controlling what interrupts can be
handled, see pragma @cite{Interrupt_State}, which subsumes the functionality
of the @cite{Unreserve_All_Interrupts} pragma.

@node Pragma Unsuppress,Pragma Use_VADS_Size,Pragma Unreserve_All_Interrupts,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-unsuppress}@anchor{d8}
@section Pragma Unsuppress


Syntax:

@example
pragma Unsuppress (IDENTIFIER [, [On =>] NAME]);
@end example

This pragma undoes the effect of a previous pragma @cite{Suppress}.  If
there is no corresponding pragma @cite{Suppress} in effect, it has no
effect.  The range of the effect is the same as for pragma
@cite{Suppress}.  The meaning of the arguments is identical to that used
in pragma @cite{Suppress}.

One important application is to ensure that checks are on in cases where
code depends on the checks for its correct functioning, so that the code
will compile correctly even if the compiler switches are set to suppress
checks. For example, in a program that depends on external names of tagged
types and wants to ensure that the duplicated tag check occurs even if all
run-time checks are suppressed by a compiler switch, the following
configuration pragma will ensure this test is not suppressed:

@example
pragma Unsuppress (Duplicated_Tag_Check);
@end example

This pragma is standard in Ada 2005. It is available in all earlier versions
of Ada as an implementation-defined pragma.

Note that in addition to the checks defined in the Ada RM, GNAT recogizes
a number of implementation-defined check names. See description of pragma
@cite{Suppress} for full details.

@node Pragma Use_VADS_Size,Pragma Validity_Checks,Pragma Unsuppress,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-use-vads-size}@anchor{d9}
@section Pragma Use_VADS_Size


@geindex Size
@geindex VADS compatibility

@geindex Rational profile

Syntax:

@example
pragma Use_VADS_Size;
@end example

This is a configuration pragma.  In a unit to which it applies, any use
of the 'Size attribute is automatically interpreted as a use of the
'VADS_Size attribute.  Note that this may result in incorrect semantic
processing of valid Ada 95 or Ada 2005 programs.  This is intended to aid in
the handling of existing code which depends on the interpretation of Size
as implemented in the VADS compiler.  See description of the VADS_Size
attribute for further details.

@node Pragma Validity_Checks,Pragma Volatile,Pragma Use_VADS_Size,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-validity-checks}@anchor{da}
@section Pragma Validity_Checks


Syntax:

@example
pragma Validity_Checks (string_LITERAL | ALL_CHECKS | On | Off);
@end example

This pragma is used in conjunction with compiler switches to control the
built-in validity checking provided by GNAT.  The compiler switches, if set
provide an initial setting for the switches, and this pragma may be used
to modify these settings, or the settings may be provided entirely by
the use of the pragma.  This pragma can be used anywhere that a pragma
is legal, including use as a configuration pragma (including use in
the @code{gnat.adc} file).

The form with a string literal specifies which validity options are to be
activated.  The validity checks are first set to include only the default
reference manual settings, and then a string of letters in the string
specifies the exact set of options required.  The form of this string
is exactly as described for the @emph{-gnatVx} compiler switch (see the
GNAT User's Guide for details).  For example the following two
methods can be used to enable validity checking for mode @cite{in} and
@cite{in out} subprogram parameters:


@itemize *

@item 
@example
pragma Validity_Checks ("im");
@end example

@item 
@example
$ gcc -c -gnatVim ...
@end example
@end itemize

The form ALL_CHECKS activates all standard checks (its use is equivalent
to the use of the @cite{gnatva} switch.

The forms with @cite{Off} and @cite{On}
can be used to temporarily disable validity checks
as shown in the following example:

@example
pragma Validity_Checks ("c"); -- validity checks for copies
pragma Validity_Checks (Off); -- turn off validity checks
A := B;                       -- B will not be validity checked
pragma Validity_Checks (On);  -- turn validity checks back on
A := C;                       -- C will be validity checked
@end example

@node Pragma Volatile,Pragma Warning_As_Error,Pragma Validity_Checks,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-volatile}@anchor{db}
@section Pragma Volatile


Syntax:

@example
pragma Volatile (LOCAL_NAME);
@end example

This pragma is defined by the Ada Reference Manual, and the GNAT
implementation is fully conformant with this definition.  The reason it
is mentioned in this section is that a pragma of the same name was supplied
in some Ada 83 compilers, including DEC Ada 83.  The Ada 95 / Ada 2005
implementation of pragma Volatile is upwards compatible with the
implementation in DEC Ada 83.

@node Pragma Warning_As_Error,Pragma Warnings,Pragma Volatile,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-warning-as-error}@anchor{dc}
@section Pragma Warning_As_Error


Syntax:

@example
pragma Warning_As_Error (static_string_EXPRESSION);
@end example

This configuration pragma allows the programmer to specify a set
of warnings that will be treated as errors. Any warning which
matches the pattern given by the pragma argument will be treated
as an error. This gives much more precise control that -gnatwe
which treats all warnings as errors.

The pattern may contain asterisks, which match zero or more characters in
the message. For example, you can use
@cite{pragma Warning_As_Error ("bits of*unused")} to treat the warning
message @cite{warning: 960 bits of "a" unused} as an error. No other regular
expression notations are permitted. All characters other than asterisk in
these three specific cases are treated as literal characters in the match.
The match is case insensitive, for example XYZ matches xyz.

Note that the pattern matches if it occurs anywhere within the warning
message string (it is not necessary to put an asterisk at the start and
the end of the message, since this is implied).

Another possibility for the static_string_EXPRESSION which works whether
or not error tags are enabled (@emph{-gnatw.d}) is to use the
@emph{-gnatw} tag string, enclosed in brackets,
as shown in the example below, to treat a class of warnings as errors.

The above use of patterns to match the message applies only to warning
messages generated by the front end. This pragma can also be applied to
warnings provided by the back end and mentioned in @ref{dd,,Pragma Warnings}.
By using a single full @emph{-Wxxx} switch in the pragma, such warnings
can also be treated as errors.

The pragma can appear either in a global configuration pragma file
(e.g. @code{gnat.adc}), or at the start of a file. Given a global
configuration pragma file containing:

@example
pragma Warning_As_Error ("[-gnatwj]");
@end example

which will treat all obsolescent feature warnings as errors, the
following program compiles as shown (compile options here are
@emph{-gnatwa.d -gnatl -gnatj55}).

@example
    1. pragma Warning_As_Error ("*never assigned*");
    2. function Warnerr return String is
    3.    X : Integer;
          |
       >>> error: variable "X" is never read and
           never assigned [-gnatwv] [warning-as-error]

    4.    Y : Integer;
          |
       >>> warning: variable "Y" is assigned but
           never read [-gnatwu]

    5. begin
    6.    Y := 0;
    7.    return %ABC%;
                 |
       >>> error: use of "%" is an obsolescent
           feature (RM J.2(4)), use """ instead
           [-gnatwj] [warning-as-error]

    8. end;

8 lines: No errors, 3 warnings (2 treated as errors)
@end example

Note that this pragma does not affect the set of warnings issued in
any way, it merely changes the effect of a matching warning if one
is produced as a result of other warnings options. As shown in this
example, if the pragma results in a warning being treated as an error,
the tag is changed from "warning:" to "error:" and the string
"[warning-as-error]" is appended to the end of the message.

@node Pragma Warnings,Pragma Weak_External,Pragma Warning_As_Error,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas id5}@anchor{de}@anchor{gnat_rm/implementation_defined_pragmas pragma-warnings}@anchor{dd}
@section Pragma Warnings


Syntax:

@example
pragma Warnings ([TOOL_NAME,] DETAILS [, REASON]);

DETAILS ::= On | Off
DETAILS ::= On | Off, local_NAME
DETAILS ::= static_string_EXPRESSION
DETAILS ::= On | Off, static_string_EXPRESSION

TOOL_NAME ::= GNAT | GNATProve

REASON ::= Reason => STRING_LITERAL @{& STRING_LITERAL@}
@end example

Note: in Ada 83 mode, a string literal may be used in place of a static string
expression (which does not exist in Ada 83).

Note if the second argument of @cite{DETAILS} is a @cite{local_NAME} then the
second form is always understood. If the intention is to use
the fourth form, then you can write @cite{NAME & ""} to force the
intepretation as a @cite{static_string_EXPRESSION}.

Note: if the first argument is a valid @cite{TOOL_NAME}, it will be interpreted
that way. The use of the @cite{TOOL_NAME} argument is relevant only to users
of SPARK and GNATprove, see last part of this section for details.

Normally warnings are enabled, with the output being controlled by
the command line switch.  Warnings (@cite{Off}) turns off generation of
warnings until a Warnings (@cite{On}) is encountered or the end of the
current unit.  If generation of warnings is turned off using this
pragma, then some or all of the warning messages are suppressed,
regardless of the setting of the command line switches.

The @cite{Reason} parameter may optionally appear as the last argument
in any of the forms of this pragma. It is intended purely for the
purposes of documenting the reason for the @cite{Warnings} pragma.
The compiler will check that the argument is a static string but
otherwise ignore this argument. Other tools may provide specialized
processing for this string.

The form with a single argument (or two arguments if Reason present),
where the first argument is @cite{ON} or @cite{OFF}
may be used as a configuration pragma.

If the @cite{LOCAL_NAME} parameter is present, warnings are suppressed for
the specified entity.  This suppression is effective from the point where
it occurs till the end of the extended scope of the variable (similar to
the scope of @cite{Suppress}). This form cannot be used as a configuration
pragma.

In the case where the first argument is other than @cite{ON} or
@cite{OFF},
the third form with a single static_string_EXPRESSION argument (and possible
reason) provides more precise
control over which warnings are active. The string is a list of letters
specifying which warnings are to be activated and which deactivated. The
code for these letters is the same as the string used in the command
line switch controlling warnings. For a brief summary, use the gnatmake
command with no arguments, which will generate usage information containing
the list of warnings switches supported. For
full details see the section on @cite{Warning Message Control} in the
@cite{GNAT User's Guide}.
This form can also be used as a configuration pragma.

The warnings controlled by the @emph{-gnatw} switch are generated by the
front end of the compiler. The GCC back end can provide additional warnings
and they are controlled by the @emph{-W} switch. Such warnings can be
identified by the appearance of a string of the form @cite{[-Wxxx]} in the
message which designates the @emph{-Wxxx} switch that controls the message.
The form with a single static_string_EXPRESSION argument also works for these
warnings, but the string must be a single full @emph{-Wxxx} switch in this
case. The above reference lists a few examples of these additional warnings.

The specified warnings will be in effect until the end of the program
or another pragma Warnings is encountered. The effect of the pragma is
cumulative. Initially the set of warnings is the standard default set
as possibly modified by compiler switches. Then each pragma Warning
modifies this set of warnings as specified. This form of the pragma may
also be used as a configuration pragma.

The fourth form, with an @cite{On|Off} parameter and a string, is used to
control individual messages, based on their text. The string argument
is a pattern that is used to match against the text of individual
warning messages (not including the initial "warning: " tag).

The pattern may contain asterisks, which match zero or more characters in
the message. For example, you can use
@cite{pragma Warnings (Off@comma{} "bits of*unused")} to suppress the warning
message @cite{warning: 960 bits of "a" unused}. No other regular
expression notations are permitted. All characters other than asterisk in
these three specific cases are treated as literal characters in the match.
The match is case insensitive, for example XYZ matches xyz.

Note that the pattern matches if it occurs anywhere within the warning
message string (it is not necessary to put an asterisk at the start and
the end of the message, since this is implied).

The above use of patterns to match the message applies only to warning
messages generated by the front end. This form of the pragma with a string
argument can also be used to control warnings provided by the back end and
mentioned above. By using a single full @emph{-Wxxx} switch in the pragma,
such warnings can be turned on and off.

There are two ways to use the pragma in this form. The OFF form can be used
as a configuration pragma. The effect is to suppress all warnings (if any)
that match the pattern string throughout the compilation (or match the
-W switch in the back end case).

The second usage is to suppress a warning locally, and in this case, two
pragmas must appear in sequence:

@example
pragma Warnings (Off, Pattern);
... code where given warning is to be suppressed
pragma Warnings (On, Pattern);
@end example

In this usage, the pattern string must match in the Off and On
pragmas, and (if @emph{-gnatw.w} is given) at least one matching
warning must be suppressed.

Note: to write a string that will match any warning, use the string
@cite{"***"}. It will not work to use a single asterisk or two
asterisks since this looks like an operator name. This form with three
asterisks is similar in effect to specifying @cite{pragma Warnings (Off)} except (if @emph{-gnatw.w} is given) that a matching
@cite{pragma Warnings (On@comma{} "***")} will be required. This can be
helpful in avoiding forgetting to turn warnings back on.

Note: the debug flag -gnatd.i (@cite{/NOWARNINGS_PRAGMAS} in VMS) can be
used to cause the compiler to entirely ignore all WARNINGS pragmas. This can
be useful in checking whether obsolete pragmas in existing programs are hiding
real problems.

Note: pragma Warnings does not affect the processing of style messages. See
separate entry for pragma Style_Checks for control of style messages.

Users of the formal verification tool GNATprove for the SPARK subset of Ada may
use the version of the pragma with a @cite{TOOL_NAME} parameter.

If present, @cite{TOOL_NAME} is the name of a tool, currently either @cite{GNAT} for the
compiler or @cite{GNATprove} for the formal verification tool. A given tool only
takes into account pragma Warnings that do not specify a tool name, or that
specify the matching tool name. This makes it possible to disable warnings
selectively for each tool, and as a consequence to detect useless pragma
Warnings with switch @cite{-gnatw.w}.

@node Pragma Weak_External,Pragma Wide_Character_Encoding,Pragma Warnings,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-weak-external}@anchor{df}
@section Pragma Weak_External


Syntax:

@example
pragma Weak_External ([Entity =>] LOCAL_NAME);
@end example

@cite{LOCAL_NAME} must refer to an object that is declared at the library
level. This pragma specifies that the given entity should be marked as a
weak symbol for the linker. It is equivalent to @cite{__attribute__((weak))}
in GNU C and causes @cite{LOCAL_NAME} to be emitted as a weak symbol instead
of a regular symbol, that is to say a symbol that does not have to be
resolved by the linker if used in conjunction with a pragma Import.

When a weak symbol is not resolved by the linker, its address is set to
zero. This is useful in writing interfaces to external modules that may
or may not be linked in the final executable, for example depending on
configuration settings.

If a program references at run time an entity to which this pragma has been
applied, and the corresponding symbol was not resolved at link time, then
the execution of the program is erroneous. It is not erroneous to take the
Address of such an entity, for example to guard potential references,
as shown in the example below.

Some file formats do not support weak symbols so not all target machines
support this pragma.

@example
--  Example of the use of pragma Weak_External

package External_Module is
  key : Integer;
  pragma Import (C, key);
  pragma Weak_External (key);
  function Present return boolean;
end External_Module;

with System; use System;
package body External_Module is
  function Present return boolean is
  begin
    return key'Address /= System.Null_Address;
  end Present;
end External_Module;
@end example

@node Pragma Wide_Character_Encoding,,Pragma Weak_External,Implementation Defined Pragmas
@anchor{gnat_rm/implementation_defined_pragmas pragma-wide-character-encoding}@anchor{e0}
@section Pragma Wide_Character_Encoding


Syntax:

@example
pragma Wide_Character_Encoding (IDENTIFIER | CHARACTER_LITERAL);
@end example

This pragma specifies the wide character encoding to be used in program
source text appearing subsequently. It is a configuration pragma, but may
also be used at any point that a pragma is allowed, and it is permissible
to have more than one such pragma in a file, allowing multiple encodings
to appear within the same file.

The argument can be an identifier or a character literal. In the identifier
case, it is one of @cite{HEX}, @cite{UPPER}, @cite{SHIFT_JIS},
@cite{EUC}, @cite{UTF8}, or @cite{BRACKETS}. In the character literal
case it is correspondingly one of the characters @code{h}, @code{u},
@code{s}, @code{e}, @code{8}, or @code{b}.

Note that when the pragma is used within a file, it affects only the
encoding within that file, and does not affect withed units, specs,
or subunits.

@node Implementation Defined Aspects,Implementation Defined Attributes,Implementation Defined Pragmas,Top
@anchor{gnat_rm/implementation_defined_aspects implementation-defined-aspects}@anchor{e1}@anchor{gnat_rm/implementation_defined_aspects doc}@anchor{e2}@anchor{gnat_rm/implementation_defined_aspects id1}@anchor{e3}
@chapter Implementation Defined Aspects


Ada defines (throughout the Ada 2012 reference manual, summarized
in Annex K) a set of aspects that can be specified for certain entities.
These language defined aspects are implemented in GNAT in Ada 2012 mode
and work as described in the Ada 2012 Reference Manual.

In addition, Ada 2012 allows implementations to define additional aspects
whose meaning is defined by the implementation.  GNAT provides
a number of these implementation-defined aspects which can be used
to extend and enhance the functionality of the compiler.  This section of
the GNAT reference manual describes these additional aspects.

Note that any program using these aspects may not be portable to
other compilers (although GNAT implements this set of aspects on all
platforms).  Therefore if portability to other compilers is an important
consideration, you should minimize the use of these aspects.

Note that for many of these aspects, the effect is essentially similar
to the use of a pragma or attribute specification with the same name
applied to the entity. For example, if we write:

@example
type R is range 1 .. 100
  with Value_Size => 10;
@end example

then the effect is the same as:

@example
type R is range 1 .. 100;
for R'Value_Size use 10;
@end example

and if we write:

@example
type R is new Integer
  with Shared => True;
@end example

then the effect is the same as:

@example
type R is new Integer;
pragma Shared (R);
@end example

In the documentation below, such cases are simply marked
as being boolean aspects equivalent to the corresponding pragma
or attribute definition clause.

@menu
* Aspect Abstract_State:: 
* Annotate:: 
* Aspect Async_Readers:: 
* Aspect Async_Writers:: 
* Aspect Contract_Cases:: 
* Aspect Depends:: 
* Aspect Dimension:: 
* Aspect Dimension_System:: 
* Aspect Effective_Reads:: 
* Aspect Effective_Writes:: 
* Aspect Favor_Top_Level:: 
* Aspect Global:: 
* Aspect Initial_Condition:: 
* Aspect Initializes:: 
* Aspect Inline_Always:: 
* Aspect Invariant:: 
* Aspect Invariant'Class:: 
* Aspect Iterable:: 
* Aspect Linker_Section:: 
* Aspect Lock_Free:: 
* Aspect No_Elaboration_Code_All:: 
* Aspect No_Tagged_Streams:: 
* Aspect Object_Size:: 
* Aspect Obsolescent:: 
* Aspect Part_Of:: 
* Aspect Persistent_BSS:: 
* Aspect Predicate:: 
* Aspect Pure_Function:: 
* Aspect Refined_Depends:: 
* Aspect Refined_Global:: 
* Aspect Refined_Post:: 
* Aspect Refined_State:: 
* Aspect Remote_Access_Type:: 
* Aspect Scalar_Storage_Order:: 
* Aspect Shared:: 
* Aspect Simple_Storage_Pool:: 
* Aspect Simple_Storage_Pool_Type:: 
* Aspect SPARK_Mode:: 
* Aspect Suppress_Debug_Info:: 
* Aspect Suppress_Initialization:: 
* Aspect Test_Case:: 
* Aspect Thread_Local_Storage:: 
* Aspect Universal_Aliasing:: 
* Aspect Universal_Data:: 
* Aspect Unmodified:: 
* Aspect Unreferenced:: 
* Aspect Unreferenced_Objects:: 
* Aspect Value_Size:: 
* Aspect Warnings:: 

@end menu

@node Aspect Abstract_State,Annotate,,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-abstract-state}@anchor{e4}
@section Aspect Abstract_State


@geindex Abstract_State

This aspect is equivalent to pragma @cite{Abstract_State}.

@node Annotate,Aspect Async_Readers,Aspect Abstract_State,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects annotate}@anchor{e5}
@section Annotate


@geindex Annotate

There are three forms of this aspect (where ID is an identifier,
and ARG is a general expression).


@table @asis

@item @emph{Annotate => ID}

Equivalent to @cite{pragma Annotate (ID@comma{} Entity => Name);}

@item @emph{Annotate => (ID)}

Equivalent to @cite{pragma Annotate (ID@comma{} Entity => Name);}

@item @emph{Annotate => (ID ,ID @{, ARG@})}

Equivalent to @cite{pragma Annotate (ID@comma{} ID @{@comma{} ARG@}@comma{} Entity => Name);}
@end table

@node Aspect Async_Readers,Aspect Async_Writers,Annotate,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-async-readers}@anchor{e6}
@section Aspect Async_Readers


@geindex Async_Readers

This boolean aspect is equivalent to pragma @cite{Async_Readers}.

@node Aspect Async_Writers,Aspect Contract_Cases,Aspect Async_Readers,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-async-writers}@anchor{e7}
@section Aspect Async_Writers


@geindex Async_Writers

This boolean aspect is equivalent to pragma @cite{Async_Writers}.

@node Aspect Contract_Cases,Aspect Depends,Aspect Async_Writers,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-contract-cases}@anchor{e8}
@section Aspect Contract_Cases


@geindex Contract_Cases

This aspect is equivalent to pragma @cite{Contract_Cases}, the sequence
of clauses being enclosed in parentheses so that syntactically it is an
aggregate.

@node Aspect Depends,Aspect Dimension,Aspect Contract_Cases,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-depends}@anchor{e9}
@section Aspect Depends


@geindex Depends

This aspect is equivalent to pragma @cite{Depends}.

@node Aspect Dimension,Aspect Dimension_System,Aspect Depends,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-dimension}@anchor{ea}
@section Aspect Dimension


@geindex Dimension

The @cite{Dimension} aspect is used to specify the dimensions of a given
subtype of a dimensioned numeric type. The aspect also specifies a symbol
used when doing formatted output of dimensioned quantities. The syntax is:

@example
with Dimension =>
  ([Symbol =>] SYMBOL, DIMENSION_VALUE @{, DIMENSION_Value@})

SYMBOL ::= STRING_LITERAL | CHARACTER_LITERAL

DIMENSION_VALUE ::=
  RATIONAL
| others               => RATIONAL
| DISCRETE_CHOICE_LIST => RATIONAL

RATIONAL ::= [-] NUMERIC_LITERAL [/ NUMERIC_LITERAL]
@end example

This aspect can only be applied to a subtype whose parent type has
a @cite{Dimension_Systen} aspect. The aspect must specify values for
all dimensions of the system. The rational values are the powers of the
corresponding dimensions that are used by the compiler to verify that
physical (numeric) computations are dimensionally consistent. For example,
the computation of a force must result in dimensions (L => 1, M => 1, T => -2).
For further examples of the usage
of this aspect, see package @cite{System.Dim.Mks}.
Note that when the dimensioned type is an integer type, then any
dimension value must be an integer literal.

@node Aspect Dimension_System,Aspect Effective_Reads,Aspect Dimension,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-dimension-system}@anchor{eb}
@section Aspect Dimension_System


@geindex Dimension_System

The @cite{Dimension_System} aspect is used to define a system of
dimensions that will be used in subsequent subtype declarations with
@cite{Dimension} aspects that reference this system. The syntax is:

@example
with Dimension_System => (DIMENSION @{, DIMENSION@});

DIMENSION ::= ([Unit_Name   =>] IDENTIFIER,
               [Unit_Symbol =>] SYMBOL,
               [Dim_Symbol  =>] SYMBOL)

SYMBOL ::= CHARACTER_LITERAL | STRING_LITERAL
@end example

This aspect is applied to a type, which must be a numeric derived type
(typically a floating-point type), that
will represent values within the dimension system. Each @cite{DIMENSION}
corresponds to one particular dimension. A maximum of 7 dimensions may
be specified. @cite{Unit_Name} is the name of the dimension (for example
@cite{Meter}). @cite{Unit_Symbol} is the shorthand used for quantities
of this dimension (for example @cite{m} for @cite{Meter}).
@cite{Dim_Symbol} gives
the identification within the dimension system (typically this is a
single letter, e.g. @cite{L} standing for length for unit name @cite{Meter}).
The @cite{Unit_Symbol} is used in formatted output of dimensioned quantities.
The @cite{Dim_Symbol} is used in error messages when numeric operations have
inconsistent dimensions.

GNAT provides the standard definition of the International MKS system in
the run-time package @cite{System.Dim.Mks}. You can easily define
similar packages for cgs units or British units, and define conversion factors
between values in different systems. The MKS system is characterized by the
following aspect:

@example
type Mks_Type is new Long_Long_Float with
  Dimension_System => (
    (Unit_Name => Meter,    Unit_Symbol => 'm',   Dim_Symbol => 'L'),
    (Unit_Name => Kilogram, Unit_Symbol => "kg",  Dim_Symbol => 'M'),
    (Unit_Name => Second,   Unit_Symbol => 's',   Dim_Symbol => 'T'),
    (Unit_Name => Ampere,   Unit_Symbol => 'A',   Dim_Symbol => 'I'),
    (Unit_Name => Kelvin,   Unit_Symbol => 'K',   Dim_Symbol => '@@'),
    (Unit_Name => Mole,     Unit_Symbol => "mol", Dim_Symbol => 'N'),
    (Unit_Name => Candela,  Unit_Symbol => "cd",  Dim_Symbol => 'J'));
@end example

Note that in the above type definition, we use the @cite{at} symbol (@code{@@}) to
represent a theta character (avoiding the use of extended Latin-1
characters in this context).

See section 'Performing Dimensionality Analysis in GNAT' in the GNAT Users
Guide for detailed examples of use of the dimension system.

@node Aspect Effective_Reads,Aspect Effective_Writes,Aspect Dimension_System,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-effective-reads}@anchor{ec}
@section Aspect Effective_Reads


@geindex Effective_Reads

This aspect is equivalent to pragma @cite{Effective_Reads}.

@node Aspect Effective_Writes,Aspect Favor_Top_Level,Aspect Effective_Reads,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-effective-writes}@anchor{ed}
@section Aspect Effective_Writes


@geindex Effective_Writes

This aspect is equivalent to pragma @cite{Effective_Writes}.

@node Aspect Favor_Top_Level,Aspect Global,Aspect Effective_Writes,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-favor-top-level}@anchor{ee}
@section Aspect Favor_Top_Level


@geindex Favor_Top_Level

This boolean aspect is equivalent to pragma @cite{Favor_Top_Level}.

@node Aspect Global,Aspect Initial_Condition,Aspect Favor_Top_Level,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-global}@anchor{ef}
@section Aspect Global


@geindex Global

This aspect is equivalent to pragma @cite{Global}.

@node Aspect Initial_Condition,Aspect Initializes,Aspect Global,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-initial-condition}@anchor{f0}
@section Aspect Initial_Condition


@geindex Initial_Condition

This aspect is equivalent to pragma @cite{Initial_Condition}.

@node Aspect Initializes,Aspect Inline_Always,Aspect Initial_Condition,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-initializes}@anchor{f1}
@section Aspect Initializes


@geindex Initializes

This aspect is equivalent to pragma @cite{Initializes}.

@node Aspect Inline_Always,Aspect Invariant,Aspect Initializes,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-inline-always}@anchor{f2}
@section Aspect Inline_Always


@geindex Inline_Always

This boolean aspect is equivalent to pragma @cite{Inline_Always}.

@node Aspect Invariant,Aspect Invariant'Class,Aspect Inline_Always,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-invariant}@anchor{f3}
@section Aspect Invariant


@geindex Invariant

This aspect is equivalent to pragma @cite{Invariant}. It is a
synonym for the language defined aspect @cite{Type_Invariant} except
that it is separately controllable using pragma @cite{Assertion_Policy}.

@node Aspect Invariant'Class,Aspect Iterable,Aspect Invariant,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-invariant-class}@anchor{f4}
@section Aspect Invariant'Class


@geindex Invariant'Class

This aspect is equivalent to pragma @cite{Type_Invariant_Class}. It is a
synonym for the language defined aspect @cite{Type_Invariant'Class} except
that it is separately controllable using pragma @cite{Assertion_Policy}.

@node Aspect Iterable,Aspect Linker_Section,Aspect Invariant'Class,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-iterable}@anchor{f5}
@section Aspect Iterable


@geindex Iterable

This aspect provides a light-weight mechanism for loops and quantified
expressions over container types, without the overhead imposed by the tampering
checks of standard Ada 2012 iterators. The value of the aspect is an aggregate
with four named components: @cite{First}, @cite{Next}, @cite{Has_Element}, and @cite{Element} (the
last one being optional). When only 3 components are specified, only the
@cite{for .. in} form of iteration over cursors is available. When all 4 components
are specified, both this form and the @cite{for .. of} form of iteration over
elements are available. The following is a typical example of use:

@example
type List is private with
    Iterable => (First        => First_Cursor,
                 Next         => Advance,
                 Has_Element  => Cursor_Has_Element,
                [Element      => Get_Element]);
@end example


@itemize *

@item 
The value denoted by @cite{First} must denote a primitive operation of the
container type that returns a @cite{Cursor}, which must a be a type declared in
the container package or visible from it. For example:
@end itemize

@example
function First_Cursor (Cont : Container) return Cursor;
@end example


@itemize *

@item 
The value of @cite{Next} is a primitive operation of the container type that takes
both a container and a cursor and yields a cursor. For example:
@end itemize

@example
function Advance (Cont : Container; Position : Cursor) return Cursor;
@end example


@itemize *

@item 
The value of @cite{Has_Element} is a primitive operation of the container type
that takes both a container and a cursor and yields a boolean. For example:
@end itemize

@example
function Cursor_Has_Element (Cont : Container; Position : Cursor) return Boolean;
@end example


@itemize *

@item 
The value of @cite{Element} is a primitive operation of the container type that
takes both a container and a cursor and yields an @cite{Element_Type}, which must
be a type declared in the container package or visible from it. For example:
@end itemize

@example
function Get_Element (Cont : Container; Position : Cursor) return Element_Type;
@end example

This aspect is used in the GNAT-defined formal container packages.

@node Aspect Linker_Section,Aspect Lock_Free,Aspect Iterable,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-linker-section}@anchor{f6}
@section Aspect Linker_Section


@geindex Linker_Section

This aspect is equivalent to an @cite{Linker_Section} pragma.

@node Aspect Lock_Free,Aspect No_Elaboration_Code_All,Aspect Linker_Section,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-lock-free}@anchor{f7}
@section Aspect Lock_Free


@geindex Lock_Free

This boolean aspect is equivalent to pragma @cite{Lock_Free}.

@node Aspect No_Elaboration_Code_All,Aspect No_Tagged_Streams,Aspect Lock_Free,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-elaboration-code-all}@anchor{f8}
@section Aspect No_Elaboration_Code_All


@geindex No_Elaboration_Code_All

This aspect is equivalent to a @cite{pragma No_Elaboration_Code_All}
statement for a program unit.

@node Aspect No_Tagged_Streams,Aspect Object_Size,Aspect No_Elaboration_Code_All,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-no-tagged-streams}@anchor{f9}
@section Aspect No_Tagged_Streams


@geindex No_Tagged_Streams

This aspect is equivalent to a @cite{pragma No_Tagged_Streams} with an
argument specifying a root tagged type (thus this aspect can only be
applied to such a type).

@node Aspect Object_Size,Aspect Obsolescent,Aspect No_Tagged_Streams,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-object-size}@anchor{fa}
@section Aspect Object_Size


@geindex Object_Size

This aspect is equivalent to an @cite{Object_Size} attribute definition
clause.

@node Aspect Obsolescent,Aspect Part_Of,Aspect Object_Size,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-obsolescent}@anchor{fb}
@section Aspect Obsolescent


@geindex Obsolsecent

This aspect is equivalent to an @cite{Obsolescent} pragma. Note that the
evaluation of this aspect happens at the point of occurrence, it is not
delayed until the freeze point.

@node Aspect Part_Of,Aspect Persistent_BSS,Aspect Obsolescent,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-part-of}@anchor{fc}
@section Aspect Part_Of


@geindex Part_Of

This aspect is equivalent to pragma @cite{Part_Of}.

@node Aspect Persistent_BSS,Aspect Predicate,Aspect Part_Of,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-persistent-bss}@anchor{fd}
@section Aspect Persistent_BSS


@geindex Persistent_BSS

This boolean aspect is equivalent to pragma @cite{Persistent_BSS}.

@node Aspect Predicate,Aspect Pure_Function,Aspect Persistent_BSS,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-predicate}@anchor{fe}
@section Aspect Predicate


@geindex Predicate

This aspect is equivalent to pragma @cite{Predicate}. It is thus
similar to the language defined aspects @cite{Dynamic_Predicate}
and @cite{Static_Predicate} except that whether the resulting
predicate is static or dynamic is controlled by the form of the
expression. It is also separately controllable using pragma
@cite{Assertion_Policy}.

@node Aspect Pure_Function,Aspect Refined_Depends,Aspect Predicate,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-pure-function}@anchor{ff}
@section Aspect Pure_Function


@geindex Pure_Function

This boolean aspect is equivalent to pragma @cite{Pure_Function}.

@node Aspect Refined_Depends,Aspect Refined_Global,Aspect Pure_Function,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-depends}@anchor{100}
@section Aspect Refined_Depends


@geindex Refined_Depends

This aspect is equivalent to pragma @cite{Refined_Depends}.

@node Aspect Refined_Global,Aspect Refined_Post,Aspect Refined_Depends,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-global}@anchor{101}
@section Aspect Refined_Global


@geindex Refined_Global

This aspect is equivalent to pragma @cite{Refined_Global}.

@node Aspect Refined_Post,Aspect Refined_State,Aspect Refined_Global,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-post}@anchor{102}
@section Aspect Refined_Post


@geindex Refined_Post

This aspect is equivalent to pragma @cite{Refined_Post}.

@node Aspect Refined_State,Aspect Remote_Access_Type,Aspect Refined_Post,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-refined-state}@anchor{103}
@section Aspect Refined_State


@geindex Refined_State

This aspect is equivalent to pragma @cite{Refined_State}.

@node Aspect Remote_Access_Type,Aspect Scalar_Storage_Order,Aspect Refined_State,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-remote-access-type}@anchor{104}
@section Aspect Remote_Access_Type


@geindex Remote_Access_Type

This aspect is equivalent to pragma @cite{Remote_Access_Type}.

@node Aspect Scalar_Storage_Order,Aspect Shared,Aspect Remote_Access_Type,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-scalar-storage-order}@anchor{105}
@section Aspect Scalar_Storage_Order


@geindex Scalar_Storage_Order

This aspect is equivalent to a @cite{Scalar_Storage_Order}
attribute definition clause.

@node Aspect Shared,Aspect Simple_Storage_Pool,Aspect Scalar_Storage_Order,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-shared}@anchor{106}
@section Aspect Shared


@geindex Shared

This boolean aspect is equivalent to pragma @cite{Shared},
and is thus a synonym for aspect @cite{Atomic}.

@node Aspect Simple_Storage_Pool,Aspect Simple_Storage_Pool_Type,Aspect Shared,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-simple-storage-pool}@anchor{107}
@section Aspect Simple_Storage_Pool


@geindex Simple_Storage_Pool

This aspect is equivalent to a @cite{Simple_Storage_Pool}
attribute definition clause.

@node Aspect Simple_Storage_Pool_Type,Aspect SPARK_Mode,Aspect Simple_Storage_Pool,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-simple-storage-pool-type}@anchor{108}
@section Aspect Simple_Storage_Pool_Type


@geindex Simple_Storage_Pool_Type

This boolean aspect is equivalent to pragma @cite{Simple_Storage_Pool_Type}.

@node Aspect SPARK_Mode,Aspect Suppress_Debug_Info,Aspect Simple_Storage_Pool_Type,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-spark-mode}@anchor{109}
@section Aspect SPARK_Mode


@geindex SPARK_Mode

This aspect is equivalent to pragma @cite{SPARK_Mode} and
may be specified for either or both of the specification and body
of a subprogram or package.

@node Aspect Suppress_Debug_Info,Aspect Suppress_Initialization,Aspect SPARK_Mode,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-suppress-debug-info}@anchor{10a}
@section Aspect Suppress_Debug_Info


@geindex Suppress_Debug_Info

This boolean aspect is equivalent to pragma @cite{Suppress_Debug_Info}.

@node Aspect Suppress_Initialization,Aspect Test_Case,Aspect Suppress_Debug_Info,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-suppress-initialization}@anchor{10b}
@section Aspect Suppress_Initialization


@geindex Suppress_Initialization

This boolean aspect is equivalent to pragma @cite{Suppress_Initialization}.

@node Aspect Test_Case,Aspect Thread_Local_Storage,Aspect Suppress_Initialization,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-test-case}@anchor{10c}
@section Aspect Test_Case


@geindex Test_Case

This aspect is equivalent to pragma @cite{Test_Case}.

@node Aspect Thread_Local_Storage,Aspect Universal_Aliasing,Aspect Test_Case,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-thread-local-storage}@anchor{10d}
@section Aspect Thread_Local_Storage


@geindex Thread_Local_Storage

This boolean aspect is equivalent to pragma @cite{Thread_Local_Storage}.

@node Aspect Universal_Aliasing,Aspect Universal_Data,Aspect Thread_Local_Storage,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-universal-aliasing}@anchor{10e}
@section Aspect Universal_Aliasing


@geindex Universal_Aliasing

This boolean aspect is equivalent to pragma @cite{Universal_Aliasing}.

@node Aspect Universal_Data,Aspect Unmodified,Aspect Universal_Aliasing,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-universal-data}@anchor{10f}
@section Aspect Universal_Data


@geindex Universal_Data

This aspect is equivalent to pragma @cite{Universal_Data}.

@node Aspect Unmodified,Aspect Unreferenced,Aspect Universal_Data,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-unmodified}@anchor{110}
@section Aspect Unmodified


@geindex Unmodified

This boolean aspect is equivalent to pragma @cite{Unmodified}.

@node Aspect Unreferenced,Aspect Unreferenced_Objects,Aspect Unmodified,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-unreferenced}@anchor{111}
@section Aspect Unreferenced


@geindex Unreferenced

This boolean aspect is equivalent to pragma @cite{Unreferenced}. Note that
in the case of formal parameters, it is not permitted to have aspects for
a formal parameter, so in this case the pragma form must be used.

@node Aspect Unreferenced_Objects,Aspect Value_Size,Aspect Unreferenced,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-unreferenced-objects}@anchor{112}
@section Aspect Unreferenced_Objects


@geindex Unreferenced_Objects

This boolean aspect is equivalent to pragma @cite{Unreferenced_Objects}.

@node Aspect Value_Size,Aspect Warnings,Aspect Unreferenced_Objects,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-value-size}@anchor{113}
@section Aspect Value_Size


@geindex Value_Size

This aspect is equivalent to a @cite{Value_Size}
attribute definition clause.

@node Aspect Warnings,,Aspect Value_Size,Implementation Defined Aspects
@anchor{gnat_rm/implementation_defined_aspects aspect-warnings}@anchor{114}
@section Aspect Warnings


@geindex Warnings

This aspect is equivalent to the two argument form of pragma @cite{Warnings},
where the first argument is @cite{ON} or @cite{OFF} and the second argument
is the entity.

@node Implementation Defined Attributes,Standard and Implementation Defined Restrictions,Implementation Defined Aspects,Top
@anchor{gnat_rm/implementation_defined_attributes doc}@anchor{115}@anchor{gnat_rm/implementation_defined_attributes implementation-defined-attributes}@anchor{8}@anchor{gnat_rm/implementation_defined_attributes id1}@anchor{116}
@chapter Implementation Defined Attributes


Ada defines (throughout the Ada reference manual,
summarized in Annex K),
a set of attributes that provide useful additional functionality in all
areas of the language.  These language defined attributes are implemented
in GNAT and work as described in the Ada Reference Manual.

In addition, Ada allows implementations to define additional
attributes whose meaning is defined by the implementation.  GNAT provides
a number of these implementation-dependent attributes which can be used
to extend and enhance the functionality of the compiler.  This section of
the GNAT reference manual describes these additional attributes.  It also
describes additional implementation-dependent features of standard
language-defined attributes.

Note that any program using these attributes may not be portable to
other compilers (although GNAT implements this set of attributes on all
platforms).  Therefore if portability to other compilers is an important
consideration, you should minimize the use of these attributes.

@menu
* Attribute Abort_Signal:: 
* Attribute Address_Size:: 
* Attribute Asm_Input:: 
* Attribute Asm_Output:: 
* Attribute Atomic_Always_Lock_Free:: 
* Attribute Bit:: 
* Attribute Bit_Position:: 
* Attribute Code_Address:: 
* Attribute Compiler_Version:: 
* Attribute Constrained:: 
* Attribute Default_Bit_Order:: 
* Attribute Default_Scalar_Storage_Order:: 
* Attribute Deref:: 
* Attribute Descriptor_Size:: 
* Attribute Elaborated:: 
* Attribute Elab_Body:: 
* Attribute Elab_Spec:: 
* Attribute Elab_Subp_Body:: 
* Attribute Emax:: 
* Attribute Enabled:: 
* Attribute Enum_Rep:: 
* Attribute Enum_Val:: 
* Attribute Epsilon:: 
* Attribute Fast_Math:: 
* Attribute Fixed_Value:: 
* Attribute From_Any:: 
* Attribute Has_Access_Values:: 
* Attribute Has_Discriminants:: 
* Attribute Img:: 
* Attribute Integer_Value:: 
* Attribute Invalid_Value:: 
* Attribute Iterable:: 
* Attribute Large:: 
* Attribute Library_Level:: 
* Attribute Lock_Free:: 
* Attribute Loop_Entry:: 
* Attribute Machine_Size:: 
* Attribute Mantissa:: 
* Attribute Maximum_Alignment:: 
* Attribute Mechanism_Code:: 
* Attribute Null_Parameter:: 
* Attribute Object_Size:: 
* Attribute Old:: 
* Attribute Passed_By_Reference:: 
* Attribute Pool_Address:: 
* Attribute Range_Length:: 
* Attribute Ref:: 
* Attribute Restriction_Set:: 
* Attribute Result:: 
* Attribute Safe_Emax:: 
* Attribute Safe_Large:: 
* Attribute Safe_Small:: 
* Attribute Scalar_Storage_Order:: 
* Attribute Simple_Storage_Pool:: 
* Attribute Small:: 
* Attribute Storage_Unit:: 
* Attribute Stub_Type:: 
* Attribute System_Allocator_Alignment:: 
* Attribute Target_Name:: 
* Attribute To_Address:: 
* Attribute To_Any:: 
* Attribute Type_Class:: 
* Attribute Type_Key:: 
* Attribute TypeCode:: 
* Attribute UET_Address:: 
* Attribute Unconstrained_Array:: 
* Attribute Universal_Literal_String:: 
* Attribute Unrestricted_Access:: 
* Attribute Update:: 
* Attribute Valid_Scalars:: 
* Attribute VADS_Size:: 
* Attribute Value_Size:: 
* Attribute Wchar_T_Size:: 
* Attribute Word_Size:: 

@end menu

@node Attribute Abort_Signal,Attribute Address_Size,,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-abort-signal}@anchor{117}
@section Attribute Abort_Signal


@geindex Abort_Signal

@cite{Standard'Abort_Signal} (@cite{Standard} is the only allowed
prefix) provides the entity for the special exception used to signal
task abort or asynchronous transfer of control.  Normally this attribute
should only be used in the tasking runtime (it is highly peculiar, and
completely outside the normal semantics of Ada, for a user program to
intercept the abort exception).

@node Attribute Address_Size,Attribute Asm_Input,Attribute Abort_Signal,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-address-size}@anchor{118}
@section Attribute Address_Size


@geindex Size of `Address`

@geindex Address_Size

@cite{Standard'Address_Size} (@cite{Standard} is the only allowed
prefix) is a static constant giving the number of bits in an
@cite{Address}. It is the same value as System.Address'Size,
but has the advantage of being static, while a direct
reference to System.Address'Size is non-static because Address
is a private type.

@node Attribute Asm_Input,Attribute Asm_Output,Attribute Address_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-asm-input}@anchor{119}
@section Attribute Asm_Input


@geindex Asm_Input

The @cite{Asm_Input} attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix.  The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g., what kind of register is required).  The second argument is the
value to be used as the input argument.  The possible values for the
constant are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
@ref{11a,,Machine Code Insertions}

@node Attribute Asm_Output,Attribute Atomic_Always_Lock_Free,Attribute Asm_Input,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-asm-output}@anchor{11b}
@section Attribute Asm_Output


@geindex Asm_Output

The @cite{Asm_Output} attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix.  The first (string)
argument is required to be a static expression and designates the
constraint for the parameter (e.g., what kind of register is
required).  The second argument is the variable to be updated with the
result.  The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end.  If there are no output operands, then this argument may
either be omitted, or explicitly given as @cite{No_Output_Operands}.
@ref{11a,,Machine Code Insertions}

@node Attribute Atomic_Always_Lock_Free,Attribute Bit,Attribute Asm_Output,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-atomic-always-lock-free}@anchor{11c}
@section Attribute Atomic_Always_Lock_Free


@geindex Atomic_Always_Lock_Free

The prefix of the @cite{Atomic_Always_Lock_Free} attribute is a type.
The result is a Boolean value which is True if the type has discriminants,
and False otherwise.  The result indicate whether atomic operations are
supported by the target for the given type.

@node Attribute Bit,Attribute Bit_Position,Attribute Atomic_Always_Lock_Free,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-bit}@anchor{11d}
@section Attribute Bit


@geindex Bit

@code{obj'Bit}, where @cite{obj} is any object, yields the bit
offset within the storage unit (byte) that contains the first bit of
storage allocated for the object.  The value of this attribute is of the
type @cite{Universal_Integer}, and is always a non-negative number not
exceeding the value of @cite{System.Storage_Unit}.

For an object that is a variable or a constant allocated in a register,
the value is zero.  (The use of this attribute does not force the
allocation of a variable to memory).

For an object that is a formal parameter, this attribute applies
to either the matching actual parameter or to a copy of the
matching actual parameter.

For an access object the value is zero.  Note that
@code{obj.all'Bit} is subject to an @cite{Access_Check} for the
designated object.  Similarly for a record component
@code{X.C'Bit} is subject to a discriminant check and
@code{X(I).Bit} and @code{X(I1..I2)'Bit}
are subject to index checks.

This attribute is designed to be compatible with the DEC Ada 83 definition
and implementation of the @cite{Bit} attribute.

@node Attribute Bit_Position,Attribute Code_Address,Attribute Bit,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-bit-position}@anchor{11e}
@section Attribute Bit_Position


@geindex Bit_Position

@code{R.C'Bit_Position}, where @cite{R} is a record object and @cite{C} is one
of the fields of the record type, yields the bit
offset within the record contains the first bit of
storage allocated for the object.  The value of this attribute is of the
type @cite{Universal_Integer}.  The value depends only on the field
@cite{C} and is independent of the alignment of
the containing record @cite{R}.

@node Attribute Code_Address,Attribute Compiler_Version,Attribute Bit_Position,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-code-address}@anchor{11f}
@section Attribute Code_Address


@geindex Code_Address

@geindex Subprogram address

@geindex Address of subprogram code

The @cite{'Address}
attribute may be applied to subprograms in Ada 95 and Ada 2005, but the
intended effect seems to be to provide
an address value which can be used to call the subprogram by means of
an address clause as in the following example:

@example
procedure K is ...

procedure L;
for L'Address use K'Address;
pragma Import (Ada, L);
@end example

A call to @cite{L} is then expected to result in a call to @cite{K}.
In Ada 83, where there were no access-to-subprogram values, this was
a common work-around for getting the effect of an indirect call.
GNAT implements the above use of @cite{Address} and the technique
illustrated by the example code works correctly.

However, for some purposes, it is useful to have the address of the start
of the generated code for the subprogram.  On some architectures, this is
not necessarily the same as the @cite{Address} value described above.
For example, the @cite{Address} value may reference a subprogram
descriptor rather than the subprogram itself.

The @cite{'Code_Address} attribute, which can only be applied to
subprogram entities, always returns the address of the start of the
generated code of the specified subprogram, which may or may not be
the same value as is returned by the corresponding @cite{'Address}
attribute.

@node Attribute Compiler_Version,Attribute Constrained,Attribute Code_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-compiler-version}@anchor{120}
@section Attribute Compiler_Version


@geindex Compiler_Version

@cite{Standard'Compiler_Version} (@cite{Standard} is the only allowed
prefix) yields a static string identifying the version of the compiler
being used to compile the unit containing the attribute reference.

@node Attribute Constrained,Attribute Default_Bit_Order,Attribute Compiler_Version,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-constrained}@anchor{121}
@section Attribute Constrained


@geindex Constrained

In addition to the usage of this attribute in the Ada RM, @cite{GNAT}
also permits the use of the @cite{'Constrained} attribute
in a generic template
for any type, including types without discriminants. The value of this
attribute in the generic instance when applied to a scalar type or a
record type without discriminants is always @cite{True}. This usage is
compatible with older Ada compilers, including notably DEC Ada.

@node Attribute Default_Bit_Order,Attribute Default_Scalar_Storage_Order,Attribute Constrained,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-default-bit-order}@anchor{122}
@section Attribute Default_Bit_Order


@geindex Big endian

@geindex Little endian

@geindex Default_Bit_Order

@cite{Standard'Default_Bit_Order} (@cite{Standard} is the only
permissible prefix), provides the value @cite{System.Default_Bit_Order}
as a @cite{Pos} value (0 for @cite{High_Order_First}, 1 for
@cite{Low_Order_First}).  This is used to construct the definition of
@cite{Default_Bit_Order} in package @cite{System}.

@node Attribute Default_Scalar_Storage_Order,Attribute Deref,Attribute Default_Bit_Order,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-default-scalar-storage-order}@anchor{123}
@section Attribute Default_Scalar_Storage_Order


@geindex Big endian

@geindex Little endian

@geindex Default_Scalar_Storage_Order

@cite{Standard'Default_Scalar_Storage_Order} (@cite{Standard} is the only
permissible prefix), provides the current value of the default scalar storage
order (as specified using pragma @cite{Default_Scalar_Storage_Order}, or
equal to @cite{Default_Bit_Order} if unspecified) as a
@cite{System.Bit_Order} value. This is a static attribute.

@node Attribute Deref,Attribute Descriptor_Size,Attribute Default_Scalar_Storage_Order,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-deref}@anchor{124}
@section Attribute Deref


@geindex Deref

The attribute @cite{typ'Deref(expr)} where @cite{expr} is of type @cite{System.Address} yields
the variable of type @cite{typ} that is located at the given address. It is similar
to @cite{(totyp (expr).all)}, where @cite{totyp} is an unchecked conversion from address to
a named access-to-@cite{typ} type, except that it yields a variable, so it can be
used on the left side of an assignment.

@node Attribute Descriptor_Size,Attribute Elaborated,Attribute Deref,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-descriptor-size}@anchor{125}
@section Attribute Descriptor_Size


@geindex Descriptor

@geindex Dope vector

@geindex Descriptor_Size

Non-static attribute @cite{Descriptor_Size} returns the size in bits of the
descriptor allocated for a type.  The result is non-zero only for unconstrained
array types and the returned value is of type universal integer.  In GNAT, an
array descriptor contains bounds information and is located immediately before
the first element of the array.

@example
type Unconstr_Array is array (Positive range <>) of Boolean;
Put_Line ("Descriptor size = " & Unconstr_Array'Descriptor_Size'Img);
@end example

The attribute takes into account any additional padding due to type alignment.
In the example above, the descriptor contains two values of type
@cite{Positive} representing the low and high bound.  Since @cite{Positive} has
a size of 31 bits and an alignment of 4, the descriptor size is @cite{2 * Positive'Size + 2} or 64 bits.

@node Attribute Elaborated,Attribute Elab_Body,Attribute Descriptor_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elaborated}@anchor{126}
@section Attribute Elaborated


@geindex Elaborated

The prefix of the @cite{'Elaborated} attribute must be a unit name.  The
value is a Boolean which indicates whether or not the given unit has been
elaborated.  This attribute is primarily intended for internal use by the
generated code for dynamic elaboration checking, but it can also be used
in user programs.  The value will always be True once elaboration of all
units has been completed.  An exception is for units which need no
elaboration, the value is always False for such units.

@node Attribute Elab_Body,Attribute Elab_Spec,Attribute Elaborated,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elab-body}@anchor{127}
@section Attribute Elab_Body


@geindex Elab_Body

This attribute can only be applied to a program unit name.  It returns
the entity for the corresponding elaboration procedure for elaborating
the body of the referenced unit.  This is used in the main generated
elaboration procedure by the binder and is not normally used in any
other context.  However, there may be specialized situations in which it
is useful to be able to call this elaboration procedure from Ada code,
e.g., if it is necessary to do selective re-elaboration to fix some
error.

@node Attribute Elab_Spec,Attribute Elab_Subp_Body,Attribute Elab_Body,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elab-spec}@anchor{128}
@section Attribute Elab_Spec


@geindex Elab_Spec

This attribute can only be applied to a program unit name.  It returns
the entity for the corresponding elaboration procedure for elaborating
the spec of the referenced unit.  This is used in the main
generated elaboration procedure by the binder and is not normally used
in any other context.  However, there may be specialized situations in
which it is useful to be able to call this elaboration procedure from
Ada code, e.g., if it is necessary to do selective re-elaboration to fix
some error.

@node Attribute Elab_Subp_Body,Attribute Emax,Attribute Elab_Spec,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-elab-subp-body}@anchor{129}
@section Attribute Elab_Subp_Body


@geindex Elab_Subp_Body

This attribute can only be applied to a library level subprogram
name and is only allowed in CodePeer mode. It returns the entity
for the corresponding elaboration procedure for elaborating the body
of the referenced subprogram unit. This is used in the main generated
elaboration procedure by the binder in CodePeer mode only and is unrecognized
otherwise.

@node Attribute Emax,Attribute Enabled,Attribute Elab_Subp_Body,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-emax}@anchor{12a}
@section Attribute Emax


@geindex Ada 83 attributes

@geindex Emax

The @cite{Emax} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Enabled,Attribute Enum_Rep,Attribute Emax,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-enabled}@anchor{12b}
@section Attribute Enabled


@geindex Enabled

The @cite{Enabled} attribute allows an application program to check at compile
time to see if the designated check is currently enabled. The prefix is a
simple identifier, referencing any predefined check name (other than
@cite{All_Checks}) or a check name introduced by pragma Check_Name. If
no argument is given for the attribute, the check is for the general state
of the check, if an argument is given, then it is an entity name, and the
check indicates whether an @cite{Suppress} or @cite{Unsuppress} has been
given naming the entity (if not, then the argument is ignored).

Note that instantiations inherit the check status at the point of the
instantiation, so a useful idiom is to have a library package that
introduces a check name with @cite{pragma Check_Name}, and then contains
generic packages or subprograms which use the @cite{Enabled} attribute
to see if the check is enabled. A user of this package can then issue
a @cite{pragma Suppress} or @cite{pragma Unsuppress} before instantiating
the package or subprogram, controlling whether the check will be present.

@node Attribute Enum_Rep,Attribute Enum_Val,Attribute Enabled,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-enum-rep}@anchor{12c}
@section Attribute Enum_Rep


@geindex Representation of enums

@geindex Enum_Rep

For every enumeration subtype @cite{S}, @code{S'Enum_Rep} denotes a
function with the following spec:

@example
function S'Enum_Rep (Arg : S'Base) return <Universal_Integer>;
@end example

It is also allowable to apply @cite{Enum_Rep} directly to an object of an
enumeration type or to a non-overloaded enumeration
literal.  In this case @code{S'Enum_Rep} is equivalent to
@code{typ'Enum_Rep(S)} where @cite{typ} is the type of the
enumeration literal or object.

The function returns the representation value for the given enumeration
value.  This will be equal to value of the @cite{Pos} attribute in the
absence of an enumeration representation clause.  This is a static
attribute (i.e.,:the result is static if the argument is static).

@code{S'Enum_Rep} can also be used with integer types and objects,
in which case it simply returns the integer value.  The reason for this
is to allow it to be used for @cite{(<>)} discrete formal arguments in
a generic unit that can be instantiated with either enumeration types
or integer types.  Note that if @cite{Enum_Rep} is used on a modular
type whose upper bound exceeds the upper bound of the largest signed
integer type, and the argument is a variable, so that the universal
integer calculation is done at run time, then the call to @cite{Enum_Rep}
may raise @cite{Constraint_Error}.

@node Attribute Enum_Val,Attribute Epsilon,Attribute Enum_Rep,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-enum-val}@anchor{12d}
@section Attribute Enum_Val


@geindex Representation of enums

@geindex Enum_Val

For every enumeration subtype @cite{S}, @code{S'Enum_Val} denotes a
function with the following spec:

@example
function S'Enum_Val (Arg : <Universal_Integer>) return S'Base;
@end example

The function returns the enumeration value whose representation matches the
argument, or raises Constraint_Error if no enumeration literal of the type
has the matching value.
This will be equal to value of the @cite{Val} attribute in the
absence of an enumeration representation clause.  This is a static
attribute (i.e., the result is static if the argument is static).

@node Attribute Epsilon,Attribute Fast_Math,Attribute Enum_Val,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-epsilon}@anchor{12e}
@section Attribute Epsilon


@geindex Ada 83 attributes

@geindex Epsilon

The @cite{Epsilon} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Fast_Math,Attribute Fixed_Value,Attribute Epsilon,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-fast-math}@anchor{12f}
@section Attribute Fast_Math


@geindex Fast_Math

@cite{Standard'Fast_Math} (@cite{Standard} is the only allowed
prefix) yields a static Boolean value that is True if pragma
@cite{Fast_Math} is active, and False otherwise.

@node Attribute Fixed_Value,Attribute From_Any,Attribute Fast_Math,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-fixed-value}@anchor{130}
@section Attribute Fixed_Value


@geindex Fixed_Value

For every fixed-point type @cite{S}, @code{S'Fixed_Value} denotes a
function with the following specification:

@example
function S'Fixed_Value (Arg : <Universal_Integer>) return S;
@end example

The value returned is the fixed-point value @cite{V} such that:

@example
V = Arg * S'Small
@end example

The effect is thus similar to first converting the argument to the
integer type used to represent @cite{S}, and then doing an unchecked
conversion to the fixed-point type.  The difference is
that there are full range checks, to ensure that the result is in range.
This attribute is primarily intended for use in implementation of the
input-output functions for fixed-point values.

@node Attribute From_Any,Attribute Has_Access_Values,Attribute Fixed_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-from-any}@anchor{131}
@section Attribute From_Any


@geindex From_Any

This internal attribute is used for the generation of remote subprogram
stubs in the context of the Distributed Systems Annex.

@node Attribute Has_Access_Values,Attribute Has_Discriminants,Attribute From_Any,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-has-access-values}@anchor{132}
@section Attribute Has_Access_Values


@geindex Access values
@geindex testing for

@geindex Has_Access_Values

The prefix of the @cite{Has_Access_Values} attribute is a type.  The result
is a Boolean value which is True if the is an access type, or is a composite
type with a component (at any nesting depth) that is an access type, and is
False otherwise.
The intended use of this attribute is in conjunction with generic
definitions.  If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has access values.

@node Attribute Has_Discriminants,Attribute Img,Attribute Has_Access_Values,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-has-discriminants}@anchor{133}
@section Attribute Has_Discriminants


@geindex Discriminants
@geindex testing for

@geindex Has_Discriminants

The prefix of the @cite{Has_Discriminants} attribute is a type.  The result
is a Boolean value which is True if the type has discriminants, and False
otherwise.  The intended use of this attribute is in conjunction with generic
definitions.  If the attribute is applied to a generic private type, it
indicates whether or not the corresponding actual type has discriminants.

@node Attribute Img,Attribute Integer_Value,Attribute Has_Discriminants,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-img}@anchor{134}
@section Attribute Img


@geindex Img

The @cite{Img} attribute differs from @cite{Image} in that it is applied
directly to an object, and yields the same result as
@cite{Image} for the subtype of the object.  This is convenient for
debugging:

@example
Put_Line ("X = " & X'Img);
@end example

has the same meaning as the more verbose:

@example
Put_Line ("X = " & T'Image (X));
@end example

where @cite{T} is the (sub)type of the object @cite{X}.

Note that technically, in analogy to @cite{Image},
@cite{X'Img} returns a parameterless function
that returns the appropriate string when called. This means that
@cite{X'Img} can be renamed as a function-returning-string, or used
in an instantiation as a function parameter.

@node Attribute Integer_Value,Attribute Invalid_Value,Attribute Img,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-integer-value}@anchor{135}
@section Attribute Integer_Value


@geindex Integer_Value

For every integer type @cite{S}, @code{S'Integer_Value} denotes a
function with the following spec:

@example
function S'Integer_Value (Arg : <Universal_Fixed>) return S;
@end example

The value returned is the integer value @cite{V}, such that:

@example
Arg = V * T'Small
@end example

where @cite{T} is the type of @cite{Arg}.
The effect is thus similar to first doing an unchecked conversion from
the fixed-point type to its corresponding implementation type, and then
converting the result to the target integer type.  The difference is
that there are full range checks, to ensure that the result is in range.
This attribute is primarily intended for use in implementation of the
standard input-output functions for fixed-point values.

@node Attribute Invalid_Value,Attribute Iterable,Attribute Integer_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-invalid-value}@anchor{136}
@section Attribute Invalid_Value


@geindex Invalid_Value

For every scalar type S, S'Invalid_Value returns an undefined value of the
type. If possible this value is an invalid representation for the type. The
value returned is identical to the value used to initialize an otherwise
uninitialized value of the type if pragma Initialize_Scalars is used,
including the ability to modify the value with the binder -Sxx flag and
relevant environment variables at run time.

@node Attribute Iterable,Attribute Large,Attribute Invalid_Value,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-iterable}@anchor{137}
@section Attribute Iterable


@geindex Iterable

Equivalent to Aspect Iterable.

@node Attribute Large,Attribute Library_Level,Attribute Iterable,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-large}@anchor{138}
@section Attribute Large


@geindex Ada 83 attributes

@geindex Large

The @cite{Large} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Library_Level,Attribute Lock_Free,Attribute Large,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-library-level}@anchor{139}
@section Attribute Library_Level


@geindex Library_Level

@cite{P'Library_Level}, where P is an entity name,
returns a Boolean value which is True if the entity is declared
at the library level, and False otherwise. Note that within a
generic instantition, the name of the generic unit denotes the
instance, which means that this attribute can be used to test
if a generic is instantiated at the library level, as shown
in this example:

@example
generic
  ...
package Gen is
  pragma Compile_Time_Error
    (not Gen'Library_Level,
     "Gen can only be instantiated at library level");
  ...
end Gen;
@end example

@node Attribute Lock_Free,Attribute Loop_Entry,Attribute Library_Level,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-lock-free}@anchor{13a}
@section Attribute Lock_Free


@geindex Lock_Free

@cite{P'Lock_Free}, where P is a protected object, returns True if a
pragma @cite{Lock_Free} applies to P.

@node Attribute Loop_Entry,Attribute Machine_Size,Attribute Lock_Free,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-loop-entry}@anchor{13b}
@section Attribute Loop_Entry


@geindex Loop_Entry

Syntax:

@example
X'Loop_Entry [(loop_name)]
@end example

The @cite{Loop_Entry} attribute is used to refer to the value that an
expression had upon entry to a given loop in much the same way that the
@cite{Old} attribute in a subprogram postcondition can be used to refer
to the value an expression had upon entry to the subprogram. The
relevant loop is either identified by the given loop name, or it is the
innermost enclosing loop when no loop name is given.

A @cite{Loop_Entry} attribute can only occur within a
@cite{Loop_Variant} or @cite{Loop_Invariant} pragma. A common use of
@cite{Loop_Entry} is to compare the current value of objects with their
initial value at loop entry, in a @cite{Loop_Invariant} pragma.

The effect of using @cite{X'Loop_Entry} is the same as declaring
a constant initialized with the initial value of @cite{X} at loop
entry. This copy is not performed if the loop is not entered, or if the
corresponding pragmas are ignored or disabled.

@node Attribute Machine_Size,Attribute Mantissa,Attribute Loop_Entry,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-machine-size}@anchor{13c}
@section Attribute Machine_Size


@geindex Machine_Size

This attribute is identical to the @cite{Object_Size} attribute.  It is
provided for compatibility with the DEC Ada 83 attribute of this name.

@node Attribute Mantissa,Attribute Maximum_Alignment,Attribute Machine_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-mantissa}@anchor{13d}
@section Attribute Mantissa


@geindex Ada 83 attributes

@geindex Mantissa

The @cite{Mantissa} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Maximum_Alignment,Attribute Mechanism_Code,Attribute Mantissa,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-maximum-alignment}@anchor{13e}@anchor{gnat_rm/implementation_defined_attributes id2}@anchor{13f}
@section Attribute Maximum_Alignment


@geindex Alignment
@geindex maximum

@geindex Maximum_Alignment

@cite{Standard'Maximum_Alignment} (@cite{Standard} is the only
permissible prefix) provides the maximum useful alignment value for the
target.  This is a static value that can be used to specify the alignment
for an object, guaranteeing that it is properly aligned in all
cases.

@node Attribute Mechanism_Code,Attribute Null_Parameter,Attribute Maximum_Alignment,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-mechanism-code}@anchor{140}
@section Attribute Mechanism_Code


@geindex Return values
@geindex passing mechanism

@geindex Parameters
@geindex passing mechanism

@geindex Mechanism_Code

@code{function'Mechanism_Code} yields an integer code for the
mechanism used for the result of function, and
@code{subprogram'Mechanism_Code (n)} yields the mechanism
used for formal parameter number @cite{n} (a static integer value with 1
meaning the first parameter) of @cite{subprogram}.  The code returned is:


@table @asis

@item @emph{1}

by copy (value)

@item @emph{2}

by reference
@end table

@node Attribute Null_Parameter,Attribute Object_Size,Attribute Mechanism_Code,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-null-parameter}@anchor{141}
@section Attribute Null_Parameter


@geindex Zero address
@geindex passing

@geindex Null_Parameter

A reference @code{T'Null_Parameter} denotes an imaginary object of
type or subtype @cite{T} allocated at machine address zero.  The attribute
is allowed only as the default expression of a formal parameter, or as
an actual expression of a subprogram call.  In either case, the
subprogram must be imported.

The identity of the object is represented by the address zero in the
argument list, independent of the passing mechanism (explicit or
default).

This capability is needed to specify that a zero address should be
passed for a record or other composite object passed by reference.
There is no way of indicating this without the @cite{Null_Parameter}
attribute.

@node Attribute Object_Size,Attribute Old,Attribute Null_Parameter,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-object-size}@anchor{142}
@section Attribute Object_Size


@geindex Size
@geindex used for objects

@geindex Object_Size

The size of an object is not necessarily the same as the size of the type
of an object.  This is because by default object sizes are increased to be
a multiple of the alignment of the object.  For example,
@cite{Natural'Size} is
31, but by default objects of type @cite{Natural} will have a size of 32 bits.
Similarly, a record containing an integer and a character:

@example
type Rec is record
   I : Integer;
   C : Character;
end record;
@end example

will have a size of 40 (that is @cite{Rec'Size} will be 40).  The
alignment will be 4, because of the
integer field, and so the default size of record objects for this type
will be 64 (8 bytes).

If the alignment of the above record is specified to be 1, then the
object size will be 40 (5 bytes). This is true by default, and also
an object size of 40 can be explicitly specified in this case.

A consequence of this capability is that different object sizes can be
given to subtypes that would otherwise be considered in Ada to be
statically matching.  But it makes no sense to consider such subtypes
as statically matching.  Consequently, in @cite{GNAT} we add a rule
to the static matching rules that requires object sizes to match.
Consider this example:

@example
 1. procedure BadAVConvert is
 2.    type R is new Integer;
 3.    subtype R1 is R range 1 .. 10;
 4.    subtype R2 is R range 1 .. 10;
 5.    for R1'Object_Size use 8;
 6.    for R2'Object_Size use 16;
 7.    type R1P is access all R1;
 8.    type R2P is access all R2;
 9.    R1PV : R1P := new R1'(4);
10.    R2PV : R2P;
11. begin
12.    R2PV := R2P (R1PV);
               |
       >>> target designated subtype not compatible with
           type "R1" defined at line 3

13. end;
@end example

In the absence of lines 5 and 6,
types @cite{R1} and @cite{R2} statically match and
hence the conversion on line 12 is legal. But since lines 5 and 6
cause the object sizes to differ, @cite{GNAT} considers that types
@cite{R1} and @cite{R2} are not statically matching, and line 12
generates the diagnostic shown above.

Similar additional checks are performed in other contexts requiring
statically matching subtypes.

@node Attribute Old,Attribute Passed_By_Reference,Attribute Object_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-old}@anchor{143}
@section Attribute Old


@geindex Old

In addition to the usage of @cite{Old} defined in the Ada 2012 RM (usage
within @cite{Post} aspect), GNAT also permits the use of this attribute
in implementation defined pragmas @cite{Postcondition},
@cite{Contract_Cases} and @cite{Test_Case}. Also usages of
@cite{Old} which would be illegal according to the Ada 2012 RM
definition are allowed under control of
implementation defined pragma @cite{Unevaluated_Use_Of_Old}.

@node Attribute Passed_By_Reference,Attribute Pool_Address,Attribute Old,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-passed-by-reference}@anchor{144}
@section Attribute Passed_By_Reference


@geindex Parameters
@geindex when passed by reference

@geindex Passed_By_Reference

@code{type'Passed_By_Reference} for any subtype @cite{type} returns
a value of type @cite{Boolean} value that is @cite{True} if the type is
normally passed by reference and @cite{False} if the type is normally
passed by copy in calls.  For scalar types, the result is always @cite{False}
and is static.  For non-scalar types, the result is non-static.

@node Attribute Pool_Address,Attribute Range_Length,Attribute Passed_By_Reference,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-pool-address}@anchor{145}
@section Attribute Pool_Address


@geindex Parameters
@geindex when passed by reference

@geindex Pool_Address

@code{X'Pool_Address} for any object @cite{X} returns the address
of X within its storage pool. This is the same as
@code{X'Address}, except that for an unconstrained array whose
bounds are allocated just before the first component,
@code{X'Pool_Address} returns the address of those bounds,
whereas @code{X'Address} returns the address of the first
component.

Here, we are interpreting 'storage pool' broadly to mean
@code{wherever the object is allocated}, which could be a
user-defined storage pool,
the global heap, on the stack, or in a static memory area.
For an object created by @cite{new}, @code{Ptr.all'Pool_Address} is
what is passed to @cite{Allocate} and returned from @cite{Deallocate}.

@node Attribute Range_Length,Attribute Ref,Attribute Pool_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-range-length}@anchor{146}
@section Attribute Range_Length


@geindex Range_Length

@code{type'Range_Length} for any discrete type @cite{type} yields
the number of values represented by the subtype (zero for a null
range).  The result is static for static subtypes.  @cite{Range_Length}
applied to the index subtype of a one dimensional array always gives the
same result as @cite{Length} applied to the array itself.

@node Attribute Ref,Attribute Restriction_Set,Attribute Range_Length,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-ref}@anchor{147}
@section Attribute Ref


@geindex Ref

@node Attribute Restriction_Set,Attribute Result,Attribute Ref,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-restriction-set}@anchor{148}
@section Attribute Restriction_Set


@geindex Restriction_Set

@geindex Restrictions

This attribute allows compile time testing of restrictions that
are currently in effect. It is primarily intended for specializing
code in the run-time based on restrictions that are active (e.g.
don't need to save fpt registers if restriction No_Floating_Point
is known to be in effect), but can be used anywhere.

There are two forms:

@example
System'Restriction_Set (partition_boolean_restriction_NAME)
System'Restriction_Set (No_Dependence => library_unit_NAME);
@end example

In the case of the first form, the only restriction names
allowed are parameterless restrictions that are checked
for consistency at bind time. For a complete list see the
subtype @cite{System.Rident.Partition_Boolean_Restrictions}.

The result returned is True if the restriction is known to
be in effect, and False if the restriction is known not to
be in effect. An important guarantee is that the value of
a Restriction_Set attribute is known to be consistent throughout
all the code of a partition.

This is trivially achieved if the entire partition is compiled
with a consistent set of restriction pragmas. However, the
compilation model does not require this. It is possible to
compile one set of units with one set of pragmas, and another
set of units with another set of pragmas. It is even possible
to compile a spec with one set of pragmas, and then WITH the
same spec with a different set of pragmas. Inconsistencies
in the actual use of the restriction are checked at bind time.

In order to achieve the guarantee of consistency for the
Restriction_Set pragma, we consider that a use of the pragma
that yields False is equivalent to a violation of the
restriction.

So for example if you write

@example
if System'Restriction_Set (No_Floating_Point) then
   ...
else
   ...
end if;
@end example

And the result is False, so that the else branch is executed,
you can assume that this restriction is not set for any unit
in the partition. This is checked by considering this use of
the restriction pragma to be a violation of the restriction
No_Floating_Point. This means that no other unit can attempt
to set this restriction (if some unit does attempt to set it,
the binder will refuse to bind the partition).

Technical note: The restriction name and the unit name are
intepreted entirely syntactically, as in the corresponding
Restrictions pragma, they are not analyzed semantically,
so they do not have a type.

@node Attribute Result,Attribute Safe_Emax,Attribute Restriction_Set,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-result}@anchor{149}
@section Attribute Result


@geindex Result

@code{function'Result} can only be used with in a Postcondition pragma
for a function. The prefix must be the name of the corresponding function. This
is used to refer to the result of the function in the postcondition expression.
For a further discussion of the use of this attribute and examples of its use,
see the description of pragma Postcondition.

@node Attribute Safe_Emax,Attribute Safe_Large,Attribute Result,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-safe-emax}@anchor{14a}
@section Attribute Safe_Emax


@geindex Ada 83 attributes

@geindex Safe_Emax

The @cite{Safe_Emax} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Safe_Large,Attribute Safe_Small,Attribute Safe_Emax,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-safe-large}@anchor{14b}
@section Attribute Safe_Large


@geindex Ada 83 attributes

@geindex Safe_Large

The @cite{Safe_Large} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Safe_Small,Attribute Scalar_Storage_Order,Attribute Safe_Large,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-safe-small}@anchor{14c}
@section Attribute Safe_Small


@geindex Ada 83 attributes

@geindex Safe_Small

The @cite{Safe_Small} attribute is provided for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute.

@node Attribute Scalar_Storage_Order,Attribute Simple_Storage_Pool,Attribute Safe_Small,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-scalar-storage-order}@anchor{14d}
@section Attribute Scalar_Storage_Order


@geindex Endianness

@geindex Scalar storage order

@geindex Scalar_Storage_Order

For every array or record type @cite{S}, the representation attribute
@cite{Scalar_Storage_Order} denotes the order in which storage elements
that make up scalar components are ordered within S. The value given must
be a static expression of type System.Bit_Order. The following is an example
of the use of this feature:

@example
--  Component type definitions

subtype Yr_Type is Natural range 0 .. 127;
subtype Mo_Type is Natural range 1 .. 12;
subtype Da_Type is Natural range 1 .. 31;

--  Record declaration

type Date is record
   Years_Since_1980 : Yr_Type;
   Month            : Mo_Type;
   Day_Of_Month     : Da_Type;
end record;

--  Record representation clause

for Date use record
   Years_Since_1980 at 0 range 0  ..  6;
   Month            at 0 range 7  .. 10;
   Day_Of_Month     at 0 range 11 .. 15;
end record;

--  Attribute definition clauses

for Date'Bit_Order use System.High_Order_First;
for Date'Scalar_Storage_Order use System.High_Order_First;
--  If Scalar_Storage_Order is specified, it must be consistent with
--  Bit_Order, so it's best to always define the latter explicitly if
--  the former is used.
@end example

Other properties are as for standard representation attribute @cite{Bit_Order},
as defined by Ada RM 13.5.3(4). The default is @cite{System.Default_Bit_Order}.

For a record type @cite{T}, if @code{T'Scalar_Storage_Order} is
specified explicitly, it shall be equal to @code{T'Bit_Order}. Note:
this means that if a @cite{Scalar_Storage_Order} attribute definition
clause is not confirming, then the type's @cite{Bit_Order} shall be
specified explicitly and set to the same value.

Derived types inherit an explicitly set scalar storage order from their parent
types. This may be overridden for the derived type by giving an explicit scalar
storage order for the derived type. For a record extension, the derived type
must have the same scalar storage order as the parent type.

If a component of @cite{T} is of a record or array type, then that type must
also have a @cite{Scalar_Storage_Order} attribute definition clause.

A component of a record or array type that is a packed array, or that
does not start on a byte boundary, must have the same scalar storage order
as the enclosing record or array type.

No component of a type that has an explicit @cite{Scalar_Storage_Order}
attribute definition may be aliased.

A confirming @cite{Scalar_Storage_Order} attribute definition clause (i.e.
with a value equal to @cite{System.Default_Bit_Order}) has no effect.

If the opposite storage order is specified, then whenever the value of
a scalar component of an object of type @cite{S} is read, the storage
elements of the enclosing machine scalar are first reversed (before
retrieving the component value, possibly applying some shift and mask
operatings on the enclosing machine scalar), and the opposite operation
is done for writes.

In that case, the restrictions set forth in 13.5.1(10.3/2) for scalar components
are relaxed. Instead, the following rules apply:


@itemize *

@item 
the underlying storage elements are those at positions
@cite{(position + first_bit / storage_element_size) .. (position + (last_bit + storage_element_size - 1) / storage_element_size)}

@item 
the sequence of underlying storage elements shall have
a size no greater than the largest machine scalar

@item 
the enclosing machine scalar is defined as the smallest machine
scalar starting at a position no greater than
@cite{position + first_bit / storage_element_size} and covering
storage elements at least up to @cite{position + (last_bit + storage_element_size - 1) / storage_element_size}

@item 
the position of the component is interpreted relative to that machine
scalar.
@end itemize

If no scalar storage order is specified for a type (either directly, or by
inheritance in the case of a derived type), then the default is normally
the native ordering of the target, but this default can be overridden using
pragma @cite{Default_Scalar_Storage_Order}.

Note that the scalar storage order only affects the in-memory data
representation. It has no effect on the representation used by stream
attributes.

@node Attribute Simple_Storage_Pool,Attribute Small,Attribute Scalar_Storage_Order,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-simple-storage-pool}@anchor{b7}@anchor{gnat_rm/implementation_defined_attributes id3}@anchor{14e}
@section Attribute Simple_Storage_Pool


@geindex Storage pool
@geindex simple

@geindex Simple storage pool

@geindex Simple_Storage_Pool

For every nonformal, nonderived access-to-object type @cite{Acc}, the
representation attribute @cite{Simple_Storage_Pool} may be specified
via an attribute_definition_clause (or by specifying the equivalent aspect):

@example
My_Pool : My_Simple_Storage_Pool_Type;

type Acc is access My_Data_Type;

for Acc'Simple_Storage_Pool use My_Pool;
@end example

The name given in an attribute_definition_clause for the
@cite{Simple_Storage_Pool} attribute shall denote a variable of
a 'simple storage pool type' (see pragma @cite{Simple_Storage_Pool_Type}).

The use of this attribute is only allowed for a prefix denoting a type
for which it has been specified. The type of the attribute is the type
of the variable specified as the simple storage pool of the access type,
and the attribute denotes that variable.

It is illegal to specify both @cite{Storage_Pool} and @cite{Simple_Storage_Pool}
for the same access type.

If the @cite{Simple_Storage_Pool} attribute has been specified for an access
type, then applying the @cite{Storage_Pool} attribute to the type is flagged
with a warning and its evaluation raises the exception @cite{Program_Error}.

If the Simple_Storage_Pool attribute has been specified for an access
type @cite{S}, then the evaluation of the attribute @code{S'Storage_Size}
returns the result of calling @code{Storage_Size (S'Simple_Storage_Pool)},
which is intended to indicate the number of storage elements reserved for
the simple storage pool. If the Storage_Size function has not been defined
for the simple storage pool type, then this attribute returns zero.

If an access type @cite{S} has a specified simple storage pool of type
@cite{SSP}, then the evaluation of an allocator for that access type calls
the primitive @cite{Allocate} procedure for type @cite{SSP}, passing
@code{S'Simple_Storage_Pool} as the pool parameter. The detailed
semantics of such allocators is the same as those defined for allocators
in section 13.11 of the @cite{Ada Reference Manual}, with the term
@cite{simple storage pool} substituted for @cite{storage pool}.

If an access type @cite{S} has a specified simple storage pool of type
@cite{SSP}, then a call to an instance of the @cite{Ada.Unchecked_Deallocation}
for that access type invokes the primitive @cite{Deallocate} procedure
for type @cite{SSP}, passing @code{S'Simple_Storage_Pool} as the pool
parameter. The detailed semantics of such unchecked deallocations is the same
as defined in section 13.11.2 of the Ada Reference Manual, except that the
term 'simple storage pool' is substituted for 'storage pool'.

@node Attribute Small,Attribute Storage_Unit,Attribute Simple_Storage_Pool,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-small}@anchor{14f}
@section Attribute Small


@geindex Ada 83 attributes

@geindex Small

The @cite{Small} attribute is defined in Ada 95 (and Ada 2005) only for
fixed-point types.
GNAT also allows this attribute to be applied to floating-point types
for compatibility with Ada 83.  See
the Ada 83 reference manual for an exact description of the semantics of
this attribute when applied to floating-point types.

@node Attribute Storage_Unit,Attribute Stub_Type,Attribute Small,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-storage-unit}@anchor{150}
@section Attribute Storage_Unit


@geindex Storage_Unit

@cite{Standard'Storage_Unit} (@cite{Standard} is the only permissible
prefix) provides the same value as @cite{System.Storage_Unit}.

@node Attribute Stub_Type,Attribute System_Allocator_Alignment,Attribute Storage_Unit,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-stub-type}@anchor{151}
@section Attribute Stub_Type


@geindex Stub_Type

The GNAT implementation of remote access-to-classwide types is
organized as described in AARM section E.4 (20.t): a value of an RACW type
(designating a remote object) is represented as a normal access
value, pointing to a "stub" object which in turn contains the
necessary information to contact the designated remote object. A
call on any dispatching operation of such a stub object does the
remote call, if necessary, using the information in the stub object
to locate the target partition, etc.

For a prefix @cite{T} that denotes a remote access-to-classwide type,
@cite{T'Stub_Type} denotes the type of the corresponding stub objects.

By construction, the layout of @cite{T'Stub_Type} is identical to that of
type @cite{RACW_Stub_Type} declared in the internal implementation-defined
unit @cite{System.Partition_Interface}. Use of this attribute will create
an implicit dependency on this unit.

@node Attribute System_Allocator_Alignment,Attribute Target_Name,Attribute Stub_Type,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-system-allocator-alignment}@anchor{152}
@section Attribute System_Allocator_Alignment


@geindex Alignment
@geindex allocator

@geindex System_Allocator_Alignment

@cite{Standard'System_Allocator_Alignment} (@cite{Standard} is the only
permissible prefix) provides the observable guaranted to be honored by
the system allocator (malloc). This is a static value that can be used
in user storage pools based on malloc either to reject allocation
with alignment too large or to enable a realignment circuitry if the
alignment request is larger than this value.

@node Attribute Target_Name,Attribute To_Address,Attribute System_Allocator_Alignment,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-target-name}@anchor{153}
@section Attribute Target_Name


@geindex Target_Name

@cite{Standard'Target_Name} (@cite{Standard} is the only permissible
prefix) provides a static string value that identifies the target
for the current compilation. For GCC implementations, this is the
standard gcc target name without the terminating slash (for
example, GNAT 5.0 on windows yields "i586-pc-mingw32msv").

@node Attribute To_Address,Attribute To_Any,Attribute Target_Name,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-to-address}@anchor{154}
@section Attribute To_Address


@geindex To_Address

The @cite{System'To_Address}
(@cite{System} is the only permissible prefix)
denotes a function identical to
@cite{System.Storage_Elements.To_Address} except that
it is a static attribute.  This means that if its argument is
a static expression, then the result of the attribute is a
static expression.  This means that such an expression can be
used in contexts (e.g., preelaborable packages) which require a
static expression and where the function call could not be used
(since the function call is always non-static, even if its
argument is static). The argument must be in the range
-(2**(m-1) .. 2**m-1, where m is the memory size
(typically 32 or 64). Negative values are intepreted in a
modular manner (e.g., -1 means the same as 16#FFFF_FFFF# on
a 32 bits machine).

@node Attribute To_Any,Attribute Type_Class,Attribute To_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-to-any}@anchor{155}
@section Attribute To_Any


@geindex To_Any

This internal attribute is used for the generation of remote subprogram
stubs in the context of the Distributed Systems Annex.

@node Attribute Type_Class,Attribute Type_Key,Attribute To_Any,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-type-class}@anchor{156}
@section Attribute Type_Class


@geindex Type_Class

@code{type'Type_Class} for any type or subtype @cite{type} yields
the value of the type class for the full type of @cite{type}.  If
@cite{type} is a generic formal type, the value is the value for the
corresponding actual subtype.  The value of this attribute is of type
@code{System.Aux_DEC.Type_Class}, which has the following definition:

@example
type Type_Class is
  (Type_Class_Enumeration,
   Type_Class_Integer,
   Type_Class_Fixed_Point,
   Type_Class_Floating_Point,
   Type_Class_Array,
   Type_Class_Record,
   Type_Class_Access,
   Type_Class_Task,
   Type_Class_Address);
@end example

Protected types yield the value @cite{Type_Class_Task}, which thus
applies to all concurrent types.  This attribute is designed to
be compatible with the DEC Ada 83 attribute of the same name.

@node Attribute Type_Key,Attribute TypeCode,Attribute Type_Class,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-type-key}@anchor{157}
@section Attribute Type_Key


@geindex Type_Key

The @cite{Type_Key} attribute is applicable to a type or subtype and
yields a value of type Standard.String containing encoded information
about the type or subtype. This provides improved compatibility with
other implementations that support this attribute.

@node Attribute TypeCode,Attribute UET_Address,Attribute Type_Key,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-typecode}@anchor{158}
@section Attribute TypeCode


@geindex TypeCode

This internal attribute is used for the generation of remote subprogram
stubs in the context of the Distributed Systems Annex.

@node Attribute UET_Address,Attribute Unconstrained_Array,Attribute TypeCode,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-uet-address}@anchor{159}
@section Attribute UET_Address


@geindex UET_Address

The @cite{UET_Address} attribute can only be used for a prefix which
denotes a library package.  It yields the address of the unit exception
table when zero cost exception handling is used.  This attribute is
intended only for use within the GNAT implementation.  See the unit
@cite{Ada.Exceptions} in files @code{a-except.ads} and @code{a-except.adb}
for details on how this attribute is used in the implementation.

@node Attribute Unconstrained_Array,Attribute Universal_Literal_String,Attribute UET_Address,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-unconstrained-array}@anchor{15a}
@section Attribute Unconstrained_Array


@geindex Unconstrained_Array

The @cite{Unconstrained_Array} attribute can be used with a prefix that
denotes any type or subtype. It is a static attribute that yields
@cite{True} if the prefix designates an unconstrained array,
and @cite{False} otherwise. In a generic instance, the result is
still static, and yields the result of applying this test to the
generic actual.

@node Attribute Universal_Literal_String,Attribute Unrestricted_Access,Attribute Unconstrained_Array,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-universal-literal-string}@anchor{15b}
@section Attribute Universal_Literal_String


@geindex Named numbers
@geindex representation of

@geindex Universal_Literal_String

The prefix of @cite{Universal_Literal_String} must be a named
number.  The static result is the string consisting of the characters of
the number as defined in the original source.  This allows the user
program to access the actual text of named numbers without intermediate
conversions and without the need to enclose the strings in quotes (which
would preclude their use as numbers).

For example, the following program prints the first 50 digits of pi:

@example
with Text_IO; use Text_IO;
with Ada.Numerics;
procedure Pi is
begin
   Put (Ada.Numerics.Pi'Universal_Literal_String);
end;
@end example

@node Attribute Unrestricted_Access,Attribute Update,Attribute Universal_Literal_String,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-unrestricted-access}@anchor{15c}
@section Attribute Unrestricted_Access


@geindex Access
@geindex unrestricted

@geindex Unrestricted_Access

The @cite{Unrestricted_Access} attribute is similar to @cite{Access}
except that all accessibility and aliased view checks are omitted.  This
is a user-beware attribute.

For objects, it is similar to @cite{Address}, for which it is a
desirable replacement where the value desired is an access type.
In other words, its effect is similar to first applying the
@cite{Address} attribute and then doing an unchecked conversion to a
desired access type.

For subprograms, @cite{P'Unrestricted_Access} may be used where
@cite{P'Access} would be illegal, to construct a value of a
less-nested named access type that designates a more-nested
subprogram. This value may be used in indirect calls, so long as the
more-nested subprogram still exists; once the subprogram containing it
has returned, such calls are erroneous. For example:

@example
package body P is

   type Less_Nested is not null access procedure;
   Global : Less_Nested;

   procedure P1 is
   begin
      Global.all;
   end P1;

   procedure P2 is
      Local_Var : Integer;

      procedure More_Nested is
      begin
         ... Local_Var ...
      end More_Nested;
   begin
      Global := More_Nested'Unrestricted_Access;
      P1;
   end P2;

end P;
@end example

When P1 is called from P2, the call via Global is OK, but if P1 were
called after P2 returns, it would be an erroneous use of a dangling
pointer.

For objects, it is possible to use @cite{Unrestricted_Access} for any
type. However, if the result is of an access-to-unconstrained array
subtype, then the resulting pointer has the same scope as the context
of the attribute, and must not be returned to some enclosing scope.
For instance, if a function uses @cite{Unrestricted_Access} to create
an access-to-unconstrained-array and returns that value to the caller,
the result will involve dangling pointers. In addition, it is only
valid to create pointers to unconstrained arrays using this attribute
if the pointer has the normal default 'fat' representation where a
pointer has two components, one points to the array and one points to
the bounds. If a size clause is used to force 'thin' representation
for a pointer to unconstrained where there is only space for a single
pointer, then the resulting pointer is not usable.

In the simple case where a direct use of Unrestricted_Access attempts
to make a thin pointer for a non-aliased object, the compiler will
reject the use as illegal, as shown in the following example:

@example
with System; use System;
procedure SliceUA2 is
   type A is access all String;
   for A'Size use Standard'Address_Size;

   procedure P (Arg : A) is
   begin
      null;
   end P;

   X : String := "hello world!";
   X2 : aliased String := "hello world!";

   AV : A := X'Unrestricted_Access;    -- ERROR
             |
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object

begin
   P (X'Unrestricted_Access);          -- ERROR
      |
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object

   P (X(7 .. 12)'Unrestricted_Access); -- ERROR
      |
>>> illegal use of Unrestricted_Access attribute
>>> attempt to generate thin pointer to unaliased object

   P (X2'Unrestricted_Access);         -- OK
end;
@end example

but other cases cannot be detected by the compiler, and are
considered to be erroneous. Consider the following example:

@example
with System; use System;
with System; use System;
procedure SliceUA is
   type AF is access all String;

   type A is access all String;
   for A'Size use Standard'Address_Size;

   procedure P (Arg : A) is
   begin
      if Arg'Length /= 6 then
         raise Program_Error;
      end if;
   end P;

   X : String := "hello world!";
   Y : AF := X (7 .. 12)'Unrestricted_Access;

begin
   P (A (Y));
end;
@end example

A normal unconstrained array value
or a constrained array object marked as aliased has the bounds in memory
just before the array, so a thin pointer can retrieve both the data and
the bounds.  But in this case, the non-aliased object @cite{X} does not have the
bounds before the string.  If the size clause for type @cite{A}
were not present, then the pointer
would be a fat pointer, where one component is a pointer to the bounds,
and all would be well.  But with the size clause present, the conversion from
fat pointer to thin pointer in the call loses the bounds, and so this
is erroneous, and the program likely raises a @cite{Program_Error} exception.

In general, it is advisable to completely
avoid mixing the use of thin pointers and the use of
@cite{Unrestricted_Access} where the designated type is an
unconstrained array.  The use of thin pointers should be restricted to
cases of porting legacy code that implicitly assumes the size of pointers,
and such code should not in any case be using this attribute.

Another erroneous situation arises if the attribute is
applied to a constant. The resulting pointer can be used to access the
constant, but the effect of trying to modify a constant in this manner
is not well-defined. Consider this example:

@example
P : constant Integer := 4;
type R is access all Integer;
RV : R := P'Unrestricted_Access;
..
RV.all := 3;
@end example

Here we attempt to modify the constant P from 4 to 3, but the compiler may
or may not notice this attempt, and subsequent references to P may yield
either the value 3 or the value 4 or the assignment may blow up if the
compiler decides to put P in read-only memory. One particular case where
@cite{Unrestricted_Access} can be used in this way is to modify the
value of an @cite{IN} parameter:

@example
procedure K (S : in String) is
   type R is access all Character;
   RV : R := S (3)'Unrestricted_Access;
begin
   RV.all := 'a';
end;
@end example

In general this is a risky approach. It may appear to "work" but such uses of
@cite{Unrestricted_Access} are potentially non-portable, even from one version
of @cite{GNAT} to another, so are best avoided if possible.

@node Attribute Update,Attribute Valid_Scalars,Attribute Unrestricted_Access,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-update}@anchor{15d}
@section Attribute Update


@geindex Update

The @cite{Update} attribute creates a copy of an array or record value
with one or more modified components. The syntax is:

@example
PREFIX'Update ( RECORD_COMPONENT_ASSOCIATION_LIST )
PREFIX'Update ( ARRAY_COMPONENT_ASSOCIATION @{, ARRAY_COMPONENT_ASSOCIATION @} )
PREFIX'Update ( MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION
                @{, MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION @} )

MULTIDIMENSIONAL_ARRAY_COMPONENT_ASSOCIATION ::= INDEX_EXPRESSION_LIST_LIST => EXPRESSION
INDEX_EXPRESSION_LIST_LIST                   ::= INDEX_EXPRESSION_LIST @{| INDEX_EXPRESSION_LIST @}
INDEX_EXPRESSION_LIST                        ::= ( EXPRESSION @{, EXPRESSION @} )
@end example

where @cite{PREFIX} is the name of an array or record object, the
association list in parentheses does not contain an @cite{others}
choice and the box symbol @cite{<>} may not appear in any
expression. The effect is to yield a copy of the array or record value
which is unchanged apart from the components mentioned in the
association list, which are changed to the indicated value. The
original value of the array or record value is not affected. For
example:

@example
type Arr is Array (1 .. 5) of Integer;
...
Avar1 : Arr := (1,2,3,4,5);
Avar2 : Arr := Avar1'Update (2 => 10, 3 .. 4 => 20);
@end example

yields a value for @cite{Avar2} of 1,10,20,20,5 with @cite{Avar1}
begin unmodified. Similarly:

@example
type Rec is A, B, C : Integer;
...
Rvar1 : Rec := (A => 1, B => 2, C => 3);
Rvar2 : Rec := Rvar1'Update (B => 20);
@end example

yields a value for @cite{Rvar2} of (A => 1, B => 20, C => 3),
with @cite{Rvar1} being unmodifed.
Note that the value of the attribute reference is computed
completely before it is used. This means that if you write:

@example
Avar1 := Avar1'Update (1 => 10, 2 => Function_Call);
@end example

then the value of @cite{Avar1} is not modified if @cite{Function_Call}
raises an exception, unlike the effect of a series of direct assignments
to elements of @cite{Avar1}. In general this requires that
two extra complete copies of the object are required, which should be
kept in mind when considering efficiency.

The @cite{Update} attribute cannot be applied to prefixes of a limited
type, and cannot reference discriminants in the case of a record type.
The accessibility level of an Update attribute result object is defined
as for an aggregate.

In the record case, no component can be mentioned more than once. In
the array case, two overlapping ranges can appear in the association list,
in which case the modifications are processed left to right.

Multi-dimensional arrays can be modified, as shown by this example:

@example
A : array (1 .. 10, 1 .. 10) of Integer;
..
A := A'Update ((1, 2) => 20, (3, 4) => 30);
@end example

which changes element (1,2) to 20 and (3,4) to 30.

@node Attribute Valid_Scalars,Attribute VADS_Size,Attribute Update,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-valid-scalars}@anchor{15e}
@section Attribute Valid_Scalars


@geindex Valid_Scalars

The @cite{'Valid_Scalars} attribute is intended to make it easier to
check the validity of scalar subcomponents of composite objects. It
is defined for any prefix @cite{X} that denotes an object.
The value of this attribute is of the predefined type Boolean.
@cite{X'Valid_Scalars} yields True if and only if evaluation of
@cite{P'Valid} yields True for every scalar part P of X or if X has
no scalar parts. It is not specified in what order the scalar parts
are checked, nor whether any more are checked after any one of them
is determined to be invalid. If the prefix @cite{X} is of a class-wide
type @cite{T'Class} (where @cite{T} is the associated specific type),
or if the prefix @cite{X} is of a specific tagged type @cite{T}, then
only the scalar parts of components of @cite{T} are traversed; in other
words, components of extensions of @cite{T} are not traversed even if
@cite{T'Class (X)'Tag /= T'Tag} . The compiler will issue a warning if it can
be determined at compile time that the prefix of the attribute has no
scalar parts (e.g., if the prefix is of an access type, an interface type,
an undiscriminated task type, or an undiscriminated protected type).

For scalar types, @cite{Valid_Scalars} is equivalent to @cite{Valid}. The use
of this attribute is not permitted for @cite{Unchecked_Union} types for which
in general it is not possible to determine the values of the discriminants.

Note: @cite{Valid_Scalars} can generate a lot of code, especially in the case
of a large variant record. If the attribute is called in many places in the
same program applied to objects of the same type, it can reduce program size
to write a function with a single use of the attribute, and then call that
function from multiple places.

@node Attribute VADS_Size,Attribute Value_Size,Attribute Valid_Scalars,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-vads-size}@anchor{15f}
@section Attribute VADS_Size


@geindex Size
@geindex VADS compatibility

@geindex VADS_Size

The @cite{'VADS_Size} attribute is intended to make it easier to port
legacy code which relies on the semantics of @cite{'Size} as implemented
by the VADS Ada 83 compiler.  GNAT makes a best effort at duplicating the
same semantic interpretation.  In particular, @cite{'VADS_Size} applied
to a predefined or other primitive type with no Size clause yields the
Object_Size (for example, @cite{Natural'Size} is 32 rather than 31 on
typical machines).  In addition @cite{'VADS_Size} applied to an object
gives the result that would be obtained by applying the attribute to
the corresponding type.

@node Attribute Value_Size,Attribute Wchar_T_Size,Attribute VADS_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-value-size}@anchor{160}
@section Attribute Value_Size


@geindex Size
@geindex setting for not-first subtype

@geindex Value_Size

@code{type'Value_Size} is the number of bits required to represent
a value of the given subtype.  It is the same as @code{type'Size},
but, unlike @cite{Size}, may be set for non-first subtypes.

@node Attribute Wchar_T_Size,Attribute Word_Size,Attribute Value_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-wchar-t-size}@anchor{161}
@section Attribute Wchar_T_Size


@geindex Wchar_T_Size

@cite{Standard'Wchar_T_Size} (@cite{Standard} is the only permissible
prefix) provides the size in bits of the C @cite{wchar_t} type
primarily for constructing the definition of this type in
package @cite{Interfaces.C}. The result is a static constant.

@node Attribute Word_Size,,Attribute Wchar_T_Size,Implementation Defined Attributes
@anchor{gnat_rm/implementation_defined_attributes attribute-word-size}@anchor{162}
@section Attribute Word_Size


@geindex Word_Size

@cite{Standard'Word_Size} (@cite{Standard} is the only permissible
prefix) provides the value @cite{System.Word_Size}. The result is
a static constant.

@node Standard and Implementation Defined Restrictions,Implementation Advice,Implementation Defined Attributes,Top
@anchor{gnat_rm/standard_and_implementation_defined_restrictions standard-and-implementation-defined-restrictions}@anchor{9}@anchor{gnat_rm/standard_and_implementation_defined_restrictions doc}@anchor{163}@anchor{gnat_rm/standard_and_implementation_defined_restrictions id1}@anchor{164}
@chapter Standard and Implementation Defined Restrictions


All Ada Reference Manual-defined Restriction identifiers are implemented:


@itemize *

@item 
language-defined restrictions (see 13.12.1)

@item 
tasking restrictions (see D.7)

@item 
high integrity restrictions (see H.4)
@end itemize

GNAT implements additional restriction identifiers. All restrictions, whether
language defined or GNAT-specific, are listed in the following.

@menu
* Partition-Wide Restrictions:: 
* Program Unit Level Restrictions:: 

@end menu

@node Partition-Wide Restrictions,Program Unit Level Restrictions,,Standard and Implementation Defined Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions partition-wide-restrictions}@anchor{165}@anchor{gnat_rm/standard_and_implementation_defined_restrictions id2}@anchor{166}
@section Partition-Wide Restrictions


There are two separate lists of restriction identifiers. The first
set requires consistency throughout a partition (in other words, if the
restriction identifier is used for any compilation unit in the partition,
then all compilation units in the partition must obey the restriction).

@menu
* Immediate_Reclamation:: 
* Max_Asynchronous_Select_Nesting:: 
* Max_Entry_Queue_Length:: 
* Max_Protected_Entries:: 
* Max_Select_Alternatives:: 
* Max_Storage_At_Blocking:: 
* Max_Task_Entries:: 
* Max_Tasks:: 
* No_Abort_Statements:: 
* No_Access_Parameter_Allocators:: 
* No_Access_Subprograms:: 
* No_Allocators:: 
* No_Anonymous_Allocators:: 
* No_Calendar:: 
* No_Coextensions:: 
* No_Default_Initialization:: 
* No_Delay:: 
* No_Dependence:: 
* No_Direct_Boolean_Operators:: 
* No_Dispatch:: 
* No_Dispatching_Calls:: 
* No_Dynamic_Attachment:: 
* No_Dynamic_Priorities:: 
* No_Entry_Calls_In_Elaboration_Code:: 
* No_Enumeration_Maps:: 
* No_Exception_Handlers:: 
* No_Exception_Propagation:: 
* No_Exception_Registration:: 
* No_Exceptions:: 
* No_Finalization:: 
* No_Fixed_Point:: 
* No_Floating_Point:: 
* No_Implicit_Conditionals:: 
* No_Implicit_Dynamic_Code:: 
* No_Implicit_Heap_Allocations:: 
* No_Implicit_Loops:: 
* No_Initialize_Scalars:: 
* No_IO:: 
* No_Local_Allocators:: 
* No_Local_Protected_Objects:: 
* No_Local_Timing_Events:: 
* No_Long_Long_Integers:: 
* No_Multiple_Elaboration:: 
* No_Nested_Finalization:: 
* No_Protected_Type_Allocators:: 
* No_Protected_Types:: 
* No_Recursion:: 
* No_Reentrancy:: 
* No_Relative_Delay:: 
* No_Requeue_Statements:: 
* No_Secondary_Stack:: 
* No_Select_Statements:: 
* No_Specific_Termination_Handlers:: 
* No_Specification_of_Aspect:: 
* No_Standard_Allocators_After_Elaboration:: 
* No_Standard_Storage_Pools:: 
* No_Stream_Optimizations:: 
* No_Streams:: 
* No_Task_Allocators:: 
* No_Task_Attributes_Package:: 
* No_Task_Hierarchy:: 
* No_Task_Termination:: 
* No_Tasking:: 
* No_Terminate_Alternatives:: 
* No_Unchecked_Access:: 
* No_Use_Of_Entity:: 
* Simple_Barriers:: 
* Static_Priorities:: 
* Static_Storage_Size:: 

@end menu

@node Immediate_Reclamation,Max_Asynchronous_Select_Nesting,,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions immediate-reclamation}@anchor{167}
@subsection Immediate_Reclamation


@geindex Immediate_Reclamation

[RM H.4] This restriction ensures that, except for storage occupied by
objects created by allocators and not deallocated via unchecked
deallocation, any storage reserved at run time for an object is
immediately reclaimed when the object no longer exists.

@node Max_Asynchronous_Select_Nesting,Max_Entry_Queue_Length,Immediate_Reclamation,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-asynchronous-select-nesting}@anchor{168}
@subsection Max_Asynchronous_Select_Nesting


@geindex Max_Asynchronous_Select_Nesting

[RM D.7] Specifies the maximum dynamic nesting level of asynchronous
selects. Violations of this restriction with a value of zero are
detected at compile time. Violations of this restriction with values
other than zero cause Storage_Error to be raised.

@node Max_Entry_Queue_Length,Max_Protected_Entries,Max_Asynchronous_Select_Nesting,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-entry-queue-length}@anchor{169}
@subsection Max_Entry_Queue_Length


@geindex Max_Entry_Queue_Length

[RM D.7] This restriction is a declaration that any protected entry compiled in
the scope of the restriction has at most the specified number of
tasks waiting on the entry at any one time, and so no queue is required.
Note that this restriction is checked at run time. Violation of this
restriction results in the raising of Program_Error exception at the point of
the call.

@geindex Max_Entry_Queue_Depth

The restriction @cite{Max_Entry_Queue_Depth} is recognized as a
synonym for @cite{Max_Entry_Queue_Length}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node Max_Protected_Entries,Max_Select_Alternatives,Max_Entry_Queue_Length,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-protected-entries}@anchor{16a}
@subsection Max_Protected_Entries


@geindex Max_Protected_Entries

[RM D.7] Specifies the maximum number of entries per protected type. The
bounds of every entry family of a protected unit shall be static, or shall be
defined by a discriminant of a subtype whose corresponding bound is static.

@node Max_Select_Alternatives,Max_Storage_At_Blocking,Max_Protected_Entries,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-select-alternatives}@anchor{16b}
@subsection Max_Select_Alternatives


@geindex Max_Select_Alternatives

[RM D.7] Specifies the maximum number of alternatives in a selective accept.

@node Max_Storage_At_Blocking,Max_Task_Entries,Max_Select_Alternatives,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-storage-at-blocking}@anchor{16c}
@subsection Max_Storage_At_Blocking


@geindex Max_Storage_At_Blocking

[RM D.7] Specifies the maximum portion (in storage elements) of a task's
Storage_Size that can be retained by a blocked task. A violation of this
restriction causes Storage_Error to be raised.

@node Max_Task_Entries,Max_Tasks,Max_Storage_At_Blocking,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-task-entries}@anchor{16d}
@subsection Max_Task_Entries


@geindex Max_Task_Entries

[RM D.7] Specifies the maximum number of entries
per task.  The bounds of every entry family
of a task unit shall be static, or shall be
defined by a discriminant of a subtype whose
corresponding bound is static.

@node Max_Tasks,No_Abort_Statements,Max_Task_Entries,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions max-tasks}@anchor{16e}
@subsection Max_Tasks


@geindex Max_Tasks

[RM D.7] Specifies the maximum number of task that may be created, not
counting the creation of the environment task.  Violations of this
restriction with a value of zero are detected at compile
time. Violations of this restriction with values other than zero cause
Storage_Error to be raised.

@node No_Abort_Statements,No_Access_Parameter_Allocators,Max_Tasks,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-abort-statements}@anchor{16f}
@subsection No_Abort_Statements


@geindex No_Abort_Statements

[RM D.7] There are no abort_statements, and there are
no calls to Task_Identification.Abort_Task.

@node No_Access_Parameter_Allocators,No_Access_Subprograms,No_Abort_Statements,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-access-parameter-allocators}@anchor{170}
@subsection No_Access_Parameter_Allocators


@geindex No_Access_Parameter_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator as the actual parameter to an access
parameter.

@node No_Access_Subprograms,No_Allocators,No_Access_Parameter_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-access-subprograms}@anchor{171}
@subsection No_Access_Subprograms


@geindex No_Access_Subprograms

[RM H.4] This restriction ensures at compile time that there are no
declarations of access-to-subprogram types.

@node No_Allocators,No_Anonymous_Allocators,No_Access_Subprograms,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-allocators}@anchor{172}
@subsection No_Allocators


@geindex No_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator.

@node No_Anonymous_Allocators,No_Calendar,No_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-anonymous-allocators}@anchor{173}
@subsection No_Anonymous_Allocators


@geindex No_Anonymous_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator of anonymous access type.

@node No_Calendar,No_Coextensions,No_Anonymous_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-calendar}@anchor{174}
@subsection No_Calendar


@geindex No_Calendar

[GNAT] This restriction ensures at compile time that there is no implicit or
explicit dependence on the package @cite{Ada.Calendar}.

@node No_Coextensions,No_Default_Initialization,No_Calendar,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-coextensions}@anchor{175}
@subsection No_Coextensions


@geindex No_Coextensions

[RM H.4] This restriction ensures at compile time that there are no
coextensions. See 3.10.2.

@node No_Default_Initialization,No_Delay,No_Coextensions,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-default-initialization}@anchor{176}
@subsection No_Default_Initialization


@geindex No_Default_Initialization

[GNAT] This restriction prohibits any instance of default initialization
of variables.  The binder implements a consistency rule which prevents
any unit compiled without the restriction from with'ing a unit with the
restriction (this allows the generation of initialization procedures to
be skipped, since you can be sure that no call is ever generated to an
initialization procedure in a unit with the restriction active). If used
in conjunction with Initialize_Scalars or Normalize_Scalars, the effect
is to prohibit all cases of variables declared without a specific
initializer (including the case of OUT scalar parameters).

@node No_Delay,No_Dependence,No_Default_Initialization,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-delay}@anchor{177}
@subsection No_Delay


@geindex No_Delay

[RM H.4] This restriction ensures at compile time that there are no
delay statements and no dependences on package Calendar.

@node No_Dependence,No_Direct_Boolean_Operators,No_Delay,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dependence}@anchor{178}
@subsection No_Dependence


@geindex No_Dependence

[RM 13.12.1] This restriction checks at compile time that there are no
dependence on a library unit.

@node No_Direct_Boolean_Operators,No_Dispatch,No_Dependence,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-direct-boolean-operators}@anchor{179}
@subsection No_Direct_Boolean_Operators


@geindex No_Direct_Boolean_Operators

[GNAT] This restriction ensures that no logical operators (and/or/xor)
are used on operands of type Boolean (or any type derived from Boolean).
This is intended for use in safety critical programs where the certification
protocol requires the use of short-circuit (and then, or else) forms for all
composite boolean operations.

@node No_Dispatch,No_Dispatching_Calls,No_Direct_Boolean_Operators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dispatch}@anchor{17a}
@subsection No_Dispatch


@geindex No_Dispatch

[RM H.4] This restriction ensures at compile time that there are no
occurrences of @cite{T'Class}, for any (tagged) subtype @cite{T}.

@node No_Dispatching_Calls,No_Dynamic_Attachment,No_Dispatch,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dispatching-calls}@anchor{17b}
@subsection No_Dispatching_Calls


@geindex No_Dispatching_Calls

[GNAT] This restriction ensures at compile time that the code generated by the
compiler involves no dispatching calls. The use of this restriction allows the
safe use of record extensions, classwide membership tests and other classwide
features not involving implicit dispatching. This restriction ensures that
the code contains no indirect calls through a dispatching mechanism. Note that
this includes internally-generated calls created by the compiler, for example
in the implementation of class-wide objects assignments. The
membership test is allowed in the presence of this restriction, because its
implementation requires no dispatching.
This restriction is comparable to the official Ada restriction
@cite{No_Dispatch} except that it is a bit less restrictive in that it allows
all classwide constructs that do not imply dispatching.
The following example indicates constructs that violate this restriction.

@example
package Pkg is
  type T is tagged record
    Data : Natural;
  end record;
  procedure P (X : T);

  type DT is new T with record
    More_Data : Natural;
  end record;
  procedure Q (X : DT);
end Pkg;

with Pkg; use Pkg;
procedure Example is
  procedure Test (O : T'Class) is
    N : Natural  := O'Size;--  Error: Dispatching call
    C : T'Class := O;      --  Error: implicit Dispatching Call
  begin
    if O in DT'Class then  --  OK   : Membership test
       Q (DT (O));         --  OK   : Type conversion plus direct call
    else
       P (O);              --  Error: Dispatching call
    end if;
  end Test;

  Obj : DT;
begin
  P (Obj);                 --  OK   : Direct call
  P (T (Obj));             --  OK   : Type conversion plus direct call
  P (T'Class (Obj));       --  Error: Dispatching call

  Test (Obj);              --  OK   : Type conversion

  if Obj in T'Class then   --  OK   : Membership test
     null;
  end if;
end Example;
@end example

@node No_Dynamic_Attachment,No_Dynamic_Priorities,No_Dispatching_Calls,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dynamic-attachment}@anchor{17c}
@subsection No_Dynamic_Attachment


@geindex No_Dynamic_Attachment

[RM D.7] This restriction ensures that there is no call to any of the
operations defined in package Ada.Interrupts
(Is_Reserved, Is_Attached, Current_Handler, Attach_Handler, Exchange_Handler,
Detach_Handler, and Reference).

@geindex No_Dynamic_Interrupts

The restriction @cite{No_Dynamic_Interrupts} is recognized as a
synonym for @cite{No_Dynamic_Attachment}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node No_Dynamic_Priorities,No_Entry_Calls_In_Elaboration_Code,No_Dynamic_Attachment,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-dynamic-priorities}@anchor{17d}
@subsection No_Dynamic_Priorities


@geindex No_Dynamic_Priorities

[RM D.7] There are no semantic dependencies on the package Dynamic_Priorities.

@node No_Entry_Calls_In_Elaboration_Code,No_Enumeration_Maps,No_Dynamic_Priorities,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-entry-calls-in-elaboration-code}@anchor{17e}
@subsection No_Entry_Calls_In_Elaboration_Code


@geindex No_Entry_Calls_In_Elaboration_Code

[GNAT] This restriction ensures at compile time that no task or protected entry
calls are made during elaboration code.  As a result of the use of this
restriction, the compiler can assume that no code past an accept statement
in a task can be executed at elaboration time.

@node No_Enumeration_Maps,No_Exception_Handlers,No_Entry_Calls_In_Elaboration_Code,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-enumeration-maps}@anchor{17f}
@subsection No_Enumeration_Maps


@geindex No_Enumeration_Maps

[GNAT] This restriction ensures at compile time that no operations requiring
enumeration maps are used (that is Image and Value attributes applied
to enumeration types).

@node No_Exception_Handlers,No_Exception_Propagation,No_Enumeration_Maps,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exception-handlers}@anchor{180}
@subsection No_Exception_Handlers


@geindex No_Exception_Handlers

[GNAT] This restriction ensures at compile time that there are no explicit
exception handlers. It also indicates that no exception propagation will
be provided. In this mode, exceptions may be raised but will result in
an immediate call to the last chance handler, a routine that the user
must define with the following profile:

@example
procedure Last_Chance_Handler
  (Source_Location : System.Address; Line : Integer);
pragma Export (C, Last_Chance_Handler,
               "__gnat_last_chance_handler");
@end example

The parameter is a C null-terminated string representing a message to be
associated with the exception (typically the source location of the raise
statement generated by the compiler). The Line parameter when nonzero
represents the line number in the source program where the raise occurs.

@node No_Exception_Propagation,No_Exception_Registration,No_Exception_Handlers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exception-propagation}@anchor{181}
@subsection No_Exception_Propagation


@geindex No_Exception_Propagation

[GNAT] This restriction guarantees that exceptions are never propagated
to an outer subprogram scope. The only case in which an exception may
be raised is when the handler is statically in the same subprogram, so
that the effect of a raise is essentially like a goto statement. Any
other raise statement (implicit or explicit) will be considered
unhandled. Exception handlers are allowed, but may not contain an
exception occurrence identifier (exception choice). In addition, use of
the package GNAT.Current_Exception is not permitted, and reraise
statements (raise with no operand) are not permitted.

@node No_Exception_Registration,No_Exceptions,No_Exception_Propagation,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exception-registration}@anchor{182}
@subsection No_Exception_Registration


@geindex No_Exception_Registration

[GNAT] This restriction ensures at compile time that no stream operations for
types Exception_Id or Exception_Occurrence are used. This also makes it
impossible to pass exceptions to or from a partition with this restriction
in a distributed environment. If this restriction is active, the generated
code is simplified by omitting the otherwise-required global registration
of exceptions when they are declared.

@node No_Exceptions,No_Finalization,No_Exception_Registration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-exceptions}@anchor{183}
@subsection No_Exceptions


@geindex No_Exceptions

[RM H.4] This restriction ensures at compile time that there are no
raise statements and no exception handlers.

@node No_Finalization,No_Fixed_Point,No_Exceptions,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-finalization}@anchor{184}
@subsection No_Finalization


@geindex No_Finalization

[GNAT] This restriction disables the language features described in
chapter 7.6 of the Ada 2005 RM as well as all form of code generation
performed by the compiler to support these features. The following types
are no longer considered controlled when this restriction is in effect:


@itemize *

@item 
@cite{Ada.Finalization.Controlled}

@item 
@cite{Ada.Finalization.Limited_Controlled}

@item 
Derivations from @cite{Controlled} or @cite{Limited_Controlled}

@item 
Class-wide types

@item 
Protected types

@item 
Task types

@item 
Array and record types with controlled components
@end itemize

The compiler no longer generates code to initialize, finalize or adjust an
object or a nested component, either declared on the stack or on the heap. The
deallocation of a controlled object no longer finalizes its contents.

@node No_Fixed_Point,No_Floating_Point,No_Finalization,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-fixed-point}@anchor{185}
@subsection No_Fixed_Point


@geindex No_Fixed_Point

[RM H.4] This restriction ensures at compile time that there are no
occurrences of fixed point types and operations.

@node No_Floating_Point,No_Implicit_Conditionals,No_Fixed_Point,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-floating-point}@anchor{186}
@subsection No_Floating_Point


@geindex No_Floating_Point

[RM H.4] This restriction ensures at compile time that there are no
occurrences of floating point types and operations.

@node No_Implicit_Conditionals,No_Implicit_Dynamic_Code,No_Floating_Point,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-conditionals}@anchor{187}
@subsection No_Implicit_Conditionals


@geindex No_Implicit_Conditionals

[GNAT] This restriction ensures that the generated code does not contain any
implicit conditionals, either by modifying the generated code where possible,
or by rejecting any construct that would otherwise generate an implicit
conditional. Note that this check does not include run time constraint
checks, which on some targets may generate implicit conditionals as
well. To control the latter, constraint checks can be suppressed in the
normal manner. Constructs generating implicit conditionals include comparisons
of composite objects and the Max/Min attributes.

@node No_Implicit_Dynamic_Code,No_Implicit_Heap_Allocations,No_Implicit_Conditionals,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-dynamic-code}@anchor{188}
@subsection No_Implicit_Dynamic_Code


@geindex No_Implicit_Dynamic_Code

@geindex trampoline

[GNAT] This restriction prevents the compiler from building 'trampolines'.
This is a structure that is built on the stack and contains dynamic
code to be executed at run time. On some targets, a trampoline is
built for the following features: @cite{Access},
@cite{Unrestricted_Access}, or @cite{Address} of a nested subprogram;
nested task bodies; primitive operations of nested tagged types.
Trampolines do not work on machines that prevent execution of stack
data. For example, on windows systems, enabling DEP (data execution
protection) will cause trampolines to raise an exception.
Trampolines are also quite slow at run time.

On many targets, trampolines have been largely eliminated. Look at the
version of system.ads for your target --- if it has
Always_Compatible_Rep equal to False, then trampolines are largely
eliminated. In particular, a trampoline is built for the following
features: @cite{Address} of a nested subprogram;
@cite{Access} or @cite{Unrestricted_Access} of a nested subprogram,
but only if pragma Favor_Top_Level applies, or the access type has a
foreign-language convention; primitive operations of nested tagged
types.

@node No_Implicit_Heap_Allocations,No_Implicit_Loops,No_Implicit_Dynamic_Code,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-heap-allocations}@anchor{189}
@subsection No_Implicit_Heap_Allocations


@geindex No_Implicit_Heap_Allocations

[RM D.7] No constructs are allowed to cause implicit heap allocation.

@node No_Implicit_Loops,No_Initialize_Scalars,No_Implicit_Heap_Allocations,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-loops}@anchor{18a}
@subsection No_Implicit_Loops


@geindex No_Implicit_Loops

[GNAT] This restriction ensures that the generated code does not contain any
implicit @cite{for} loops, either by modifying
the generated code where possible,
or by rejecting any construct that would otherwise generate an implicit
@cite{for} loop. If this restriction is active, it is possible to build
large array aggregates with all static components without generating an
intermediate temporary, and without generating a loop to initialize individual
components. Otherwise, a loop is created for arrays larger than about 5000
scalar components.

@node No_Initialize_Scalars,No_IO,No_Implicit_Loops,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-initialize-scalars}@anchor{18b}
@subsection No_Initialize_Scalars


@geindex No_Initialize_Scalars

[GNAT] This restriction ensures that no unit in the partition is compiled with
pragma Initialize_Scalars. This allows the generation of more efficient
code, and in particular eliminates dummy null initialization routines that
are otherwise generated for some record and array types.

@node No_IO,No_Local_Allocators,No_Initialize_Scalars,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-io}@anchor{18c}
@subsection No_IO


@geindex No_IO

[RM H.4] This restriction ensures at compile time that there are no
dependences on any of the library units Sequential_IO, Direct_IO,
Text_IO, Wide_Text_IO, Wide_Wide_Text_IO, or Stream_IO.

@node No_Local_Allocators,No_Local_Protected_Objects,No_IO,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-allocators}@anchor{18d}
@subsection No_Local_Allocators


@geindex No_Local_Allocators

[RM H.4] This restriction ensures at compile time that there are no
occurrences of an allocator in subprograms, generic subprograms, tasks,
and entry bodies.

@node No_Local_Protected_Objects,No_Local_Timing_Events,No_Local_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-protected-objects}@anchor{18e}
@subsection No_Local_Protected_Objects


@geindex No_Local_Protected_Objects

[RM D.7] This restriction ensures at compile time that protected objects are
only declared at the library level.

@node No_Local_Timing_Events,No_Long_Long_Integers,No_Local_Protected_Objects,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-local-timing-events}@anchor{18f}
@subsection No_Local_Timing_Events


@geindex No_Local_Timing_Events

[RM D.7] All objects of type Ada.Timing_Events.Timing_Event are
declared at the library level.

@node No_Long_Long_Integers,No_Multiple_Elaboration,No_Local_Timing_Events,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-long-long-integers}@anchor{190}
@subsection No_Long_Long_Integers


@geindex No_Long_Long_Integers

[GNAT] This partition-wide restriction forbids any explicit reference to
type Standard.Long_Long_Integer, and also forbids declaring range types whose
implicit base type is Long_Long_Integer, and modular types whose size exceeds
Long_Integer'Size.

@node No_Multiple_Elaboration,No_Nested_Finalization,No_Long_Long_Integers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-multiple-elaboration}@anchor{191}
@subsection No_Multiple_Elaboration


@geindex No_Multiple_Elaboration

[GNAT] Normally each package contains a 16-bit counter used to check for access
before elaboration, and to control multiple elaboration attempts.
This counter is eliminated for units compiled with the static model
of elaboration if restriction @cite{No_Elaboration_Code}
is active but because of
the need to check for multiple elaboration in the general case, these
counters cannot be eliminated if elaboration code may be present. The
restriction @cite{No_Multiple_Elaboration}
allows suppression of these counters
in static elaboration units even if they do have elaboration code. If this
restriction is used, then the situations in which multiple elaboration is
possible, including non-Ada main programs, and Stand Alone libraries, are not
permitted, and will be diagnosed by the binder.

@node No_Nested_Finalization,No_Protected_Type_Allocators,No_Multiple_Elaboration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-nested-finalization}@anchor{192}
@subsection No_Nested_Finalization


@geindex No_Nested_Finalization

[RM D.7] All objects requiring finalization are declared at the library level.

@node No_Protected_Type_Allocators,No_Protected_Types,No_Nested_Finalization,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-protected-type-allocators}@anchor{193}
@subsection No_Protected_Type_Allocators


@geindex No_Protected_Type_Allocators

[RM D.7] This restriction ensures at compile time that there are no allocator
expressions that attempt to allocate protected objects.

@node No_Protected_Types,No_Recursion,No_Protected_Type_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-protected-types}@anchor{194}
@subsection No_Protected_Types


@geindex No_Protected_Types

[RM H.4] This restriction ensures at compile time that there are no
declarations of protected types or protected objects.

@node No_Recursion,No_Reentrancy,No_Protected_Types,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-recursion}@anchor{195}
@subsection No_Recursion


@geindex No_Recursion

[RM H.4] A program execution is erroneous if a subprogram is invoked as
part of its execution.

@node No_Reentrancy,No_Relative_Delay,No_Recursion,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-reentrancy}@anchor{196}
@subsection No_Reentrancy


@geindex No_Reentrancy

[RM H.4] A program execution is erroneous if a subprogram is executed by
two tasks at the same time.

@node No_Relative_Delay,No_Requeue_Statements,No_Reentrancy,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-relative-delay}@anchor{197}
@subsection No_Relative_Delay


@geindex No_Relative_Delay

[RM D.7] This restriction ensures at compile time that there are no delay
relative statements and prevents expressions such as @cite{delay 1.23;} from
appearing in source code.

@node No_Requeue_Statements,No_Secondary_Stack,No_Relative_Delay,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-requeue-statements}@anchor{198}
@subsection No_Requeue_Statements


@geindex No_Requeue_Statements

[RM D.7] This restriction ensures at compile time that no requeue statements
are permitted and prevents keyword @cite{requeue} from being used in source
code.

@geindex No_Requeue

The restriction @cite{No_Requeue} is recognized as a
synonym for @cite{No_Requeue_Statements}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on oNobsolescent features are activated).

@node No_Secondary_Stack,No_Select_Statements,No_Requeue_Statements,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-secondary-stack}@anchor{199}
@subsection No_Secondary_Stack


@geindex No_Secondary_Stack

[GNAT] This restriction ensures at compile time that the generated code
does not contain any reference to the secondary stack.  The secondary
stack is used to implement functions returning unconstrained objects
(arrays or records) on some targets.

@node No_Select_Statements,No_Specific_Termination_Handlers,No_Secondary_Stack,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-select-statements}@anchor{19a}
@subsection No_Select_Statements


@geindex No_Select_Statements

[RM D.7] This restriction ensures at compile time no select statements of any
kind are permitted, that is the keyword @cite{select} may not appear.

@node No_Specific_Termination_Handlers,No_Specification_of_Aspect,No_Select_Statements,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-specific-termination-handlers}@anchor{19b}
@subsection No_Specific_Termination_Handlers


@geindex No_Specific_Termination_Handlers

[RM D.7] There are no calls to Ada.Task_Termination.Set_Specific_Handler
or to Ada.Task_Termination.Specific_Handler.

@node No_Specification_of_Aspect,No_Standard_Allocators_After_Elaboration,No_Specific_Termination_Handlers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-specification-of-aspect}@anchor{19c}
@subsection No_Specification_of_Aspect


@geindex No_Specification_of_Aspect

[RM 13.12.1] This restriction checks at compile time that no aspect
specification, attribute definition clause, or pragma is given for a
given aspect.

@node No_Standard_Allocators_After_Elaboration,No_Standard_Storage_Pools,No_Specification_of_Aspect,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-standard-allocators-after-elaboration}@anchor{19d}
@subsection No_Standard_Allocators_After_Elaboration


@geindex No_Standard_Allocators_After_Elaboration

[RM D.7] Specifies that an allocator using a standard storage pool
should never be evaluated at run time after the elaboration of the
library items of the partition has completed. Otherwise, Storage_Error
is raised.

@node No_Standard_Storage_Pools,No_Stream_Optimizations,No_Standard_Allocators_After_Elaboration,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-standard-storage-pools}@anchor{19e}
@subsection No_Standard_Storage_Pools


@geindex No_Standard_Storage_Pools

[GNAT] This restriction ensures at compile time that no access types
use the standard default storage pool.  Any access type declared must
have an explicit Storage_Pool attribute defined specifying a
user-defined storage pool.

@node No_Stream_Optimizations,No_Streams,No_Standard_Storage_Pools,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-stream-optimizations}@anchor{19f}
@subsection No_Stream_Optimizations


@geindex No_Stream_Optimizations

[GNAT] This restriction affects the performance of stream operations on types
@cite{String}, @cite{Wide_String} and @cite{Wide_Wide_String}. By default, the
compiler uses block reads and writes when manipulating @cite{String} objects
due to their supperior performance. When this restriction is in effect, the
compiler performs all IO operations on a per-character basis.

@node No_Streams,No_Task_Allocators,No_Stream_Optimizations,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-streams}@anchor{1a0}
@subsection No_Streams


@geindex No_Streams

[GNAT] This restriction ensures at compile/bind time that there are no
stream objects created and no use of stream attributes.
This restriction does not forbid dependences on the package
@cite{Ada.Streams}. So it is permissible to with
@cite{Ada.Streams} (or another package that does so itself)
as long as no actual stream objects are created and no
stream attributes are used.

Note that the use of restriction allows optimization of tagged types,
since they do not need to worry about dispatching stream operations.
To take maximum advantage of this space-saving optimization, any
unit declaring a tagged type should be compiled with the restriction,
though this is not required.

@node No_Task_Allocators,No_Task_Attributes_Package,No_Streams,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-allocators}@anchor{1a1}
@subsection No_Task_Allocators


@geindex No_Task_Allocators

[RM D.7] There are no allocators for task types
or types containing task subcomponents.

@node No_Task_Attributes_Package,No_Task_Hierarchy,No_Task_Allocators,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-attributes-package}@anchor{1a2}
@subsection No_Task_Attributes_Package


@geindex No_Task_Attributes_Package

[GNAT] This restriction ensures at compile time that there are no implicit or
explicit dependencies on the package @cite{Ada.Task_Attributes}.

@geindex No_Task_Attributes

The restriction @cite{No_Task_Attributes} is recognized as a synonym
for @cite{No_Task_Attributes_Package}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node No_Task_Hierarchy,No_Task_Termination,No_Task_Attributes_Package,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-hierarchy}@anchor{1a3}
@subsection No_Task_Hierarchy


@geindex No_Task_Hierarchy

[RM D.7] All (non-environment) tasks depend
directly on the environment task of the partition.

@node No_Task_Termination,No_Tasking,No_Task_Hierarchy,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-task-termination}@anchor{1a4}
@subsection No_Task_Termination


@geindex No_Task_Termination

[RM D.7] Tasks that terminate are erroneous.

@node No_Tasking,No_Terminate_Alternatives,No_Task_Termination,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-tasking}@anchor{1a5}
@subsection No_Tasking


@geindex No_Tasking

[GNAT] This restriction prevents the declaration of tasks or task types
throughout the partition.  It is similar in effect to the use of
@cite{Max_Tasks => 0} except that violations are caught at compile time
and cause an error message to be output either by the compiler or
binder.

@node No_Terminate_Alternatives,No_Unchecked_Access,No_Tasking,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-terminate-alternatives}@anchor{1a6}
@subsection No_Terminate_Alternatives


@geindex No_Terminate_Alternatives

[RM D.7] There are no selective accepts with terminate alternatives.

@node No_Unchecked_Access,No_Use_Of_Entity,No_Terminate_Alternatives,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-unchecked-access}@anchor{1a7}
@subsection No_Unchecked_Access


@geindex No_Unchecked_Access

[RM H.4] This restriction ensures at compile time that there are no
occurrences of the Unchecked_Access attribute.

@node No_Use_Of_Entity,Simple_Barriers,No_Unchecked_Access,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-use-of-entity}@anchor{1a8}
@subsection No_Use_Of_Entity


@geindex No_Use_Of_Entity

[GNAT] This restriction ensures at compile time that there are no references
to the entity given in the form

@example
No_Use_Of_Entity => Name
@end example

where @code{Name} is the fully qualified entity, for example

@example
No_Use_Of_Entity => Ada.Text_IO.Put_Line
@end example

@node Simple_Barriers,Static_Priorities,No_Use_Of_Entity,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions simple-barriers}@anchor{1a9}
@subsection Simple_Barriers


@geindex Simple_Barriers

[RM D.7] This restriction ensures at compile time that barriers in entry
declarations for protected types are restricted to either static boolean
expressions or references to simple boolean variables defined in the private
part of the protected type.  No other form of entry barriers is permitted.

@geindex Boolean_Entry_Barriers

The restriction @cite{Boolean_Entry_Barriers} is recognized as a
synonym for @cite{Simple_Barriers}. This is retained for historical
compatibility purposes (and a warning will be generated for its use if
warnings on obsolescent features are activated).

@node Static_Priorities,Static_Storage_Size,Simple_Barriers,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions static-priorities}@anchor{1aa}
@subsection Static_Priorities


@geindex Static_Priorities

[GNAT] This restriction ensures at compile time that all priority expressions
are static, and that there are no dependences on the package
@cite{Ada.Dynamic_Priorities}.

@node Static_Storage_Size,,Static_Priorities,Partition-Wide Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions static-storage-size}@anchor{1ab}
@subsection Static_Storage_Size


@geindex Static_Storage_Size

[GNAT] This restriction ensures at compile time that any expression appearing
in a Storage_Size pragma or attribute definition clause is static.

@node Program Unit Level Restrictions,,Partition-Wide Restrictions,Standard and Implementation Defined Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions program-unit-level-restrictions}@anchor{1ac}@anchor{gnat_rm/standard_and_implementation_defined_restrictions id3}@anchor{1ad}
@section Program Unit Level Restrictions


The second set of restriction identifiers
does not require partition-wide consistency.
The restriction may be enforced for a single
compilation unit without any effect on any of the
other compilation units in the partition.

@menu
* No_Elaboration_Code:: 
* No_Entry_Queue:: 
* No_Implementation_Aspect_Specifications:: 
* No_Implementation_Attributes:: 
* No_Implementation_Identifiers:: 
* No_Implementation_Pragmas:: 
* No_Implementation_Restrictions:: 
* No_Implementation_Units:: 
* No_Implicit_Aliasing:: 
* No_Obsolescent_Features:: 
* No_Wide_Characters:: 
* SPARK_05:: 

@end menu

@node No_Elaboration_Code,No_Entry_Queue,,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-elaboration-code}@anchor{1ae}
@subsection No_Elaboration_Code


@geindex No_Elaboration_Code

[GNAT] This restriction ensures at compile time that no elaboration code is
generated.  Note that this is not the same condition as is enforced
by pragma @cite{Preelaborate}.  There are cases in which pragma
@cite{Preelaborate} still permits code to be generated (e.g., code
to initialize a large array to all zeroes), and there are cases of units
which do not meet the requirements for pragma @cite{Preelaborate},
but for which no elaboration code is generated.  Generally, it is
the case that preelaborable units will meet the restrictions, with
the exception of large aggregates initialized with an others_clause,
and exception declarations (which generate calls to a run-time
registry procedure).  This restriction is enforced on
a unit by unit basis, it need not be obeyed consistently
throughout a partition.

In the case of aggregates with others, if the aggregate has a dynamic
size, there is no way to eliminate the elaboration code (such dynamic
bounds would be incompatible with @cite{Preelaborate} in any case). If
the bounds are static, then use of this restriction actually modifies
the code choice of the compiler to avoid generating a loop, and instead
generate the aggregate statically if possible, no matter how many times
the data for the others clause must be repeatedly generated.

It is not possible to precisely document
the constructs which are compatible with this restriction, since,
unlike most other restrictions, this is not a restriction on the
source code, but a restriction on the generated object code. For
example, if the source contains a declaration:

@example
Val : constant Integer := X;
@end example

where X is not a static constant, it may be possible, depending
on complex optimization circuitry, for the compiler to figure
out the value of X at compile time, in which case this initialization
can be done by the loader, and requires no initialization code. It
is not possible to document the precise conditions under which the
optimizer can figure this out.

Note that this the implementation of this restriction requires full
code generation. If it is used in conjunction with "semantics only"
checking, then some cases of violations may be missed.

@node No_Entry_Queue,No_Implementation_Aspect_Specifications,No_Elaboration_Code,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-entry-queue}@anchor{1af}
@subsection No_Entry_Queue


@geindex No_Entry_Queue

[GNAT] This restriction is a declaration that any protected entry compiled in
the scope of the restriction has at most one task waiting on the entry
at any one time, and so no queue is required.  This restriction is not
checked at compile time.  A program execution is erroneous if an attempt
is made to queue a second task on such an entry.

@node No_Implementation_Aspect_Specifications,No_Implementation_Attributes,No_Entry_Queue,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-aspect-specifications}@anchor{1b0}
@subsection No_Implementation_Aspect_Specifications


@geindex No_Implementation_Aspect_Specifications

[RM 13.12.1] This restriction checks at compile time that no
GNAT-defined aspects are present.  With this restriction, the only
aspects that can be used are those defined in the Ada Reference Manual.

@node No_Implementation_Attributes,No_Implementation_Identifiers,No_Implementation_Aspect_Specifications,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-attributes}@anchor{1b1}
@subsection No_Implementation_Attributes


@geindex No_Implementation_Attributes

[RM 13.12.1] This restriction checks at compile time that no
GNAT-defined attributes are present.  With this restriction, the only
attributes that can be used are those defined in the Ada Reference
Manual.

@node No_Implementation_Identifiers,No_Implementation_Pragmas,No_Implementation_Attributes,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-identifiers}@anchor{1b2}
@subsection No_Implementation_Identifiers


@geindex No_Implementation_Identifiers

[RM 13.12.1] This restriction checks at compile time that no
implementation-defined identifiers (marked with pragma Implementation_Defined)
occur within language-defined packages.

@node No_Implementation_Pragmas,No_Implementation_Restrictions,No_Implementation_Identifiers,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-pragmas}@anchor{1b3}
@subsection No_Implementation_Pragmas


@geindex No_Implementation_Pragmas

[RM 13.12.1] This restriction checks at compile time that no
GNAT-defined pragmas are present.  With this restriction, the only
pragmas that can be used are those defined in the Ada Reference Manual.

@node No_Implementation_Restrictions,No_Implementation_Units,No_Implementation_Pragmas,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-restrictions}@anchor{1b4}
@subsection No_Implementation_Restrictions


@geindex No_Implementation_Restrictions

[GNAT] This restriction checks at compile time that no GNAT-defined restriction
identifiers (other than @cite{No_Implementation_Restrictions} itself)
are present.  With this restriction, the only other restriction identifiers
that can be used are those defined in the Ada Reference Manual.

@node No_Implementation_Units,No_Implicit_Aliasing,No_Implementation_Restrictions,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implementation-units}@anchor{1b5}
@subsection No_Implementation_Units


@geindex No_Implementation_Units

[RM 13.12.1] This restriction checks at compile time that there is no
mention in the context clause of any implementation-defined descendants
of packages Ada, Interfaces, or System.

@node No_Implicit_Aliasing,No_Obsolescent_Features,No_Implementation_Units,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-implicit-aliasing}@anchor{1b6}
@subsection No_Implicit_Aliasing


@geindex No_Implicit_Aliasing

[GNAT] This restriction, which is not required to be partition-wide consistent,
requires an explicit aliased keyword for an object to which 'Access,
'Unchecked_Access, or 'Address is applied, and forbids entirely the use of
the 'Unrestricted_Access attribute for objects. Note: the reason that
Unrestricted_Access is forbidden is that it would require the prefix
to be aliased, and in such cases, it can always be replaced by
the standard attribute Unchecked_Access which is preferable.

@node No_Obsolescent_Features,No_Wide_Characters,No_Implicit_Aliasing,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-obsolescent-features}@anchor{1b7}
@subsection No_Obsolescent_Features


@geindex No_Obsolescent_Features

[RM 13.12.1] This restriction checks at compile time that no obsolescent
features are used, as defined in Annex J of the Ada Reference Manual.

@node No_Wide_Characters,SPARK_05,No_Obsolescent_Features,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions no-wide-characters}@anchor{1b8}
@subsection No_Wide_Characters


@geindex No_Wide_Characters

[GNAT] This restriction ensures at compile time that no uses of the types
@cite{Wide_Character} or @cite{Wide_String} or corresponding wide
wide types
appear, and that no wide or wide wide string or character literals
appear in the program (that is literals representing characters not in
type @cite{Character}).

@node SPARK_05,,No_Wide_Characters,Program Unit Level Restrictions
@anchor{gnat_rm/standard_and_implementation_defined_restrictions spark-05}@anchor{1b9}
@subsection SPARK_05


@geindex SPARK_05

[GNAT] This restriction checks at compile time that some constructs
forbidden in SPARK 2005 are not present. Error messages related to
SPARK restriction have the form:

@example
violation of restriction "SPARK_05" at <source-location>
 <error message>
@end example

@geindex SPARK

The restriction @cite{SPARK} is recognized as a
synonym for @cite{SPARK_05}. This is retained for historical
compatibility purposes (and an unconditional warning will be generated
for its use, advising replacement by @cite{SPARK}).

This is not a replacement for the semantic checks performed by the
SPARK Examiner tool, as the compiler currently only deals with code,
not SPARK 2005 annotations, and does not guarantee catching all
cases of constructs forbidden by SPARK 2005.

Thus it may well be the case that code which passes the compiler with
the SPARK restriction is rejected by the SPARK Examiner, e.g. due to
the different visibility rules of the Examiner based on SPARK 2005
@cite{inherit} annotations.

This restriction can be useful in providing an initial filter for code
developed using SPARK 2005, or in examining legacy code to see how far
it is from meeting SPARK restrictions.

The list below summarizes the checks that are performed when this
restriction is in force:


@itemize *

@item 
No block statements

@item 
No case statements with only an others clause

@item 
Exit statements in loops must respect the SPARK 2005 language restrictions

@item 
No goto statements

@item 
Return can only appear as last statement in function

@item 
Function must have return statement

@item 
Loop parameter specification must include subtype mark

@item 
Prefix of expanded name cannot be a loop statement

@item 
Abstract subprogram not allowed

@item 
User-defined operators not allowed

@item 
Access type parameters not allowed

@item 
Default expressions for parameters not allowed

@item 
Default expressions for record fields not allowed

@item 
No tasking constructs allowed

@item 
Label needed at end of subprograms and packages

@item 
No mixing of positional and named parameter association

@item 
No access types as result type

@item 
No unconstrained arrays as result types

@item 
No null procedures

@item 
Initial and later declarations must be in correct order (declaration can't come after body)

@item 
No attributes on private types if full declaration not visible

@item 
No package declaration within package specification

@item 
No controlled types

@item 
No discriminant types

@item 
No overloading

@item 
Selector name cannot be operator symbol (i.e. operator symbol cannot be prefixed)

@item 
Access attribute not allowed

@item 
Allocator not allowed

@item 
Result of catenation must be String

@item 
Operands of catenation must be string literal, static char or another catenation

@item 
No conditional expressions

@item 
No explicit dereference

@item 
Quantified expression not allowed

@item 
Slicing not allowed

@item 
No exception renaming

@item 
No generic renaming

@item 
No object renaming

@item 
No use clause

@item 
Aggregates must be qualified

@item 
Non-static choice in array aggregates not allowed

@item 
The only view conversions which are allowed as in-out parameters are conversions of a tagged type to an ancestor type

@item 
No mixing of positional and named association in aggregate, no multi choice

@item 
AND, OR and XOR for arrays only allowed when operands have same static bounds

@item 
Fixed point operands to * or / must be qualified or converted

@item 
Comparison operators not allowed for Booleans or arrays (except strings)

@item 
Equality not allowed for arrays with non-matching static bounds (except strings)

@item 
Conversion / qualification not allowed for arrays with non-matching static bounds

@item 
Subprogram declaration only allowed in package spec (unless followed by import)

@item 
Access types not allowed

@item 
Incomplete type declaration not allowed

@item 
Object and subtype declarations must respect SPARK restrictions

@item 
Digits or delta constraint not allowed

@item 
Decimal fixed point type not allowed

@item 
Aliasing of objects not allowed

@item 
Modular type modulus must be power of 2

@item 
Base not allowed on subtype mark

@item 
Unary operators not allowed on modular types (except not)

@item 
Untagged record cannot be null

@item 
No class-wide operations

@item 
Initialization expressions must respect SPARK restrictions

@item 
Non-static ranges not allowed except in iteration schemes

@item 
String subtypes must have lower bound of 1

@item 
Subtype of Boolean cannot have constraint

@item 
At most one tagged type or extension per package

@item 
Interface is not allowed

@item 
Character literal cannot be prefixed (selector name cannot be character literal)

@item 
Record aggregate cannot contain 'others'

@item 
Component association in record aggregate must contain a single choice

@item 
Ancestor part cannot be a type mark

@item 
Attributes 'Image, 'Width and 'Value not allowed

@item 
Functions may not update globals

@item 
Subprograms may not contain direct calls to themselves (prevents recursion within unit)

@item 
Call to subprogram not allowed in same unit before body has been seen (prevents recursion within unit)
@end itemize

The following restrictions are enforced, but note that they are actually more
strict that the latest SPARK 2005 language definition:


@itemize *

@item 
No derived types other than tagged type extensions

@item 
Subtype of unconstrained array must have constraint
@end itemize

This list summarises the main SPARK 2005 language rules that are not
currently checked by the SPARK_05 restriction:


@itemize *

@item 
SPARK annotations are treated as comments so are not checked at all

@item 
Based real literals not allowed

@item 
Objects cannot be initialized at declaration by calls to user-defined functions

@item 
Objects cannot be initialized at declaration by assignments from variables

@item 
Objects cannot be initialized at declaration by assignments from indexed/selected components

@item 
Ranges shall not be null

@item 
A fixed point delta expression must be a simple expression

@item 
Restrictions on where renaming declarations may be placed

@item 
Externals of mode 'out' cannot be referenced

@item 
Externals of mode 'in' cannot be updated

@item 
Loop with no iteration scheme or exits only allowed as last statement in main program or task

@item 
Subprogram cannot have parent unit name

@item 
SPARK 2005 inherited subprogram must be prefixed with overriding

@item 
External variables (or functions that reference them) may not be passed as actual parameters

@item 
Globals must be explicitly mentioned in contract

@item 
Deferred constants cannot be completed by pragma Import

@item 
Package initialization cannot read/write variables from other packages

@item 
Prefix not allowed for entities that are directly visible

@item 
Identifier declaration can't override inherited package name

@item 
Cannot use Standard or other predefined packages as identifiers

@item 
After renaming, cannot use the original name

@item 
Subprograms can only be renamed to remove package prefix

@item 
Pragma import must be immediately after entity it names

@item 
No mutual recursion between multiple units (this can be checked with gnatcheck)
@end itemize

Note that if a unit is compiled in Ada 95 mode with the SPARK restriction,
violations will be reported for constructs forbidden in SPARK 95,
instead of SPARK 2005.

@node Implementation Advice,Implementation Defined Characteristics,Standard and Implementation Defined Restrictions,Top
@anchor{gnat_rm/implementation_advice doc}@anchor{1ba}@anchor{gnat_rm/implementation_advice implementation-advice}@anchor{a}@anchor{gnat_rm/implementation_advice id1}@anchor{1bb}
@chapter Implementation Advice


The main text of the Ada Reference Manual describes the required
behavior of all Ada compilers, and the GNAT compiler conforms to
these requirements.

In addition, there are sections throughout the Ada Reference Manual headed
by the phrase 'Implementation advice'.  These sections are not normative,
i.e., they do not specify requirements that all compilers must
follow.  Rather they provide advice on generally desirable behavior.
They are not requirements, because they describe behavior that cannot
be provided on all systems, or may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice in
the Ada Reference Manual.  Each such RM section corresponds to a section
in this chapter whose title specifies the
RM section number and paragraph number and the subject of
the advice.  The contents of each section consists of the RM text within
quotation marks,
followed by the GNAT interpretation of the advice.  Most often, this simply says
'followed', which means that GNAT follows the advice.  However, in a
number of cases, GNAT deliberately deviates from this advice, in which
case the text describes what GNAT does and why.

@geindex Error detection

@menu
* RM 1.1.3(20); Error Detection: RM 1 1 3 20 Error Detection. 
* RM 1.1.3(31); Child Units: RM 1 1 3 31 Child Units. 
* RM 1.1.5(12); Bounded Errors: RM 1 1 5 12 Bounded Errors. 
* RM 2.8(16); Pragmas: RM 2 8 16 Pragmas. 
* RM 2.8(17-19); Pragmas: RM 2 8 17-19 Pragmas. 
* RM 3.5.2(5); Alternative Character Sets: RM 3 5 2 5 Alternative Character Sets. 
* RM 3.5.4(28); Integer Types: RM 3 5 4 28 Integer Types. 
* RM 3.5.4(29); Integer Types: RM 3 5 4 29 Integer Types. 
* RM 3.5.5(8); Enumeration Values: RM 3 5 5 8 Enumeration Values. 
* RM 3.5.7(17); Float Types: RM 3 5 7 17 Float Types. 
* RM 3.6.2(11); Multidimensional Arrays: RM 3 6 2 11 Multidimensional Arrays. 
* RM 9.6(30-31); Duration'Small: RM 9 6 30-31 Duration'Small. 
* RM 10.2.1(12); Consistent Representation: RM 10 2 1 12 Consistent Representation. 
* RM 11.4.1(19); Exception Information: RM 11 4 1 19 Exception Information. 
* RM 11.5(28); Suppression of Checks: RM 11 5 28 Suppression of Checks. 
* RM 13.1 (21-24); Representation Clauses: RM 13 1 21-24 Representation Clauses. 
* RM 13.2(6-8); Packed Types: RM 13 2 6-8 Packed Types. 
* RM 13.3(14-19); Address Clauses: RM 13 3 14-19 Address Clauses. 
* RM 13.3(29-35); Alignment Clauses: RM 13 3 29-35 Alignment Clauses. 
* RM 13.3(42-43); Size Clauses: RM 13 3 42-43 Size Clauses. 
* RM 13.3(50-56); Size Clauses: RM 13 3 50-56 Size Clauses. 
* RM 13.3(71-73); Component Size Clauses: RM 13 3 71-73 Component Size Clauses. 
* RM 13.4(9-10); Enumeration Representation Clauses: RM 13 4 9-10 Enumeration Representation Clauses. 
* RM 13.5.1(17-22); Record Representation Clauses: RM 13 5 1 17-22 Record Representation Clauses. 
* RM 13.5.2(5); Storage Place Attributes: RM 13 5 2 5 Storage Place Attributes. 
* RM 13.5.3(7-8); Bit Ordering: RM 13 5 3 7-8 Bit Ordering. 
* RM 13.7(37); Address as Private: RM 13 7 37 Address as Private. 
* RM 13.7.1(16); Address Operations: RM 13 7 1 16 Address Operations. 
* RM 13.9(14-17); Unchecked Conversion: RM 13 9 14-17 Unchecked Conversion. 
* RM 13.11(23-25); Implicit Heap Usage: RM 13 11 23-25 Implicit Heap Usage. 
* RM 13.11.2(17); Unchecked Deallocation: RM 13 11 2 17 Unchecked Deallocation. 
* RM 13.13.2(17); Stream Oriented Attributes: RM 13 13 2 17 Stream Oriented Attributes. 
* RM A.1(52); Names of Predefined Numeric Types: RM A 1 52 Names of Predefined Numeric Types. 
* RM A.3.2(49); Ada.Characters.Handling: RM A 3 2 49 Ada Characters Handling. 
* RM A.4.4(106); Bounded-Length String Handling: RM A 4 4 106 Bounded-Length String Handling. 
* RM A.5.2(46-47); Random Number Generation: RM A 5 2 46-47 Random Number Generation. 
* RM A.10.7(23); Get_Immediate: RM A 10 7 23 Get_Immediate. 
* RM B.1(39-41); Pragma Export: RM B 1 39-41 Pragma Export. 
* RM B.2(12-13); Package Interfaces: RM B 2 12-13 Package Interfaces. 
* RM B.3(63-71); Interfacing with C: RM B 3 63-71 Interfacing with C. 
* RM B.4(95-98); Interfacing with COBOL: RM B 4 95-98 Interfacing with COBOL. 
* RM B.5(22-26); Interfacing with Fortran: RM B 5 22-26 Interfacing with Fortran. 
* RM C.1(3-5); Access to Machine Operations: RM C 1 3-5 Access to Machine Operations. 
* RM C.1(10-16); Access to Machine Operations: RM C 1 10-16 Access to Machine Operations. 
* RM C.3(28); Interrupt Support: RM C 3 28 Interrupt Support. 
* RM C.3.1(20-21); Protected Procedure Handlers: RM C 3 1 20-21 Protected Procedure Handlers. 
* RM C.3.2(25); Package Interrupts: RM C 3 2 25 Package Interrupts. 
* RM C.4(14); Pre-elaboration Requirements: RM C 4 14 Pre-elaboration Requirements. 
* RM C.5(8); Pragma Discard_Names: RM C 5 8 Pragma Discard_Names. 
* RM C.7.2(30); The Package Task_Attributes: RM C 7 2 30 The Package Task_Attributes. 
* RM D.3(17); Locking Policies: RM D 3 17 Locking Policies. 
* RM D.4(16); Entry Queuing Policies: RM D 4 16 Entry Queuing Policies. 
* RM D.6(9-10); Preemptive Abort: RM D 6 9-10 Preemptive Abort. 
* RM D.7(21); Tasking Restrictions: RM D 7 21 Tasking Restrictions. 
* RM D.8(47-49); Monotonic Time: RM D 8 47-49 Monotonic Time. 
* RM E.5(28-29); Partition Communication Subsystem: RM E 5 28-29 Partition Communication Subsystem. 
* RM F(7); COBOL Support: RM F 7 COBOL Support. 
* RM F.1(2); Decimal Radix Support: RM F 1 2 Decimal Radix Support. 
* RM G; Numerics: RM G Numerics. 
* RM G.1.1(56-58); Complex Types: RM G 1 1 56-58 Complex Types. 
* RM G.1.2(49); Complex Elementary Functions: RM G 1 2 49 Complex Elementary Functions. 
* RM G.2.4(19); Accuracy Requirements: RM G 2 4 19 Accuracy Requirements. 
* RM G.2.6(15); Complex Arithmetic Accuracy: RM G 2 6 15 Complex Arithmetic Accuracy. 
* RM H.6(15/2); Pragma Partition_Elaboration_Policy: RM H 6 15/2 Pragma Partition_Elaboration_Policy. 

@end menu

@node RM 1 1 3 20 Error Detection,RM 1 1 3 31 Child Units,,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-1-1-3-20-error-detection}@anchor{1bc}
@section RM 1.1.3(20): Error Detection


@quotation

"If an implementation detects the use of an unsupported Specialized Needs
Annex feature at run time, it should raise @cite{Program_Error} if
feasible."
@end quotation

Not relevant.  All specialized needs annex features are either supported,
or diagnosed at compile time.

@geindex Child Units

@node RM 1 1 3 31 Child Units,RM 1 1 5 12 Bounded Errors,RM 1 1 3 20 Error Detection,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-1-1-3-31-child-units}@anchor{1bd}
@section RM 1.1.3(31): Child Units


@quotation

"If an implementation wishes to provide implementation-defined
extensions to the functionality of a language-defined library unit, it
should normally do so by adding children to the library unit."
@end quotation

Followed.

@geindex Bounded errors

@node RM 1 1 5 12 Bounded Errors,RM 2 8 16 Pragmas,RM 1 1 3 31 Child Units,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-1-1-5-12-bounded-errors}@anchor{1be}
@section RM 1.1.5(12): Bounded Errors


@quotation

"If an implementation detects a bounded error or erroneous
execution, it should raise @cite{Program_Error}."
@end quotation

Followed in all cases in which the implementation detects a bounded
error or erroneous execution.  Not all such situations are detected at
runtime.

@geindex Pragmas

@node RM 2 8 16 Pragmas,RM 2 8 17-19 Pragmas,RM 1 1 5 12 Bounded Errors,Implementation Advice
@anchor{gnat_rm/implementation_advice id2}@anchor{1bf}@anchor{gnat_rm/implementation_advice rm-2-8-16-pragmas}@anchor{1c0}
@section RM 2.8(16): Pragmas


@quotation

"Normally, implementation-defined pragmas should have no semantic effect
for error-free programs; that is, if the implementation-defined pragmas
are removed from a working program, the program should still be legal,
and should still have the same semantics."
@end quotation

The following implementation defined pragmas are exceptions to this
rule:


@multitable {xxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxx} 
@headitem

Pragma

@tab

Explanation

@item

@emph{Abort_Defer}

@tab

Affects semantics

@item

@emph{Ada_83}

@tab

Affects legality

@item

@emph{Assert}

@tab

Affects semantics

@item

@emph{CPP_Class}

@tab

Affects semantics

@item

@emph{CPP_Constructor}

@tab

Affects semantics

@item

@emph{Debug}

@tab

Affects semantics

@item

@emph{Interface_Name}

@tab

Affects semantics

@item

@emph{Machine_Attribute}

@tab

Affects semantics

@item

@emph{Unimplemented_Unit}

@tab

Affects legality

@item

@emph{Unchecked_Union}

@tab

Affects semantics

@end multitable


In each of the above cases, it is essential to the purpose of the pragma
that this advice not be followed.  For details see
@ref{7,,Implementation Defined Pragmas}.

@node RM 2 8 17-19 Pragmas,RM 3 5 2 5 Alternative Character Sets,RM 2 8 16 Pragmas,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-2-8-17-19-pragmas}@anchor{1c1}
@section RM 2.8(17-19): Pragmas


@quotation

"Normally, an implementation should not define pragmas that can
make an illegal program legal, except as follows:


@itemize *

@item 
A pragma used to complete a declaration, such as a pragma @cite{Import};

@item 
A pragma used to configure the environment by adding, removing, or
replacing @cite{library_items}."
@end itemize
@end quotation

See @ref{1c0,,RM 2.8(16); Pragmas}.

@geindex Character Sets

@geindex Alternative Character Sets

@node RM 3 5 2 5 Alternative Character Sets,RM 3 5 4 28 Integer Types,RM 2 8 17-19 Pragmas,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-2-5-alternative-character-sets}@anchor{1c2}
@section RM 3.5.2(5): Alternative Character Sets


@quotation

"If an implementation supports a mode with alternative interpretations
for @cite{Character} and @cite{Wide_Character}, the set of graphic
characters of @cite{Character} should nevertheless remain a proper
subset of the set of graphic characters of @cite{Wide_Character}.  Any
character set 'localizations' should be reflected in the results of
the subprograms defined in the language-defined package
@cite{Characters.Handling} (see A.3) available in such a mode.  In a mode with
an alternative interpretation of @cite{Character}, the implementation should
also support a corresponding change in what is a legal
@cite{identifier_letter}."
@end quotation

Not all wide character modes follow this advice, in particular the JIS
and IEC modes reflect standard usage in Japan, and in these encoding,
the upper half of the Latin-1 set is not part of the wide-character
subset, since the most significant bit is used for wide character
encoding.  However, this only applies to the external forms.  Internally
there is no such restriction.

@geindex Integer types

@node RM 3 5 4 28 Integer Types,RM 3 5 4 29 Integer Types,RM 3 5 2 5 Alternative Character Sets,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-4-28-integer-types}@anchor{1c3}
@section RM 3.5.4(28): Integer Types


@quotation

"An implementation should support @cite{Long_Integer} in addition to
@cite{Integer} if the target machine supports 32-bit (or longer)
arithmetic.  No other named integer subtypes are recommended for package
@cite{Standard}.  Instead, appropriate named integer subtypes should be
provided in the library package @cite{Interfaces} (see B.2)."
@end quotation

@cite{Long_Integer} is supported.  Other standard integer types are supported
so this advice is not fully followed.  These types
are supported for convenient interface to C, and so that all hardware
types of the machine are easily available.

@node RM 3 5 4 29 Integer Types,RM 3 5 5 8 Enumeration Values,RM 3 5 4 28 Integer Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-4-29-integer-types}@anchor{1c4}
@section RM 3.5.4(29): Integer Types


@quotation

"An implementation for a two's complement machine should support
modular types with a binary modulus up to @code{System.Max_Int*2+2}.  An
implementation should support a non-binary modules up to @cite{Integer'Last}."
@end quotation

Followed.

@geindex Enumeration values

@node RM 3 5 5 8 Enumeration Values,RM 3 5 7 17 Float Types,RM 3 5 4 29 Integer Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-5-8-enumeration-values}@anchor{1c5}
@section RM 3.5.5(8): Enumeration Values


@quotation

"For the evaluation of a call on @code{S'Pos} for an enumeration
subtype, if the value of the operand does not correspond to the internal
code for any enumeration literal of its type (perhaps due to an
un-initialized variable), then the implementation should raise
@cite{Program_Error}.  This is particularly important for enumeration
types with noncontiguous internal codes specified by an
enumeration_representation_clause."
@end quotation

Followed.

@geindex Float types

@node RM 3 5 7 17 Float Types,RM 3 6 2 11 Multidimensional Arrays,RM 3 5 5 8 Enumeration Values,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-5-7-17-float-types}@anchor{1c6}
@section RM 3.5.7(17): Float Types


@quotation

"An implementation should support @cite{Long_Float} in addition to
@cite{Float} if the target machine supports 11 or more digits of
precision.  No other named floating point subtypes are recommended for
package @cite{Standard}.  Instead, appropriate named floating point subtypes
should be provided in the library package @cite{Interfaces} (see B.2)."
@end quotation

@cite{Short_Float} and @cite{Long_Long_Float} are also provided.  The
former provides improved compatibility with other implementations
supporting this type.  The latter corresponds to the highest precision
floating-point type supported by the hardware.  On most machines, this
will be the same as @cite{Long_Float}, but on some machines, it will
correspond to the IEEE extended form.  The notable case is all ia32
(x86) implementations, where @cite{Long_Long_Float} corresponds to
the 80-bit extended precision format supported in hardware on this
processor.  Note that the 128-bit format on SPARC is not supported,
since this is a software rather than a hardware format.

@geindex Multidimensional arrays

@geindex Arrays
@geindex multidimensional

@node RM 3 6 2 11 Multidimensional Arrays,RM 9 6 30-31 Duration'Small,RM 3 5 7 17 Float Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-3-6-2-11-multidimensional-arrays}@anchor{1c7}
@section RM 3.6.2(11): Multidimensional Arrays


@quotation

"An implementation should normally represent multidimensional arrays in
row-major order, consistent with the notation used for multidimensional
array aggregates (see 4.3.3).  However, if a pragma @cite{Convention}
(@cite{Fortran}, ...) applies to a multidimensional array type, then
column-major order should be used instead (see B.5, @cite{Interfacing with Fortran})."
@end quotation

Followed.

@geindex Duration'Small

@node RM 9 6 30-31 Duration'Small,RM 10 2 1 12 Consistent Representation,RM 3 6 2 11 Multidimensional Arrays,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-9-6-30-31-duration-small}@anchor{1c8}
@section RM 9.6(30-31): Duration'Small


@quotation

"Whenever possible in an implementation, the value of @cite{Duration'Small}
should be no greater than 100 microseconds."
@end quotation

Followed.  (@cite{Duration'Small} = 10**(-9)).

@quotation

"The time base for @cite{delay_relative_statements} should be monotonic;
it need not be the same time base as used for @cite{Calendar.Clock}."
@end quotation

Followed.

@node RM 10 2 1 12 Consistent Representation,RM 11 4 1 19 Exception Information,RM 9 6 30-31 Duration'Small,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-10-2-1-12-consistent-representation}@anchor{1c9}
@section RM 10.2.1(12): Consistent Representation


@quotation

"In an implementation, a type declared in a pre-elaborated package should
have the same representation in every elaboration of a given version of
the package, whether the elaborations occur in distinct executions of
the same program, or in executions of distinct programs or partitions
that include the given version."
@end quotation

Followed, except in the case of tagged types.  Tagged types involve
implicit pointers to a local copy of a dispatch table, and these pointers
have representations which thus depend on a particular elaboration of the
package.  It is not easy to see how it would be possible to follow this
advice without severely impacting efficiency of execution.

@geindex Exception information

@node RM 11 4 1 19 Exception Information,RM 11 5 28 Suppression of Checks,RM 10 2 1 12 Consistent Representation,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-11-4-1-19-exception-information}@anchor{1ca}
@section RM 11.4.1(19): Exception Information


@quotation

"@cite{Exception_Message} by default and @cite{Exception_Information}
should produce information useful for
debugging.  @cite{Exception_Message} should be short, about one
line.  @cite{Exception_Information} can be long.  @cite{Exception_Message}
should not include the
@cite{Exception_Name}.  @cite{Exception_Information} should include both
the @cite{Exception_Name} and the @cite{Exception_Message}."
@end quotation

Followed.  For each exception that doesn't have a specified
@cite{Exception_Message}, the compiler generates one containing the location
of the raise statement.  This location has the form 'file_name:line', where
file_name is the short file name (without path information) and line is the line
number in the file.  Note that in the case of the Zero Cost Exception
mechanism, these messages become redundant with the Exception_Information that
contains a full backtrace of the calling sequence, so they are disabled.
To disable explicitly the generation of the source location message, use the
Pragma @cite{Discard_Names}.

@geindex Suppression of checks

@geindex Checks
@geindex suppression of

@node RM 11 5 28 Suppression of Checks,RM 13 1 21-24 Representation Clauses,RM 11 4 1 19 Exception Information,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-11-5-28-suppression-of-checks}@anchor{1cb}
@section RM 11.5(28): Suppression of Checks


@quotation

"The implementation should minimize the code executed for checks that
have been suppressed."
@end quotation

Followed.

@geindex Representation clauses

@node RM 13 1 21-24 Representation Clauses,RM 13 2 6-8 Packed Types,RM 11 5 28 Suppression of Checks,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-1-21-24-representation-clauses}@anchor{1cc}
@section RM 13.1 (21-24): Representation Clauses


@quotation

"The recommended level of support for all representation items is
qualified as follows:

An implementation need not support representation items containing
non-static expressions, except that an implementation should support a
representation item for a given entity if each non-static expression in
the representation item is a name that statically denotes a constant
declared before the entity."
@end quotation

Followed.  In fact, GNAT goes beyond the recommended level of support
by allowing nonstatic expressions in some representation clauses even
without the need to declare constants initialized with the values of
such expressions.
For example:

@example
  X : Integer;
  Y : Float;
  for Y'Address use X'Address;>>


"An implementation need not support a specification for the `Size`
for a given composite subtype, nor the size or storage place for an
object (including a component) of a given composite subtype, unless the
constraints on the subtype and its composite subcomponents (if any) are
all static constraints."
@end example

Followed.  Size Clauses are not permitted on non-static components, as
described above.

@quotation

"An aliased component, or a component whose type is by-reference, should
always be allocated at an addressable location."
@end quotation

Followed.

@geindex Packed types

@node RM 13 2 6-8 Packed Types,RM 13 3 14-19 Address Clauses,RM 13 1 21-24 Representation Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-2-6-8-packed-types}@anchor{1cd}
@section RM 13.2(6-8): Packed Types


@quotation

"If a type is packed, then the implementation should try to minimize
storage allocated to objects of the type, possibly at the expense of
speed of accessing components, subject to reasonable complexity in
addressing calculations.

The recommended level of support pragma @cite{Pack} is:

For a packed record type, the components should be packed as tightly as
possible subject to the Sizes of the component subtypes, and subject to
any @cite{record_representation_clause} that applies to the type; the
implementation may, but need not, reorder components or cross aligned
word boundaries to improve the packing.  A component whose @cite{Size} is
greater than the word size may be allocated an integral number of words."
@end quotation

Followed.  Tight packing of arrays is supported for all component sizes
up to 64-bits. If the array component size is 1 (that is to say, if
the component is a boolean type or an enumeration type with two values)
then values of the type are implicitly initialized to zero. This
happens both for objects of the packed type, and for objects that have a
subcomponent of the packed type.

@quotation

"An implementation should support Address clauses for imported
subprograms."
@end quotation

Followed.

@geindex Address clauses

@node RM 13 3 14-19 Address Clauses,RM 13 3 29-35 Alignment Clauses,RM 13 2 6-8 Packed Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-14-19-address-clauses}@anchor{1ce}
@section RM 13.3(14-19): Address Clauses


@quotation

"For an array @cite{X}, @code{X'Address} should point at the first
component of the array, and not at the array bounds."
@end quotation

Followed.

@quotation

"The recommended level of support for the @cite{Address} attribute is:

@code{X'Address} should produce a useful result if @cite{X} is an
object that is aliased or of a by-reference type, or is an entity whose
@cite{Address} has been specified."
@end quotation

Followed.  A valid address will be produced even if none of those
conditions have been met.  If necessary, the object is forced into
memory to ensure the address is valid.

@quotation

"An implementation should support @cite{Address} clauses for imported
subprograms."
@end quotation

Followed.

@quotation

"Objects (including subcomponents) that are aliased or of a by-reference
type should be allocated on storage element boundaries."
@end quotation

Followed.

@quotation

"If the @cite{Address} of an object is specified, or it is imported or exported,
then the implementation should not perform optimizations based on
assumptions of no aliases."
@end quotation

Followed.

@geindex Alignment clauses

@node RM 13 3 29-35 Alignment Clauses,RM 13 3 42-43 Size Clauses,RM 13 3 14-19 Address Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-29-35-alignment-clauses}@anchor{1cf}
@section RM 13.3(29-35): Alignment Clauses


@quotation

"The recommended level of support for the @cite{Alignment} attribute for
subtypes is:

An implementation should support specified Alignments that are factors
and multiples of the number of storage elements per word, subject to the
following:"
@end quotation

Followed.

@quotation

"An implementation need not support specified Alignments for
combinations of Sizes and Alignments that cannot be easily
loaded and stored by available machine instructions."
@end quotation

Followed.

@quotation

"An implementation need not support specified Alignments that are
greater than the maximum @cite{Alignment} the implementation ever returns by
default."
@end quotation

Followed.

@quotation

"The recommended level of support for the @cite{Alignment} attribute for
objects is:

Same as above, for subtypes, but in addition:"
@end quotation

Followed.

@quotation

"For stand-alone library-level objects of statically constrained
subtypes, the implementation should support all alignments
supported by the target linker.  For example, page alignment is likely to
be supported for such objects, but not for subtypes."
@end quotation

Followed.

@geindex Size clauses

@node RM 13 3 42-43 Size Clauses,RM 13 3 50-56 Size Clauses,RM 13 3 29-35 Alignment Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-42-43-size-clauses}@anchor{1d0}
@section RM 13.3(42-43): Size Clauses


@quotation

"The recommended level of support for the @cite{Size} attribute of
objects is:

A @cite{Size} clause should be supported for an object if the specified
@cite{Size} is at least as large as its subtype's @cite{Size}, and
corresponds to a size in storage elements that is a multiple of the
object's @cite{Alignment} (if the @cite{Alignment} is nonzero)."
@end quotation

Followed.

@node RM 13 3 50-56 Size Clauses,RM 13 3 71-73 Component Size Clauses,RM 13 3 42-43 Size Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-50-56-size-clauses}@anchor{1d1}
@section RM 13.3(50-56): Size Clauses


@quotation

"If the @cite{Size} of a subtype is specified, and allows for efficient
independent addressability (see 9.10) on the target architecture, then
the @cite{Size} of the following objects of the subtype should equal the
@cite{Size} of the subtype:

Aliased objects (including components)."
@end quotation

Followed.

@quotation

"@cite{Size} clause on a composite subtype should not affect the
internal layout of components."
@end quotation

Followed. But note that this can be overridden by use of the implementation
pragma Implicit_Packing in the case of packed arrays.

@quotation

"The recommended level of support for the @cite{Size} attribute of subtypes is:

The @cite{Size} (if not specified) of a static discrete or fixed point
subtype should be the number of bits needed to represent each value
belonging to the subtype using an unbiased representation, leaving space
for a sign bit only if the subtype contains negative values.  If such a
subtype is a first subtype, then an implementation should support a
specified @cite{Size} for it that reflects this representation."
@end quotation

Followed.

@quotation

"For a subtype implemented with levels of indirection, the @cite{Size}
should include the size of the pointers, but not the size of what they
point at."
@end quotation

Followed.

@geindex Component_Size clauses

@node RM 13 3 71-73 Component Size Clauses,RM 13 4 9-10 Enumeration Representation Clauses,RM 13 3 50-56 Size Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-3-71-73-component-size-clauses}@anchor{1d2}
@section RM 13.3(71-73): Component Size Clauses


@quotation

"The recommended level of support for the @cite{Component_Size}
attribute is:

An implementation need not support specified @cite{Component_Sizes} that are
less than the @cite{Size} of the component subtype."
@end quotation

Followed.

@quotation

"An implementation should support specified Component_Sizes that
are factors and multiples of the word size.  For such
Component_Sizes, the array should contain no gaps between
components.  For other Component_Sizes (if supported), the array
should contain no gaps between components when packing is also
specified; the implementation should forbid this combination in cases
where it cannot support a no-gaps representation."
@end quotation

Followed.

@geindex Enumeration representation clauses

@geindex Representation clauses
@geindex enumeration

@node RM 13 4 9-10 Enumeration Representation Clauses,RM 13 5 1 17-22 Record Representation Clauses,RM 13 3 71-73 Component Size Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-4-9-10-enumeration-representation-clauses}@anchor{1d3}
@section RM 13.4(9-10): Enumeration Representation Clauses


@quotation

"The recommended level of support for enumeration representation clauses
is:

An implementation need not support enumeration representation clauses
for boolean types, but should at minimum support the internal codes in
the range @cite{System.Min_Int .. System.Max_Int}."
@end quotation

Followed.

@geindex Record representation clauses

@geindex Representation clauses
@geindex records

@node RM 13 5 1 17-22 Record Representation Clauses,RM 13 5 2 5 Storage Place Attributes,RM 13 4 9-10 Enumeration Representation Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-5-1-17-22-record-representation-clauses}@anchor{1d4}
@section RM 13.5.1(17-22): Record Representation Clauses


@quotation

"The recommended level of support for
@cite{record_representation_clauses} is:

An implementation should support storage places that can be extracted
with a load, mask, shift sequence of machine code, and set with a load,
shift, mask, store sequence, given the available machine instructions
and run-time model."
@end quotation

Followed.

@quotation

"A storage place should be supported if its size is equal to the
@cite{Size} of the component subtype, and it starts and ends on a
boundary that obeys the @cite{Alignment} of the component subtype."
@end quotation

Followed.

@quotation

"If the default bit ordering applies to the declaration of a given type,
then for a component whose subtype's @cite{Size} is less than the word
size, any storage place that does not cross an aligned word boundary
should be supported."
@end quotation

Followed.

@quotation

"An implementation may reserve a storage place for the tag field of a
tagged type, and disallow other components from overlapping that place."
@end quotation

Followed.  The storage place for the tag field is the beginning of the tagged
record, and its size is Address'Size.  GNAT will reject an explicit component
clause for the tag field.

@quotation

"An implementation need not support a @cite{component_clause} for a
component of an extension part if the storage place is not after the
storage places of all components of the parent type, whether or not
those storage places had been specified."
@end quotation

Followed.  The above advice on record representation clauses is followed,
and all mentioned features are implemented.

@geindex Storage place attributes

@node RM 13 5 2 5 Storage Place Attributes,RM 13 5 3 7-8 Bit Ordering,RM 13 5 1 17-22 Record Representation Clauses,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-5-2-5-storage-place-attributes}@anchor{1d5}
@section RM 13.5.2(5): Storage Place Attributes


@quotation

"If a component is represented using some form of pointer (such as an
offset) to the actual data of the component, and this data is contiguous
with the rest of the object, then the storage place attributes should
reflect the place of the actual data, not the pointer.  If a component is
allocated discontinuously from the rest of the object, then a warning
should be generated upon reference to one of its storage place
attributes."
@end quotation

Followed.  There are no such components in GNAT.

@geindex Bit ordering

@node RM 13 5 3 7-8 Bit Ordering,RM 13 7 37 Address as Private,RM 13 5 2 5 Storage Place Attributes,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-5-3-7-8-bit-ordering}@anchor{1d6}
@section RM 13.5.3(7-8): Bit Ordering


@quotation

"The recommended level of support for the non-default bit ordering is:

If @cite{Word_Size} = @cite{Storage_Unit}, then the implementation
should support the non-default bit ordering in addition to the default
bit ordering."
@end quotation

Followed.  Word size does not equal storage size in this implementation.
Thus non-default bit ordering is not supported.

@geindex Address
@geindex as private type

@node RM 13 7 37 Address as Private,RM 13 7 1 16 Address Operations,RM 13 5 3 7-8 Bit Ordering,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-7-37-address-as-private}@anchor{1d7}
@section RM 13.7(37): Address as Private


@quotation

"@cite{Address} should be of a private type."
@end quotation

Followed.

@geindex Operations
@geindex on `Address`

@geindex Address
@geindex operations of

@node RM 13 7 1 16 Address Operations,RM 13 9 14-17 Unchecked Conversion,RM 13 7 37 Address as Private,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-7-1-16-address-operations}@anchor{1d8}
@section RM 13.7.1(16): Address Operations


@quotation

"Operations in @cite{System} and its children should reflect the target
environment semantics as closely as is reasonable.  For example, on most
machines, it makes sense for address arithmetic to 'wrap around'.
Operations that do not make sense should raise @cite{Program_Error}."
@end quotation

Followed.  Address arithmetic is modular arithmetic that wraps around.  No
operation raises @cite{Program_Error}, since all operations make sense.

@geindex Unchecked conversion

@node RM 13 9 14-17 Unchecked Conversion,RM 13 11 23-25 Implicit Heap Usage,RM 13 7 1 16 Address Operations,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-9-14-17-unchecked-conversion}@anchor{1d9}
@section RM 13.9(14-17): Unchecked Conversion


@quotation

"The @cite{Size} of an array object should not include its bounds; hence,
the bounds should not be part of the converted data."
@end quotation

Followed.

@quotation

"The implementation should not generate unnecessary run-time checks to
ensure that the representation of @cite{S} is a representation of the
target type.  It should take advantage of the permission to return by
reference when possible.  Restrictions on unchecked conversions should be
avoided unless required by the target environment."
@end quotation

Followed.  There are no restrictions on unchecked conversion.  A warning is
generated if the source and target types do not have the same size since
the semantics in this case may be target dependent.

@quotation

"The recommended level of support for unchecked conversions is:

Unchecked conversions should be supported and should be reversible in
the cases where this clause defines the result.  To enable meaningful use
of unchecked conversion, a contiguous representation should be used for
elementary subtypes, for statically constrained array subtypes whose
component subtype is one of the subtypes described in this paragraph,
and for record subtypes without discriminants whose component subtypes
are described in this paragraph."
@end quotation

Followed.

@geindex Heap usage
@geindex implicit

@node RM 13 11 23-25 Implicit Heap Usage,RM 13 11 2 17 Unchecked Deallocation,RM 13 9 14-17 Unchecked Conversion,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-11-23-25-implicit-heap-usage}@anchor{1da}
@section RM 13.11(23-25): Implicit Heap Usage


@quotation

"An implementation should document any cases in which it dynamically
allocates heap storage for a purpose other than the evaluation of an
allocator."
@end quotation

Followed, the only other points at which heap storage is dynamically
allocated are as follows:


@itemize *

@item 
At initial elaboration time, to allocate dynamically sized global
objects.

@item 
To allocate space for a task when a task is created.

@item 
To extend the secondary stack dynamically when needed.  The secondary
stack is used for returning variable length results.
@end itemize


@quotation

"A default (implementation-provided) storage pool for an
access-to-constant type should not have overhead to support deallocation of
individual objects."
@end quotation

Followed.

@quotation

"A storage pool for an anonymous access type should be created at the
point of an allocator for the type, and be reclaimed when the designated
object becomes inaccessible."
@end quotation

Followed.

@geindex Unchecked deallocation

@node RM 13 11 2 17 Unchecked Deallocation,RM 13 13 2 17 Stream Oriented Attributes,RM 13 11 23-25 Implicit Heap Usage,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-11-2-17-unchecked-deallocation}@anchor{1db}
@section RM 13.11.2(17): Unchecked Deallocation


@quotation

"For a standard storage pool, @cite{Free} should actually reclaim the
storage."
@end quotation

Followed.

@geindex Stream oriented attributes

@node RM 13 13 2 17 Stream Oriented Attributes,RM A 1 52 Names of Predefined Numeric Types,RM 13 11 2 17 Unchecked Deallocation,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-13-13-2-17-stream-oriented-attributes}@anchor{1dc}
@section RM 13.13.2(17): Stream Oriented Attributes


@quotation

"If a stream element is the same size as a storage element, then the
normal in-memory representation should be used by @cite{Read} and
@cite{Write} for scalar objects.  Otherwise, @cite{Read} and @cite{Write}
should use the smallest number of stream elements needed to represent
all values in the base range of the scalar type."
@end quotation

Followed.  By default, GNAT uses the interpretation suggested by AI-195,
which specifies using the size of the first subtype.
However, such an implementation is based on direct binary
representations and is therefore target- and endianness-dependent.
To address this issue, GNAT also supplies an alternate implementation
of the stream attributes @cite{Read} and @cite{Write},
which uses the target-independent XDR standard representation
for scalar types.

@geindex XDR representation

@geindex Read attribute

@geindex Write attribute

@geindex Stream oriented attributes

The XDR implementation is provided as an alternative body of the
@cite{System.Stream_Attributes} package, in the file
@code{s-stratt-xdr.adb} in the GNAT library.
There is no @code{s-stratt-xdr.ads} file.
In order to install the XDR implementation, do the following:


@itemize *

@item 
Replace the default implementation of the
@cite{System.Stream_Attributes} package with the XDR implementation.
For example on a Unix platform issue the commands:

@example
$ mv s-stratt.adb s-stratt-default.adb
$ mv s-stratt-xdr.adb s-stratt.adb
@end example

@item 
Rebuild the GNAT run-time library as documented in
the @cite{GNAT and Libraries} section of the @cite{GNAT User's Guide}.
@end itemize

@node RM A 1 52 Names of Predefined Numeric Types,RM A 3 2 49 Ada Characters Handling,RM 13 13 2 17 Stream Oriented Attributes,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-1-52-names-of-predefined-numeric-types}@anchor{1dd}
@section RM A.1(52): Names of Predefined Numeric Types


@quotation

"If an implementation provides additional named predefined integer types,
then the names should end with @code{Integer} as in
@code{Long_Integer}.  If an implementation provides additional named
predefined floating point types, then the names should end with
@code{Float} as in @code{Long_Float}."
@end quotation

Followed.

@geindex Ada.Characters.Handling

@node RM A 3 2 49 Ada Characters Handling,RM A 4 4 106 Bounded-Length String Handling,RM A 1 52 Names of Predefined Numeric Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-3-2-49-ada-characters-handling}@anchor{1de}
@section RM A.3.2(49): @cite{Ada.Characters.Handling}


@quotation

"If an implementation provides a localized definition of @cite{Character}
or @cite{Wide_Character}, then the effects of the subprograms in
@cite{Characters.Handling} should reflect the localizations.
See also 3.5.2."
@end quotation

Followed.  GNAT provides no such localized definitions.

@geindex Bounded-length strings

@node RM A 4 4 106 Bounded-Length String Handling,RM A 5 2 46-47 Random Number Generation,RM A 3 2 49 Ada Characters Handling,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-4-4-106-bounded-length-string-handling}@anchor{1df}
@section RM A.4.4(106): Bounded-Length String Handling


@quotation

"Bounded string objects should not be implemented by implicit pointers
and dynamic allocation."
@end quotation

Followed.  No implicit pointers or dynamic allocation are used.

@geindex Random number generation

@node RM A 5 2 46-47 Random Number Generation,RM A 10 7 23 Get_Immediate,RM A 4 4 106 Bounded-Length String Handling,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-5-2-46-47-random-number-generation}@anchor{1e0}
@section RM A.5.2(46-47): Random Number Generation


@quotation

"Any storage associated with an object of type @cite{Generator} should be
reclaimed on exit from the scope of the object."
@end quotation

Followed.

@quotation

"If the generator period is sufficiently long in relation to the number
of distinct initiator values, then each possible value of
@cite{Initiator} passed to @cite{Reset} should initiate a sequence of
random numbers that does not, in a practical sense, overlap the sequence
initiated by any other value.  If this is not possible, then the mapping
between initiator values and generator states should be a rapidly
varying function of the initiator value."
@end quotation

Followed.  The generator period is sufficiently long for the first
condition here to hold true.

@geindex Get_Immediate

@node RM A 10 7 23 Get_Immediate,RM B 1 39-41 Pragma Export,RM A 5 2 46-47 Random Number Generation,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-a-10-7-23-get-immediate}@anchor{1e1}
@section RM A.10.7(23): @cite{Get_Immediate}


@quotation

"The @cite{Get_Immediate} procedures should be implemented with
unbuffered input.  For a device such as a keyboard, input should be
available if a key has already been typed, whereas for a disk
file, input should always be available except at end of file.  For a file
associated with a keyboard-like device, any line-editing features of the
underlying operating system should be disabled during the execution of
@cite{Get_Immediate}."
@end quotation

Followed on all targets except VxWorks. For VxWorks, there is no way to
provide this functionality that does not result in the input buffer being
flushed before the @cite{Get_Immediate} call. A special unit
@cite{Interfaces.Vxworks.IO} is provided that contains routines to enable
this functionality.

@geindex Export

@node RM B 1 39-41 Pragma Export,RM B 2 12-13 Package Interfaces,RM A 10 7 23 Get_Immediate,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-1-39-41-pragma-export}@anchor{1e2}
@section RM B.1(39-41): Pragma @cite{Export}


@quotation

"If an implementation supports pragma @cite{Export} to a given language,
then it should also allow the main subprogram to be written in that
language.  It should support some mechanism for invoking the elaboration
of the Ada library units included in the system, and for invoking the
finalization of the environment task.  On typical systems, the
recommended mechanism is to provide two subprograms whose link names are
@cite{adainit} and @cite{adafinal}.  @cite{adainit} should contain the
elaboration code for library units.  @cite{adafinal} should contain the
finalization code.  These subprograms should have no effect the second
and subsequent time they are called."
@end quotation

Followed.

@quotation

"Automatic elaboration of pre-elaborated packages should be
provided when pragma @cite{Export} is supported."
@end quotation

Followed when the main program is in Ada.  If the main program is in a
foreign language, then
@cite{adainit} must be called to elaborate pre-elaborated
packages.

@quotation

"For each supported convention @cite{L} other than @cite{Intrinsic}, an
implementation should support @cite{Import} and @cite{Export} pragmas
for objects of @cite{L}-compatible types and for subprograms, and pragma
@cite{Convention} for @cite{L}-eligible types and for subprograms,
presuming the other language has corresponding features.  Pragma
@cite{Convention} need not be supported for scalar types."
@end quotation

Followed.

@geindex Package Interfaces

@geindex Interfaces

@node RM B 2 12-13 Package Interfaces,RM B 3 63-71 Interfacing with C,RM B 1 39-41 Pragma Export,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-2-12-13-package-interfaces}@anchor{1e3}
@section RM B.2(12-13): Package @cite{Interfaces}


@quotation

"For each implementation-defined convention identifier, there should be a
child package of package Interfaces with the corresponding name.  This
package should contain any declarations that would be useful for
interfacing to the language (implementation) represented by the
convention.  Any declarations useful for interfacing to any language on
the given hardware architecture should be provided directly in
@cite{Interfaces}."
@end quotation

Followed.

@quotation

"An implementation supporting an interface to C, COBOL, or Fortran should
provide the corresponding package or packages described in the following
clauses."
@end quotation

Followed.  GNAT provides all the packages described in this section.

@geindex C
@geindex interfacing with

@node RM B 3 63-71 Interfacing with C,RM B 4 95-98 Interfacing with COBOL,RM B 2 12-13 Package Interfaces,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-3-63-71-interfacing-with-c}@anchor{1e4}
@section RM B.3(63-71): Interfacing with C


@quotation

"An implementation should support the following interface correspondences
between Ada and C."
@end quotation

Followed.

@quotation

"An Ada procedure corresponds to a void-returning C function."
@end quotation

Followed.

@quotation

"An Ada function corresponds to a non-void C function."
@end quotation

Followed.

@quotation

"An Ada @cite{in} scalar parameter is passed as a scalar argument to a C
function."
@end quotation

Followed.

@quotation

"An Ada @cite{in} parameter of an access-to-object type with designated
type @cite{T} is passed as a @code{t*} argument to a C function,
where @code{t} is the C type corresponding to the Ada type @cite{T}."
@end quotation

Followed.

@quotation

"An Ada access @cite{T} parameter, or an Ada @cite{out} or @cite{in out}
parameter of an elementary type @cite{T}, is passed as a @code{t*}
argument to a C function, where @code{t} is the C type corresponding to
the Ada type @cite{T}.  In the case of an elementary @cite{out} or
@cite{in out} parameter, a pointer to a temporary copy is used to
preserve by-copy semantics."
@end quotation

Followed.

@quotation

"An Ada parameter of a record type @cite{T}, of any mode, is passed as a
@code{t*} argument to a C function, where @code{t} is the C
structure corresponding to the Ada type @cite{T}."
@end quotation

Followed.  This convention may be overridden by the use of the C_Pass_By_Copy
pragma, or Convention, or by explicitly specifying the mechanism for a given
call using an extended import or export pragma.

@quotation

"An Ada parameter of an array type with component type @cite{T}, of any
mode, is passed as a @code{t*} argument to a C function, where
@code{t} is the C type corresponding to the Ada type @cite{T}."
@end quotation

Followed.

@quotation

"An Ada parameter of an access-to-subprogram type is passed as a pointer
to a C function whose prototype corresponds to the designated
subprogram's specification."
@end quotation

Followed.

@geindex COBOL
@geindex interfacing with

@node RM B 4 95-98 Interfacing with COBOL,RM B 5 22-26 Interfacing with Fortran,RM B 3 63-71 Interfacing with C,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-4-95-98-interfacing-with-cobol}@anchor{1e5}
@section RM B.4(95-98): Interfacing with COBOL


@quotation

"An Ada implementation should support the following interface
correspondences between Ada and COBOL."
@end quotation

Followed.

@quotation

"An Ada access @cite{T} parameter is passed as a @code{BY REFERENCE} data item of
the COBOL type corresponding to @cite{T}."
@end quotation

Followed.

@quotation

"An Ada in scalar parameter is passed as a @code{BY CONTENT} data item of
the corresponding COBOL type."
@end quotation

Followed.

@quotation

"Any other Ada parameter is passed as a @code{BY REFERENCE} data item of the
COBOL type corresponding to the Ada parameter type; for scalars, a local
copy is used if necessary to ensure by-copy semantics."
@end quotation

Followed.

@geindex Fortran
@geindex interfacing with

@node RM B 5 22-26 Interfacing with Fortran,RM C 1 3-5 Access to Machine Operations,RM B 4 95-98 Interfacing with COBOL,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-b-5-22-26-interfacing-with-fortran}@anchor{1e6}
@section RM B.5(22-26): Interfacing with Fortran


@quotation

"An Ada implementation should support the following interface
correspondences between Ada and Fortran:"
@end quotation

Followed.

@quotation

"An Ada procedure corresponds to a Fortran subroutine."
@end quotation

Followed.

@quotation

"An Ada function corresponds to a Fortran function."
@end quotation

Followed.

@quotation

"An Ada parameter of an elementary, array, or record type @cite{T} is
passed as a @cite{T} argument to a Fortran procedure, where @cite{T} is
the Fortran type corresponding to the Ada type @cite{T}, and where the
INTENT attribute of the corresponding dummy argument matches the Ada
formal parameter mode; the Fortran implementation's parameter passing
conventions are used.  For elementary types, a local copy is used if
necessary to ensure by-copy semantics."
@end quotation

Followed.

@quotation

"An Ada parameter of an access-to-subprogram type is passed as a
reference to a Fortran procedure whose interface corresponds to the
designated subprogram's specification."
@end quotation

Followed.

@geindex Machine operations

@node RM C 1 3-5 Access to Machine Operations,RM C 1 10-16 Access to Machine Operations,RM B 5 22-26 Interfacing with Fortran,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-1-3-5-access-to-machine-operations}@anchor{1e7}
@section RM C.1(3-5): Access to Machine Operations


@quotation

"The machine code or intrinsic support should allow access to all
operations normally available to assembly language programmers for the
target environment, including privileged instructions, if any."
@end quotation

Followed.

@quotation

"The interfacing pragmas (see Annex B) should support interface to
assembler; the default assembler should be associated with the
convention identifier @cite{Assembler}."
@end quotation

Followed.

@quotation

"If an entity is exported to assembly language, then the implementation
should allocate it at an addressable location, and should ensure that it
is retained by the linking process, even if not otherwise referenced
from the Ada code.  The implementation should assume that any call to a
machine code or assembler subprogram is allowed to read or update every
object that is specified as exported."
@end quotation

Followed.

@node RM C 1 10-16 Access to Machine Operations,RM C 3 28 Interrupt Support,RM C 1 3-5 Access to Machine Operations,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-1-10-16-access-to-machine-operations}@anchor{1e8}
@section RM C.1(10-16): Access to Machine Operations


@quotation

"The implementation should ensure that little or no overhead is
associated with calling intrinsic and machine-code subprograms."
@end quotation

Followed for both intrinsics and machine-code subprograms.

@quotation

"It is recommended that intrinsic subprograms be provided for convenient
access to any machine operations that provide special capabilities or
efficiency and that are not otherwise available through the language
constructs."
@end quotation

Followed.  A full set of machine operation intrinsic subprograms is provided.

@quotation

"Atomic read-modify-write operations---e.g., test and set, compare and
swap, decrement and test, enqueue/dequeue."
@end quotation

Followed on any target supporting such operations.

@quotation

"Standard numeric functions---e.g.:, sin, log."
@end quotation

Followed on any target supporting such operations.

@quotation

"String manipulation operations---e.g.:, translate and test."
@end quotation

Followed on any target supporting such operations.

@quotation

"Vector operations---e.g.:, compare vector against thresholds."
@end quotation

Followed on any target supporting such operations.

@quotation

"Direct operations on I/O ports."
@end quotation

Followed on any target supporting such operations.

@geindex Interrupt support

@node RM C 3 28 Interrupt Support,RM C 3 1 20-21 Protected Procedure Handlers,RM C 1 10-16 Access to Machine Operations,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-3-28-interrupt-support}@anchor{1e9}
@section RM C.3(28): Interrupt Support


@quotation

"If the @cite{Ceiling_Locking} policy is not in effect, the
implementation should provide means for the application to specify which
interrupts are to be blocked during protected actions, if the underlying
system allows for a finer-grain control of interrupt blocking."
@end quotation

Followed.  The underlying system does not allow for finer-grain control
of interrupt blocking.

@geindex Protected procedure handlers

@node RM C 3 1 20-21 Protected Procedure Handlers,RM C 3 2 25 Package Interrupts,RM C 3 28 Interrupt Support,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-3-1-20-21-protected-procedure-handlers}@anchor{1ea}
@section RM C.3.1(20-21): Protected Procedure Handlers


@quotation

"Whenever possible, the implementation should allow interrupt handlers to
be called directly by the hardware."
@end quotation

Followed on any target where the underlying operating system permits
such direct calls.

@quotation

"Whenever practical, violations of any
implementation-defined restrictions should be detected before run time."
@end quotation

Followed.  Compile time warnings are given when possible.

@geindex Package `Interrupts`

@geindex Interrupts

@node RM C 3 2 25 Package Interrupts,RM C 4 14 Pre-elaboration Requirements,RM C 3 1 20-21 Protected Procedure Handlers,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-3-2-25-package-interrupts}@anchor{1eb}
@section RM C.3.2(25): Package @cite{Interrupts}


@quotation

"If implementation-defined forms of interrupt handler procedures are
supported, such as protected procedures with parameters, then for each
such form of a handler, a type analogous to @cite{Parameterless_Handler}
should be specified in a child package of @cite{Interrupts}, with the
same operations as in the predefined package Interrupts."
@end quotation

Followed.

@geindex Pre-elaboration requirements

@node RM C 4 14 Pre-elaboration Requirements,RM C 5 8 Pragma Discard_Names,RM C 3 2 25 Package Interrupts,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-4-14-pre-elaboration-requirements}@anchor{1ec}
@section RM C.4(14): Pre-elaboration Requirements


@quotation

"It is recommended that pre-elaborated packages be implemented in such a
way that there should be little or no code executed at run time for the
elaboration of entities not already covered by the Implementation
Requirements."
@end quotation

Followed.  Executable code is generated in some cases, e.g., loops
to initialize large arrays.

@node RM C 5 8 Pragma Discard_Names,RM C 7 2 30 The Package Task_Attributes,RM C 4 14 Pre-elaboration Requirements,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-5-8-pragma-discard-names}@anchor{1ed}
@section RM C.5(8): Pragma @cite{Discard_Names}


@quotation

"If the pragma applies to an entity, then the implementation should
reduce the amount of storage used for storing names associated with that
entity."
@end quotation

Followed.

@geindex Package Task_Attributes

@geindex Task_Attributes

@node RM C 7 2 30 The Package Task_Attributes,RM D 3 17 Locking Policies,RM C 5 8 Pragma Discard_Names,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-c-7-2-30-the-package-task-attributes}@anchor{1ee}
@section RM C.7.2(30): The Package Task_Attributes


@quotation

"Some implementations are targeted to domains in which memory use at run
time must be completely deterministic.  For such implementations, it is
recommended that the storage for task attributes will be pre-allocated
statically and not from the heap.  This can be accomplished by either
placing restrictions on the number and the size of the task's
attributes, or by using the pre-allocated storage for the first @cite{N}
attribute objects, and the heap for the others.  In the latter case,
@cite{N} should be documented."
@end quotation

Not followed.  This implementation is not targeted to such a domain.

@geindex Locking Policies

@node RM D 3 17 Locking Policies,RM D 4 16 Entry Queuing Policies,RM C 7 2 30 The Package Task_Attributes,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-3-17-locking-policies}@anchor{1ef}
@section RM D.3(17): Locking Policies


@quotation

"The implementation should use names that end with @code{_Locking} for
locking policies defined by the implementation."
@end quotation

Followed.  Two implementation-defined locking policies are defined,
whose names (@cite{Inheritance_Locking} and
@cite{Concurrent_Readers_Locking}) follow this suggestion.

@geindex Entry queuing policies

@node RM D 4 16 Entry Queuing Policies,RM D 6 9-10 Preemptive Abort,RM D 3 17 Locking Policies,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-4-16-entry-queuing-policies}@anchor{1f0}
@section RM D.4(16): Entry Queuing Policies


@quotation

"Names that end with @code{_Queuing} should be used
for all implementation-defined queuing policies."
@end quotation

Followed.  No such implementation-defined queuing policies exist.

@geindex Preemptive abort

@node RM D 6 9-10 Preemptive Abort,RM D 7 21 Tasking Restrictions,RM D 4 16 Entry Queuing Policies,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-6-9-10-preemptive-abort}@anchor{1f1}
@section RM D.6(9-10): Preemptive Abort


@quotation

"Even though the @cite{abort_statement} is included in the list of
potentially blocking operations (see 9.5.1), it is recommended that this
statement be implemented in a way that never requires the task executing
the @cite{abort_statement} to block."
@end quotation

Followed.

@quotation

"On a multi-processor, the delay associated with aborting a task on
another processor should be bounded; the implementation should use
periodic polling, if necessary, to achieve this."
@end quotation

Followed.

@geindex Tasking restrictions

@node RM D 7 21 Tasking Restrictions,RM D 8 47-49 Monotonic Time,RM D 6 9-10 Preemptive Abort,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-7-21-tasking-restrictions}@anchor{1f2}
@section RM D.7(21): Tasking Restrictions


@quotation

"When feasible, the implementation should take advantage of the specified
restrictions to produce a more efficient implementation."
@end quotation

GNAT currently takes advantage of these restrictions by providing an optimized
run time when the Ravenscar profile and the GNAT restricted run time set
of restrictions are specified.  See pragma @cite{Profile (Ravenscar)} and
pragma @cite{Profile (Restricted)} for more details.

@geindex Time
@geindex monotonic

@node RM D 8 47-49 Monotonic Time,RM E 5 28-29 Partition Communication Subsystem,RM D 7 21 Tasking Restrictions,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-d-8-47-49-monotonic-time}@anchor{1f3}
@section RM D.8(47-49): Monotonic Time


@quotation

"When appropriate, implementations should provide configuration
mechanisms to change the value of @cite{Tick}."
@end quotation

Such configuration mechanisms are not appropriate to this implementation
and are thus not supported.

@quotation

"It is recommended that @cite{Calendar.Clock} and @cite{Real_Time.Clock}
be implemented as transformations of the same time base."
@end quotation

Followed.

@quotation

"It is recommended that the best time base which exists in
the underlying system be available to the application through
@cite{Clock}.  @cite{Best} may mean highest accuracy or largest range."
@end quotation

Followed.

@geindex Partition communication subsystem

@geindex PCS

@node RM E 5 28-29 Partition Communication Subsystem,RM F 7 COBOL Support,RM D 8 47-49 Monotonic Time,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-e-5-28-29-partition-communication-subsystem}@anchor{1f4}
@section RM E.5(28-29): Partition Communication Subsystem


@quotation

"Whenever possible, the PCS on the called partition should allow for
multiple tasks to call the RPC-receiver with different messages and
should allow them to block until the corresponding subprogram body
returns."
@end quotation

Followed by GLADE, a separately supplied PCS that can be used with
GNAT.

@quotation

"The @cite{Write} operation on a stream of type @cite{Params_Stream_Type}
should raise @cite{Storage_Error} if it runs out of space trying to
write the @cite{Item} into the stream."
@end quotation

Followed by GLADE, a separately supplied PCS that can be used with
GNAT.

@geindex COBOL support

@node RM F 7 COBOL Support,RM F 1 2 Decimal Radix Support,RM E 5 28-29 Partition Communication Subsystem,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-f-7-cobol-support}@anchor{1f5}
@section RM F(7): COBOL Support


@quotation

"If COBOL (respectively, C) is widely supported in the target
environment, implementations supporting the Information Systems Annex
should provide the child package @cite{Interfaces.COBOL} (respectively,
@cite{Interfaces.C}) specified in Annex B and should support a
@cite{convention_identifier} of COBOL (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language."
@end quotation

Followed.

@geindex Decimal radix support

@node RM F 1 2 Decimal Radix Support,RM G Numerics,RM F 7 COBOL Support,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-f-1-2-decimal-radix-support}@anchor{1f6}
@section RM F.1(2): Decimal Radix Support


@quotation

"Packed decimal should be used as the internal representation for objects
of subtype @cite{S} when @cite{S}'Machine_Radix = 10."
@end quotation

Not followed.  GNAT ignores @cite{S}'Machine_Radix and always uses binary
representations.

@geindex Numerics

@node RM G Numerics,RM G 1 1 56-58 Complex Types,RM F 1 2 Decimal Radix Support,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-numerics}@anchor{1f7}
@section RM G: Numerics


@quotation

"If Fortran (respectively, C) is widely supported in the target
environment, implementations supporting the Numerics Annex
should provide the child package @cite{Interfaces.Fortran} (respectively,
@cite{Interfaces.C}) specified in Annex B and should support a
@cite{convention_identifier} of Fortran (respectively, C) in the interfacing
pragmas (see Annex B), thus allowing Ada programs to interface with
programs written in that language."
@end quotation

Followed.

@geindex Complex types

@node RM G 1 1 56-58 Complex Types,RM G 1 2 49 Complex Elementary Functions,RM G Numerics,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-1-1-56-58-complex-types}@anchor{1f8}
@section RM G.1.1(56-58): Complex Types


@quotation

"Because the usual mathematical meaning of multiplication of a complex
operand and a real operand is that of the scaling of both components of
the former by the latter, an implementation should not perform this
operation by first promoting the real operand to complex type and then
performing a full complex multiplication.  In systems that, in the
future, support an Ada binding to IEC 559:1989, the latter technique
will not generate the required result when one of the components of the
complex operand is infinite.  (Explicit multiplication of the infinite
component by the zero component obtained during promotion yields a NaN
that propagates into the final result.) Analogous advice applies in the
case of multiplication of a complex operand and a pure-imaginary
operand, and in the case of division of a complex operand by a real or
pure-imaginary operand."
@end quotation

Not followed.

@quotation

"Similarly, because the usual mathematical meaning of addition of a
complex operand and a real operand is that the imaginary operand remains
unchanged, an implementation should not perform this operation by first
promoting the real operand to complex type and then performing a full
complex addition.  In implementations in which the @cite{Signed_Zeros}
attribute of the component type is @cite{True} (and which therefore
conform to IEC 559:1989 in regard to the handling of the sign of zero in
predefined arithmetic operations), the latter technique will not
generate the required result when the imaginary component of the complex
operand is a negatively signed zero.  (Explicit addition of the negative
zero to the zero obtained during promotion yields a positive zero.)
Analogous advice applies in the case of addition of a complex operand
and a pure-imaginary operand, and in the case of subtraction of a
complex operand and a real or pure-imaginary operand."
@end quotation

Not followed.

@quotation

"Implementations in which @cite{Real'Signed_Zeros} is @cite{True} should
attempt to provide a rational treatment of the signs of zero results and
result components.  As one example, the result of the @cite{Argument}
function should have the sign of the imaginary component of the
parameter @cite{X} when the point represented by that parameter lies on
the positive real axis; as another, the sign of the imaginary component
of the @cite{Compose_From_Polar} function should be the same as
(respectively, the opposite of) that of the @cite{Argument} parameter when that
parameter has a value of zero and the @cite{Modulus} parameter has a
nonnegative (respectively, negative) value."
@end quotation

Followed.

@geindex Complex elementary functions

@node RM G 1 2 49 Complex Elementary Functions,RM G 2 4 19 Accuracy Requirements,RM G 1 1 56-58 Complex Types,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-1-2-49-complex-elementary-functions}@anchor{1f9}
@section RM G.1.2(49): Complex Elementary Functions


@quotation

"Implementations in which @cite{Complex_Types.Real'Signed_Zeros} is
@cite{True} should attempt to provide a rational treatment of the signs
of zero results and result components.  For example, many of the complex
elementary functions have components that are odd functions of one of
the parameter components; in these cases, the result component should
have the sign of the parameter component at the origin.  Other complex
elementary functions have zero components whose sign is opposite that of
a parameter component at the origin, or is always positive or always
negative."
@end quotation

Followed.

@geindex Accuracy requirements

@node RM G 2 4 19 Accuracy Requirements,RM G 2 6 15 Complex Arithmetic Accuracy,RM G 1 2 49 Complex Elementary Functions,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-2-4-19-accuracy-requirements}@anchor{1fa}
@section RM G.2.4(19): Accuracy Requirements


@quotation

"The versions of the forward trigonometric functions without a
@cite{Cycle} parameter should not be implemented by calling the
corresponding version with a @cite{Cycle} parameter of
@cite{2.0*Numerics.Pi}, since this will not provide the required
accuracy in some portions of the domain.  For the same reason, the
version of @cite{Log} without a @cite{Base} parameter should not be
implemented by calling the corresponding version with a @cite{Base}
parameter of @cite{Numerics.e}."
@end quotation

Followed.

@geindex Complex arithmetic accuracy

@geindex Accuracy
@geindex complex arithmetic

@node RM G 2 6 15 Complex Arithmetic Accuracy,RM H 6 15/2 Pragma Partition_Elaboration_Policy,RM G 2 4 19 Accuracy Requirements,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-g-2-6-15-complex-arithmetic-accuracy}@anchor{1fb}
@section RM G.2.6(15): Complex Arithmetic Accuracy


@quotation

"The version of the @cite{Compose_From_Polar} function without a
@cite{Cycle} parameter should not be implemented by calling the
corresponding version with a @cite{Cycle} parameter of
@cite{2.0*Numerics.Pi}, since this will not provide the required
accuracy in some portions of the domain."
@end quotation

Followed.

@geindex Sequential elaboration policy

@node RM H 6 15/2 Pragma Partition_Elaboration_Policy,,RM G 2 6 15 Complex Arithmetic Accuracy,Implementation Advice
@anchor{gnat_rm/implementation_advice rm-h-6-15-2-pragma-partition-elaboration-policy}@anchor{1fc}
@section RM H.6(15/2): Pragma Partition_Elaboration_Policy


@quotation

"If the partition elaboration policy is @cite{Sequential} and the
Environment task becomes permanently blocked during elaboration then the
partition is deadlocked and it is recommended that the partition be
immediately terminated."
@end quotation

Not followed.

@node Implementation Defined Characteristics,Intrinsic Subprograms,Implementation Advice,Top
@anchor{gnat_rm/implementation_defined_characteristics implementation-defined-characteristics}@anchor{b}@anchor{gnat_rm/implementation_defined_characteristics doc}@anchor{1fd}@anchor{gnat_rm/implementation_defined_characteristics id1}@anchor{1fe}
@chapter Implementation Defined Characteristics


In addition to the implementation dependent pragmas and attributes, and the
implementation advice, there are a number of other Ada features that are
potentially implementation dependent and are designated as
implementation-defined. These are mentioned throughout the Ada Reference
Manual, and are summarized in Annex M.

A requirement for conforming Ada compilers is that they provide
documentation describing how the implementation deals with each of these
issues.  In this chapter you will find each point in Annex M listed,
followed by a description of how GNAT
handles the implementation dependence.

You can use this chapter as a guide to minimizing implementation
dependent features in your programs if portability to other compilers
and other operating systems is an important consideration.  The numbers
in each entry below correspond to the paragraph numbers in the Ada
Reference Manual.


@itemize *

@item 
"Whether or not each recommendation given in Implementation
Advice is followed.  See 1.1.2(37)."
@end itemize

See @ref{a,,Implementation Advice}.


@itemize *

@item 
"Capacity limitations of the implementation.  See 1.1.3(3)."
@end itemize

The complexity of programs that can be processed is limited only by the
total amount of available virtual memory, and disk space for the
generated object files.


@itemize *

@item 
"Variations from the standard that are impractical to avoid
given the implementation's execution environment.  See 1.1.3(6)."
@end itemize

There are no variations from the standard.


@itemize *

@item 
"Which code_statements cause external
interactions.  See 1.1.3(10)."
@end itemize

Any @cite{code_statement} can potentially cause external interactions.


@itemize *

@item 
"The coded representation for the text of an Ada
program.  See 2.1(4)."
@end itemize

See separate section on source representation.


@itemize *

@item 
"The control functions allowed in comments.  See 2.1(14)."
@end itemize

See separate section on source representation.


@itemize *

@item 
"The representation for an end of line.  See 2.2(2)."
@end itemize

See separate section on source representation.


@itemize *

@item 
"Maximum supported line length and lexical element
length.  See 2.2(15)."
@end itemize

The maximum line length is 255 characters and the maximum length of
a lexical element is also 255 characters. This is the default setting
if not overridden by the use of compiler switch @emph{-gnaty} (which
sets the maximum to 79) or @emph{-gnatyMnn} which allows the maximum
line length to be specified to be any value up to 32767. The maximum
length of a lexical element is the same as the maximum line length.


@itemize *

@item 
"Implementation defined pragmas.  See 2.8(14)."
@end itemize

See @ref{7,,Implementation Defined Pragmas}.


@itemize *

@item 
"Effect of pragma @cite{Optimize}.  See 2.8(27)."
@end itemize

Pragma @cite{Optimize}, if given with a @cite{Time} or @cite{Space}
parameter, checks that the optimization flag is set, and aborts if it is
not.


@itemize *

@item 
"The sequence of characters of the value returned by
@code{S'Image} when some of the graphic characters of
@code{S'Wide_Image} are not defined in @cite{Character}.  See
3.5(37)."
@end itemize

The sequence of characters is as defined by the wide character encoding
method used for the source.  See section on source representation for
further details.


@itemize *

@item 
"The predefined integer types declared in
@cite{Standard}.  See 3.5.4(25)."
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Type

@tab

Representation

@item

@emph{Short_Short_Integer}

@tab

8 bit signed

@item

@emph{Short_Integer}

@tab

(Short) 16 bit signed

@item

@emph{Integer}

@tab

32 bit signed

@item

@emph{Long_Integer}

@tab

64 bit signed (on most 64 bit targets,
depending on the C definition of long).
32 bit signed (all other targets)

@item

@emph{Long_Long_Integer}

@tab

64 bit signed

@end multitable



@itemize *

@item 
"Any nonstandard integer types and the operators defined
for them.  See 3.5.4(26)."
@end itemize

There are no nonstandard integer types.


@itemize *

@item 
"Any nonstandard real types and the operators defined for
them.  See 3.5.6(8)."
@end itemize

There are no nonstandard real types.


@itemize *

@item 
"What combinations of requested decimal precision and range
are supported for floating point types.  See 3.5.7(7)."
@end itemize

The precision and range is as defined by the IEEE standard.


@itemize *

@item 
"The predefined floating point types declared in
@cite{Standard}.  See 3.5.7(16)."
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Type

@tab

Representation

@item

@emph{Short_Float}

@tab

32 bit IEEE short

@item

@emph{Float}

@tab

(Short) 32 bit IEEE short

@item

@emph{Long_Float}

@tab

64 bit IEEE long

@item

@emph{Long_Long_Float}

@tab

64 bit IEEE long (80 bit IEEE long on x86 processors)

@end multitable



@itemize *

@item 
"The small of an ordinary fixed point type.  See 3.5.9(8)."
@end itemize

@cite{Fine_Delta} is 2**(-63)


@itemize *

@item 
"What combinations of small, range, and digits are
supported for fixed point types.  See 3.5.9(10)."
@end itemize

Any combinations are permitted that do not result in a small less than
@cite{Fine_Delta} and do not result in a mantissa larger than 63 bits.
If the mantissa is larger than 53 bits on machines where Long_Long_Float
is 64 bits (true of all architectures except ia32), then the output from
Text_IO is accurate to only 53 bits, rather than the full mantissa.  This
is because floating-point conversions are used to convert fixed point.


@itemize *

@item 
"The result of @cite{Tags.Expanded_Name} for types declared
within an unnamed @cite{block_statement}.  See 3.9(10)."
@end itemize

Block numbers of the form @cite{B`nnn`}, where @cite{nnn} is a
decimal integer are allocated.


@itemize *

@item 
"Implementation-defined attributes.  See 4.1.4(12)."
@end itemize

See @ref{8,,Implementation Defined Attributes}.


@itemize *

@item 
"Any implementation-defined time types.  See 9.6(6)."
@end itemize

There are no implementation-defined time types.


@itemize *

@item 
"The time base associated with relative delays."
@end itemize

See 9.6(20).  The time base used is that provided by the C library
function @cite{gettimeofday}.


@itemize *

@item 
"The time base of the type @cite{Calendar.Time}.  See
9.6(23)."
@end itemize

The time base used is that provided by the C library function
@cite{gettimeofday}.


@itemize *

@item 
"The time zone used for package @cite{Calendar}
operations.  See 9.6(24)."
@end itemize

The time zone used by package @cite{Calendar} is the current system time zone
setting for local time, as accessed by the C library function
@cite{localtime}.


@itemize *

@item 
"Any limit on @cite{delay_until_statements} of
@cite{select_statements}.  See 9.6(29)."
@end itemize

There are no such limits.


@itemize *

@item 
"Whether or not two non-overlapping parts of a composite
object are independently addressable, in the case where packing, record
layout, or @cite{Component_Size} is specified for the object.  See
9.10(1)."
@end itemize

Separate components are independently addressable if they do not share
overlapping storage units.


@itemize *

@item 
"The representation for a compilation.  See 10.1(2)."
@end itemize

A compilation is represented by a sequence of files presented to the
compiler in a single invocation of the @emph{gcc} command.


@itemize *

@item 
"Any restrictions on compilations that contain multiple
compilation_units.  See 10.1(4)."
@end itemize

No single file can contain more than one compilation unit, but any
sequence of files can be presented to the compiler as a single
compilation.


@itemize *

@item 
"The mechanisms for creating an environment and for adding
and replacing compilation units.  See 10.1.4(3)."
@end itemize

See separate section on compilation model.


@itemize *

@item 
"The manner of explicitly assigning library units to a
partition.  See 10.2(2)."
@end itemize

If a unit contains an Ada main program, then the Ada units for the partition
are determined by recursive application of the rules in the Ada Reference
Manual section 10.2(2-6).  In other words, the Ada units will be those that
are needed by the main program, and then this definition of need is applied
recursively to those units, and the partition contains the transitive
closure determined by this relationship.  In short, all the necessary units
are included, with no need to explicitly specify the list.  If additional
units are required, e.g., by foreign language units, then all units must be
mentioned in the context clause of one of the needed Ada units.

If the partition contains no main program, or if the main program is in
a language other than Ada, then GNAT
provides the binder options @emph{-z} and @emph{-n} respectively, and in
this case a list of units can be explicitly supplied to the binder for
inclusion in the partition (all units needed by these units will also
be included automatically).  For full details on the use of these
options, refer to the @cite{GNAT Make Program gnatmake} in the
@cite{GNAT User's Guide}.


@itemize *

@item 
"The implementation-defined means, if any, of specifying
which compilation units are needed by a given compilation unit.  See
10.2(2)."
@end itemize

The units needed by a given compilation unit are as defined in
the Ada Reference Manual section 10.2(2-6).  There are no
implementation-defined pragmas or other implementation-defined
means for specifying needed units.


@itemize *

@item 
"The manner of designating the main subprogram of a
partition.  See 10.2(7)."
@end itemize

The main program is designated by providing the name of the
corresponding @code{ALI} file as the input parameter to the binder.


@itemize *

@item 
"The order of elaboration of @cite{library_items}.  See
10.2(18)."
@end itemize

The first constraint on ordering is that it meets the requirements of
Chapter 10 of the Ada Reference Manual.  This still leaves some
implementation dependent choices, which are resolved by first
elaborating bodies as early as possible (i.e., in preference to specs
where there is a choice), and second by evaluating the immediate with
clauses of a unit to determine the probably best choice, and
third by elaborating in alphabetical order of unit names
where a choice still remains.


@itemize *

@item 
"Parameter passing and function return for the main
subprogram.  See 10.2(21)."
@end itemize

The main program has no parameters.  It may be a procedure, or a function
returning an integer type.  In the latter case, the returned integer
value is the return code of the program (overriding any value that
may have been set by a call to @cite{Ada.Command_Line.Set_Exit_Status}).


@itemize *

@item 
"The mechanisms for building and running partitions.  See
10.2(24)."
@end itemize

GNAT itself supports programs with only a single partition.  The GNATDIST
tool provided with the GLADE package (which also includes an implementation
of the PCS) provides a completely flexible method for building and running
programs consisting of multiple partitions.  See the separate GLADE manual
for details.


@itemize *

@item 
"The details of program execution, including program
termination.  See 10.2(25)."
@end itemize

See separate section on compilation model.


@itemize *

@item 
"The semantics of any non-active partitions supported by the
implementation.  See 10.2(28)."
@end itemize

Passive partitions are supported on targets where shared memory is
provided by the operating system.  See the GLADE reference manual for
further details.


@itemize *

@item 
"The information returned by @cite{Exception_Message}.  See
11.4.1(10)."
@end itemize

Exception message returns the null string unless a specific message has
been passed by the program.


@itemize *

@item 
"The result of @cite{Exceptions.Exception_Name} for types
declared within an unnamed @cite{block_statement}.  See 11.4.1(12)."
@end itemize

Blocks have implementation defined names of the form @cite{B`nnn`}
where @cite{nnn} is an integer.


@itemize *

@item 
"The information returned by
@cite{Exception_Information}.  See 11.4.1(13)."
@end itemize

@cite{Exception_Information} returns a string in the following format:

@example
*Exception_Name:* nnnnn
*Message:* mmmmm
*PID:* ppp
*Load address:* 0xhhhh
*Call stack traceback locations:*
0xhhhh 0xhhhh 0xhhhh ... 0xhhh
@end example

where

@quotation


@itemize *

@item 
@cite{nnnn} is the fully qualified name of the exception in all upper
case letters. This line is always present.

@item 
@cite{mmmm} is the message (this line present only if message is non-null)

@item 
@cite{ppp} is the Process Id value as a decimal integer (this line is
present only if the Process Id is nonzero). Currently we are
not making use of this field.

@item 
The Load address line, the Call stack traceback locations line and the
following values are present only if at least one traceback location was
recorded. The Load address indicates the address at which the main executable
was loaded; this line may not be present if operating system hasn't relocated
the main executable. The values are given in C style format, with lower case
letters for a-f, and only as many digits present as are necessary.
The line terminator sequence at the end of each line, including
the last line is a single @cite{LF} character (@cite{16#0A#}).
@end itemize
@end quotation


@itemize *

@item 
"Implementation-defined check names.  See 11.5(27)."
@end itemize

The implementation defined check name Alignment_Check controls checking of
address clause values for proper alignment (that is, the address supplied
must be consistent with the alignment of the type).

The implementation defined check name Predicate_Check controls whether
predicate checks are generated.

The implementation defined check name Validity_Check controls whether
validity checks are generated.

In addition, a user program can add implementation-defined check names
by means of the pragma Check_Name.


@itemize *

@item 
"The interpretation of each aspect of representation.  See
13.1(20)."
@end itemize

See separate section on data representations.


@itemize *

@item 
"Any restrictions placed upon representation items.  See
13.1(20)."
@end itemize

See separate section on data representations.


@itemize *

@item 
"The meaning of @cite{Size} for indefinite subtypes.  See
13.3(48)."
@end itemize

Size for an indefinite subtype is the maximum possible size, except that
for the case of a subprogram parameter, the size of the parameter object
is the actual size.


@itemize *

@item 
"The default external representation for a type tag.  See
13.3(75)."
@end itemize

The default external representation for a type tag is the fully expanded
name of the type in upper case letters.


@itemize *

@item 
"What determines whether a compilation unit is the same in
two different partitions.  See 13.3(76)."
@end itemize

A compilation unit is the same in two different partitions if and only
if it derives from the same source file.


@itemize *

@item 
"Implementation-defined components.  See 13.5.1(15)."
@end itemize

The only implementation defined component is the tag for a tagged type,
which contains a pointer to the dispatching table.


@itemize *

@item 
"If @cite{Word_Size} = @cite{Storage_Unit}, the default bit
ordering.  See 13.5.3(5)."
@end itemize

@cite{Word_Size} (32) is not the same as @cite{Storage_Unit} (8) for this
implementation, so no non-default bit ordering is supported.  The default
bit ordering corresponds to the natural endianness of the target architecture.


@itemize *

@item 
"The contents of the visible part of package @cite{System}
and its language-defined children.  See 13.7(2)."
@end itemize

See the definition of these packages in files @code{system.ads} and
@code{s-stoele.ads}. Note that two declarations are added to package
System.

@example
Max_Priority           : constant Positive := Priority'Last;
Max_Interrupt_Priority : constant Positive := Interrupt_Priority'Last;
@end example


@itemize *

@item 
"The contents of the visible part of package
@cite{System.Machine_Code}, and the meaning of
@cite{code_statements}.  See 13.8(7)."
@end itemize

See the definition and documentation in file @code{s-maccod.ads}.


@itemize *

@item 
"The effect of unchecked conversion.  See 13.9(11)."
@end itemize

Unchecked conversion between types of the same size
results in an uninterpreted transmission of the bits from one type
to the other.  If the types are of unequal sizes, then in the case of
discrete types, a shorter source is first zero or sign extended as
necessary, and a shorter target is simply truncated on the left.
For all non-discrete types, the source is first copied if necessary
to ensure that the alignment requirements of the target are met, then
a pointer is constructed to the source value, and the result is obtained
by dereferencing this pointer after converting it to be a pointer to the
target type. Unchecked conversions where the target subtype is an
unconstrained array are not permitted. If the target alignment is
greater than the source alignment, then a copy of the result is
made with appropriate alignment


@itemize *

@item 
"The semantics of operations on invalid representations.
See 13.9.2(10-11)."
@end itemize

For assignments and other operations where the use of invalid values cannot
result in erroneous behavior, the compiler ignores the possibility of invalid
values. An exception is raised at the point where an invalid value would
result in erroneous behavior. For example executing:

@example
procedure invalidvals is
  X : Integer := -1;
  Y : Natural range 1 .. 10;
  for Y'Address use X'Address;
  Z : Natural range 1 .. 10;
  A : array (Natural range 1 .. 10) of Integer;
begin
  Z := Y;     -- no exception
  A (Z) := 3; -- exception raised;
end;
@end example

As indicated, an exception is raised on the array assignment, but not
on the simple assignment of the invalid negative value from Y to Z.


@itemize *

@item 
"The manner of choosing a storage pool for an access type
when @cite{Storage_Pool} is not specified for the type.  See 13.11(17)."
@end itemize

There are 3 different standard pools used by the compiler when
@cite{Storage_Pool} is not specified depending whether the type is local
to a subprogram or defined at the library level and whether
@cite{Storage_Size`is specified or not. See documentation in the runtime library units `System.Pool_Global}, @cite{System.Pool_Size} and
@cite{System.Pool_Local} in files @code{s-poosiz.ads},
@code{s-pooglo.ads} and @code{s-pooloc.ads} for full details on the
default pools used.


@itemize *

@item 
"Whether or not the implementation provides user-accessible
names for the standard pool type(s).  See 13.11(17)."
@end itemize

See documentation in the sources of the run time mentioned in the previous
paragraph.  All these pools are accessible by means of @cite{with}'ing
these units.


@itemize *

@item 
"The meaning of @cite{Storage_Size}.  See 13.11(18)."
@end itemize

@cite{Storage_Size} is measured in storage units, and refers to the
total space available for an access type collection, or to the primary
stack space for a task.


@itemize *

@item 
"Implementation-defined aspects of storage pools.  See
13.11(22)."
@end itemize

See documentation in the sources of the run time mentioned in the
paragraph about standard storage pools above
for details on GNAT-defined aspects of storage pools.


@itemize *

@item 
"The set of restrictions allowed in a pragma
@cite{Restrictions}.  See 13.12(7)."
@end itemize

See @ref{9,,Standard and Implementation Defined Restrictions}.


@itemize *

@item 
"The consequences of violating limitations on
@cite{Restrictions} pragmas.  See 13.12(9)."
@end itemize

Restrictions that can be checked at compile time result in illegalities
if violated.  Currently there are no other consequences of violating
restrictions.


@itemize *

@item 
"The representation used by the @cite{Read} and
@cite{Write} attributes of elementary types in terms of stream
elements.  See 13.13.2(9)."
@end itemize

The representation is the in-memory representation of the base type of
the type, using the number of bits corresponding to the
@code{type'Size} value, and the natural ordering of the machine.


@itemize *

@item 
"The names and characteristics of the numeric subtypes
declared in the visible part of package @cite{Standard}.  See A.1(3)."
@end itemize

See items describing the integer and floating-point types supported.


@itemize *

@item 
"The string returned by @cite{Character_Set_Version}.
See A.3.5(3)."
@end itemize

@cite{Ada.Wide_Characters.Handling.Character_Set_Version} returns
the string "Unicode 4.0", referring to version 4.0 of the
Unicode specification.


@itemize *

@item 
"The accuracy actually achieved by the elementary
functions.  See A.5.1(1)."
@end itemize

The elementary functions correspond to the functions available in the C
library.  Only fast math mode is implemented.


@itemize *

@item 
"The sign of a zero result from some of the operators or
functions in @cite{Numerics.Generic_Elementary_Functions}, when
@cite{Float_Type'Signed_Zeros} is @cite{True}.  See A.5.1(46)."
@end itemize

The sign of zeroes follows the requirements of the IEEE 754 standard on
floating-point.


@itemize *

@item 
"The value of
@cite{Numerics.Float_Random.Max_Image_Width}.  See A.5.2(27)."
@end itemize

Maximum image width is 6864, see library file @code{s-rannum.ads}.


@itemize *

@item 
"The value of
@cite{Numerics.Discrete_Random.Max_Image_Width}.  See A.5.2(27)."
@end itemize

Maximum image width is 6864, see library file @code{s-rannum.ads}.


@itemize *

@item 
"The algorithms for random number generation.  See
A.5.2(32)."
@end itemize

The algorithm is the Mersenne Twister, as documented in the source file
@code{s-rannum.adb}. This version of the algorithm has a period of
2**19937-1.


@itemize *

@item 
"The string representation of a random number generator's
state.  See A.5.2(38)."
@end itemize

The value returned by the Image function is the concatenation of
the fixed-width decimal representations of the 624 32-bit integers
of the state vector.


@itemize *

@item 
"The minimum time interval between calls to the
time-dependent Reset procedure that are guaranteed to initiate different
random number sequences.  See A.5.2(45)."
@end itemize

The minimum period between reset calls to guarantee distinct series of
random numbers is one microsecond.


@itemize *

@item 
"The values of the @cite{Model_Mantissa},
@cite{Model_Emin}, @cite{Model_Epsilon}, @cite{Model},
@cite{Safe_First}, and @cite{Safe_Last} attributes, if the Numerics
Annex is not supported.  See A.5.3(72)."
@end itemize

Run the compiler with @emph{-gnatS} to produce a listing of package
@cite{Standard}, has the values of all numeric attributes.


@itemize *

@item 
"Any implementation-defined characteristics of the
input-output packages.  See A.7(14)."
@end itemize

There are no special implementation defined characteristics for these
packages.


@itemize *

@item 
"The value of @cite{Buffer_Size} in @cite{Storage_IO}.  See
A.9(10)."
@end itemize

All type representations are contiguous, and the @cite{Buffer_Size} is
the value of @code{type'Size} rounded up to the next storage unit
boundary.


@itemize *

@item 
"External files for standard input, standard output, and
standard error See A.10(5)."
@end itemize

These files are mapped onto the files provided by the C streams
libraries.  See source file @code{i-cstrea.ads} for further details.


@itemize *

@item 
"The accuracy of the value produced by @cite{Put}.  See
A.10.9(36)."
@end itemize

If more digits are requested in the output than are represented by the
precision of the value, zeroes are output in the corresponding least
significant digit positions.


@itemize *

@item 
"The meaning of @cite{Argument_Count}, @cite{Argument}, and
@cite{Command_Name}.  See A.15(1)."
@end itemize

These are mapped onto the @cite{argv} and @cite{argc} parameters of the
main program in the natural manner.


@itemize *

@item 
"The interpretation of the @cite{Form} parameter in procedure
@cite{Create_Directory}.  See A.16(56)."
@end itemize

The @cite{Form} parameter is not used.


@itemize *

@item 
"The interpretation of the @cite{Form} parameter in procedure
@cite{Create_Path}.  See A.16(60)."
@end itemize

The @cite{Form} parameter is not used.


@itemize *

@item 
"The interpretation of the @cite{Form} parameter in procedure
@cite{Copy_File}.  See A.16(68)."
@end itemize

The @cite{Form} parameter is case-insensitive.
Two fields are recognized in the @cite{Form} parameter:

@example
*preserve=<value>*
*mode=<value>*
@end example

<value> starts immediately after the character '=' and ends with the
character immediately preceding the next comma (',') or with the last
character of the parameter.

The only possible values for preserve= are:


@multitable {xxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Value

@tab

Meaning

@item

@emph{no_attributes}

@tab

Do not try to preserve any file attributes. This is the
default if no preserve= is found in Form.

@item

@emph{all_attributes}

@tab

Try to preserve all file attributes (timestamps, access rights).

@item

@emph{timestamps}

@tab

Preserve the timestamp of the copied file, but not the other
file attributes.

@end multitable


The only possible values for mode= are:


@multitable {xxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Value

@tab

Meaning

@item

@emph{copy}

@tab

Only do the copy if the destination file does not already exist.
If it already exists, Copy_File fails.

@item

@emph{overwrite}

@tab

Copy the file in all cases. Overwrite an already existing destination file.

@item

@emph{append}

@tab

Append the original file to the destination file. If the destination file
does not exist, the destination file is a copy of the source file.
When mode=append, the field preserve=, if it exists, is not taken into account.

@end multitable


If the Form parameter includes one or both of the fields and the value or
values are incorrect, Copy_file fails with Use_Error.

Examples of correct Forms:

@example
Form => "preserve=no_attributes,mode=overwrite" (the default)
Form => "mode=append"
Form => "mode=copy, preserve=all_attributes"
@end example

Examples of incorrect Forms:

@example
Form => "preserve=junk"
Form => "mode=internal, preserve=timestamps"
@end example


@itemize *

@item 
"The interpretation of the @cite{Pattern} parameter, when not the null string,
in the @cite{Start_Search} and @cite{Search} procedures.
See A.16(104) and A.16(112)."
@end itemize

When the @cite{Pattern} parameter is not the null string, it is interpreted
according to the syntax of regular expressions as defined in the
@cite{GNAT.Regexp} package.

See @ref{1ff,,GNAT.Regexp (g-regexp.ads)}.


@itemize *

@item 
"Implementation-defined convention names.  See B.1(11)."
@end itemize

The following convention names are supported


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Convention Name

@tab

Interpretation

@item

@emph{Ada}

@tab

Ada

@item

@emph{Ada_Pass_By_Copy}

@tab

Allowed for any types except by-reference types such as limited
records. Compatible with convention Ada, but causes any parameters
with this convention to be passed by copy.

@item

@emph{Ada_Pass_By_Reference}

@tab

Allowed for any types except by-copy types such as scalars.
Compatible with convention Ada, but causes any parameters
with this convention to be passed by reference.

@item

@emph{Assembler}

@tab

Assembly language

@item

@emph{Asm}

@tab

Synonym for Assembler

@item

@emph{Assembly}

@tab

Synonym for Assembler

@item

@emph{C}

@tab

C

@item

@emph{C_Pass_By_Copy}

@tab

Allowed only for record types, like C, but also notes that record
is to be passed by copy rather than reference.

@item

@emph{COBOL}

@tab

COBOL

@item

@emph{C_Plus_Plus (or CPP)}

@tab

C++

@item

@emph{Default}

@tab

Treated the same as C

@item

@emph{External}

@tab

Treated the same as C

@item

@emph{Fortran}

@tab

Fortran

@item

@emph{Intrinsic}

@tab

For support of pragma @cite{Import} with convention Intrinsic, see
separate section on Intrinsic Subprograms.

@item

@emph{Stdcall}

@tab

Stdcall (used for Windows implementations only).  This convention correspond
to the WINAPI (previously called Pascal convention) C/C++ convention under
Windows.  A routine with this convention cleans the stack before
exit. This pragma cannot be applied to a dispatching call.

@item

@emph{DLL}

@tab

Synonym for Stdcall

@item

@emph{Win32}

@tab

Synonym for Stdcall

@item

@emph{Stubbed}

@tab

Stubbed is a special convention used to indicate that the body of the
subprogram will be entirely ignored.  Any call to the subprogram
is converted into a raise of the @cite{Program_Error} exception.  If a
pragma @cite{Import} specifies convention @cite{stubbed} then no body need
be present at all.  This convention is useful during development for the
inclusion of subprograms whose body has not yet been written.
In addition, all otherwise unrecognized convention names are also
treated as being synonymous with convention C.  In all implementations
except for VMS, use of such other names results in a warning.  In VMS
implementations, these names are accepted silently.

@end multitable



@itemize *

@item 
"The meaning of link names.  See B.1(36)."
@end itemize

Link names are the actual names used by the linker.


@itemize *

@item 
"The manner of choosing link names when neither the link
name nor the address of an imported or exported entity is specified.  See
B.1(36)."
@end itemize

The default linker name is that which would be assigned by the relevant
external language, interpreting the Ada name as being in all lower case
letters.


@itemize *

@item 
"The effect of pragma @cite{Linker_Options}.  See B.1(37)."
@end itemize

The string passed to @cite{Linker_Options} is presented uninterpreted as
an argument to the link command, unless it contains ASCII.NUL characters.
NUL characters if they appear act as argument separators, so for example

@example
pragma Linker_Options ("-labc" & ASCII.NUL & "-ldef");
@end example

causes two separate arguments @cite{-labc} and @cite{-ldef} to be passed to the
linker. The order of linker options is preserved for a given unit. The final
list of options passed to the linker is in reverse order of the elaboration
order. For example, linker options for a body always appear before the options
from the corresponding package spec.


@itemize *

@item 
"The contents of the visible part of package
@cite{Interfaces} and its language-defined descendants.  See B.2(1)."
@end itemize

See files with prefix @code{i-} in the distributed library.


@itemize *

@item 
"Implementation-defined children of package
@cite{Interfaces}.  The contents of the visible part of package
@cite{Interfaces}.  See B.2(11)."
@end itemize

See files with prefix @code{i-} in the distributed library.


@itemize *

@item 
"The types @cite{Floating}, @cite{Long_Floating},
@cite{Binary}, @cite{Long_Binary}, @cite{Decimal_ Element}, and
@cite{COBOL_Character}; and the initialization of the variables
@cite{Ada_To_COBOL} and @cite{COBOL_To_Ada}, in
@cite{Interfaces.COBOL}.  See B.4(50)."
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

COBOL

@tab

Ada

@item

@emph{Floating}

@tab

Float

@item

@emph{Long_Floating}

@tab

(Floating) Long_Float

@item

@emph{Binary}

@tab

Integer

@item

@emph{Long_Binary}

@tab

Long_Long_Integer

@item

@emph{Decimal_Element}

@tab

Character

@item

@emph{COBOL_Character}

@tab

Character

@end multitable


For initialization, see the file @code{i-cobol.ads} in the distributed library.


@itemize *

@item 
"Support for access to machine instructions.  See C.1(1)."
@end itemize

See documentation in file @code{s-maccod.ads} in the distributed library.


@itemize *

@item 
"Implementation-defined aspects of access to machine
operations.  See C.1(9)."
@end itemize

See documentation in file @code{s-maccod.ads} in the distributed library.


@itemize *

@item 
"Implementation-defined aspects of interrupts.  See C.3(2)."
@end itemize

Interrupts are mapped to signals or conditions as appropriate.  See
definition of unit
@cite{Ada.Interrupt_Names} in source file @code{a-intnam.ads} for details
on the interrupts supported on a particular target.


@itemize *

@item 
"Implementation-defined aspects of pre-elaboration.  See
C.4(13)."
@end itemize

GNAT does not permit a partition to be restarted without reloading,
except under control of the debugger.


@itemize *

@item 
"The semantics of pragma @cite{Discard_Names}.  See C.5(7)."
@end itemize

Pragma @cite{Discard_Names} causes names of enumeration literals to
be suppressed.  In the presence of this pragma, the Image attribute
provides the image of the Pos of the literal, and Value accepts
Pos values.


@itemize *

@item 
"The result of the @cite{Task_Identification.Image}
attribute.  See C.7.1(7)."
@end itemize

The result of this attribute is a string that identifies
the object or component that denotes a given task. If a variable @cite{Var}
has a task type, the image for this task will have the form @cite{Var_`XXXXXXXX`},
where the suffix
is the hexadecimal representation of the virtual address of the corresponding
task control block. If the variable is an array of tasks, the image of each
task will have the form of an indexed component indicating the position of a
given task in the array, e.g., @cite{Group(5)_`XXXXXXX`}. If the task is a
component of a record, the image of the task will have the form of a selected
component. These rules are fully recursive, so that the image of a task that
is a subcomponent of a composite object corresponds to the expression that
designates this task.

If a task is created by an allocator, its image depends on the context. If the
allocator is part of an object declaration, the rules described above are used
to construct its image, and this image is not affected by subsequent
assignments. If the allocator appears within an expression, the image
includes only the name of the task type.

If the configuration pragma Discard_Names is present, or if the restriction
No_Implicit_Heap_Allocation is in effect,  the image reduces to
the numeric suffix, that is to say the hexadecimal representation of the
virtual address of the control block of the task.


@itemize *

@item 
"The value of @cite{Current_Task} when in a protected entry
or interrupt handler.  See C.7.1(17)."
@end itemize

Protected entries or interrupt handlers can be executed by any
convenient thread, so the value of @cite{Current_Task} is undefined.


@itemize *

@item 
"The effect of calling @cite{Current_Task} from an entry
body or interrupt handler.  See C.7.1(19)."
@end itemize

The effect of calling @cite{Current_Task} from an entry body or
interrupt handler is to return the identification of the task currently
executing the code.


@itemize *

@item 
"Implementation-defined aspects of
@cite{Task_Attributes}.  See C.7.2(19)."
@end itemize

There are no implementation-defined aspects of @cite{Task_Attributes}.


@itemize *

@item 
"Values of all @cite{Metrics}.  See D(2)."
@end itemize

The metrics information for GNAT depends on the performance of the
underlying operating system.  The sources of the run-time for tasking
implementation, together with the output from @emph{-gnatG} can be
used to determine the exact sequence of operating systems calls made
to implement various tasking constructs.  Together with appropriate
information on the performance of the underlying operating system,
on the exact target in use, this information can be used to determine
the required metrics.


@itemize *

@item 
"The declarations of @cite{Any_Priority} and
@cite{Priority}.  See D.1(11)."
@end itemize

See declarations in file @code{system.ads}.


@itemize *

@item 
"Implementation-defined execution resources.  See D.1(15)."
@end itemize

There are no implementation-defined execution resources.


@itemize *

@item 
"Whether, on a multiprocessor, a task that is waiting for
access to a protected object keeps its processor busy.  See D.2.1(3)."
@end itemize

On a multi-processor, a task that is waiting for access to a protected
object does not keep its processor busy.


@itemize *

@item 
"The affect of implementation defined execution resources
on task dispatching.  See D.2.1(9)."
@end itemize

Tasks map to threads in the threads package used by GNAT.  Where possible
and appropriate, these threads correspond to native threads of the
underlying operating system.


@itemize *

@item 
"Implementation-defined @cite{policy_identifiers} allowed
in a pragma @cite{Task_Dispatching_Policy}.  See D.2.2(3)."
@end itemize

There are no implementation-defined policy-identifiers allowed in this
pragma.


@itemize *

@item 
"Implementation-defined aspects of priority inversion.  See
D.2.2(16)."
@end itemize

Execution of a task cannot be preempted by the implementation processing
of delay expirations for lower priority tasks.


@itemize *

@item 
"Implementation-defined task dispatching.  See D.2.2(18)."
@end itemize

The policy is the same as that of the underlying threads implementation.


@itemize *

@item 
"Implementation-defined @cite{policy_identifiers} allowed
in a pragma @cite{Locking_Policy}.  See D.3(4)."
@end itemize

The two implementation defined policies permitted in GNAT are
@cite{Inheritance_Locking} and  @cite{Conccurent_Readers_Locking}.  On
targets that support the @cite{Inheritance_Locking} policy, locking is
implemented by inheritance, i.e., the task owning the lock operates
at a priority equal to the highest priority of any task currently
requesting the lock.  On targets that support the
@cite{Conccurent_Readers_Locking} policy, locking is implemented with a
read/write lock allowing multiple propected object functions to enter
concurrently.


@itemize *

@item 
"Default ceiling priorities.  See D.3(10)."
@end itemize

The ceiling priority of protected objects of the type
@cite{System.Interrupt_Priority'Last} as described in the Ada
Reference Manual D.3(10),


@itemize *

@item 
"The ceiling of any protected object used internally by
the implementation.  See D.3(16)."
@end itemize

The ceiling priority of internal protected objects is
@cite{System.Priority'Last}.


@itemize *

@item 
"Implementation-defined queuing policies.  See D.4(1)."
@end itemize

There are no implementation-defined queuing policies.


@itemize *

@item 
"On a multiprocessor, any conditions that cause the
completion of an aborted construct to be delayed later than what is
specified for a single processor.  See D.6(3)."
@end itemize

The semantics for abort on a multi-processor is the same as on a single
processor, there are no further delays.


@itemize *

@item 
"Any operations that implicitly require heap storage
allocation.  See D.7(8)."
@end itemize

The only operation that implicitly requires heap storage allocation is
task creation.


@itemize *

@item 
"What happens when a task terminates in the presence of
pragma @cite{No_Task_Termination}. See D.7(15)."
@end itemize

Execution is erroneous in that case.


@itemize *

@item 
"Implementation-defined aspects of pragma
@cite{Restrictions}.  See D.7(20)."
@end itemize

There are no such implementation-defined aspects.


@itemize *

@item 
"Implementation-defined aspects of package
@cite{Real_Time}.  See D.8(17)."
@end itemize

There are no implementation defined aspects of package @cite{Real_Time}.


@itemize *

@item 
"Implementation-defined aspects of
@cite{delay_statements}.  See D.9(8)."
@end itemize

Any difference greater than one microsecond will cause the task to be
delayed (see D.9(7)).


@itemize *

@item 
"The upper bound on the duration of interrupt blocking
caused by the implementation.  See D.12(5)."
@end itemize

The upper bound is determined by the underlying operating system.  In
no cases is it more than 10 milliseconds.


@itemize *

@item 
"The means for creating and executing distributed
programs.  See E(5)."
@end itemize

The GLADE package provides a utility GNATDIST for creating and executing
distributed programs.  See the GLADE reference manual for further details.


@itemize *

@item 
"Any events that can result in a partition becoming
inaccessible.  See E.1(7)."
@end itemize

See the GLADE reference manual for full details on such events.


@itemize *

@item 
"The scheduling policies, treatment of priorities, and
management of shared resources between partitions in certain cases.  See
E.1(11)."
@end itemize

See the GLADE reference manual for full details on these aspects of
multi-partition execution.


@itemize *

@item 
"Events that cause the version of a compilation unit to
change.  See E.3(5)."
@end itemize

Editing the source file of a compilation unit, or the source files of
any units on which it is dependent in a significant way cause the version
to change.  No other actions cause the version number to change.  All changes
are significant except those which affect only layout, capitalization or
comments.


@itemize *

@item 
"Whether the execution of the remote subprogram is
immediately aborted as a result of cancellation.  See E.4(13)."
@end itemize

See the GLADE reference manual for details on the effect of abort in
a distributed application.


@itemize *

@item 
"Implementation-defined aspects of the PCS.  See E.5(25)."
@end itemize

See the GLADE reference manual for a full description of all implementation
defined aspects of the PCS.


@itemize *

@item 
"Implementation-defined interfaces in the PCS.  See
E.5(26)."
@end itemize

See the GLADE reference manual for a full description of all
implementation defined interfaces.


@itemize *

@item 
"The values of named numbers in the package
@cite{Decimal}.  See F.2(7)."
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxx} 
@headitem

Named Number

@tab

Value

@item

@emph{Max_Scale}

@tab

+18

@item

@emph{Min_Scale}

@tab

-18

@item

@emph{Min_Delta}

@tab

1.0E-18

@item

@emph{Max_Delta}

@tab

1.0E+18

@item

@emph{Max_Decimal_Digits}

@tab

18

@end multitable



@itemize *

@item 
"The value of @cite{Max_Picture_Length} in the package
@cite{Text_IO.Editing}.  See F.3.3(16)."
@end itemize

64


@itemize *

@item 
"The value of @cite{Max_Picture_Length} in the package
@cite{Wide_Text_IO.Editing}.  See F.3.4(5)."
@end itemize

64


@itemize *

@item 
"The accuracy actually achieved by the complex elementary
functions and by other complex arithmetic operations.  See G.1(1)."
@end itemize

Standard library functions are used for the complex arithmetic
operations.  Only fast math mode is currently supported.


@itemize *

@item 
"The sign of a zero result (or a component thereof) from
any operator or function in @cite{Numerics.Generic_Complex_Types}, when
@cite{Real'Signed_Zeros} is True.  See G.1.1(53)."
@end itemize

The signs of zero values are as recommended by the relevant
implementation advice.


@itemize *

@item 
"The sign of a zero result (or a component thereof) from
any operator or function in
@cite{Numerics.Generic_Complex_Elementary_Functions}, when
@cite{Real'Signed_Zeros} is @cite{True}.  See G.1.2(45)."
@end itemize

The signs of zero values are as recommended by the relevant
implementation advice.


@itemize *

@item 
"Whether the strict mode or the relaxed mode is the
default.  See G.2(2)."
@end itemize

The strict mode is the default.  There is no separate relaxed mode.  GNAT
provides a highly efficient implementation of strict mode.


@itemize *

@item 
"The result interval in certain cases of fixed-to-float
conversion.  See G.2.1(10)."
@end itemize

For cases where the result interval is implementation dependent, the
accuracy is that provided by performing all operations in 64-bit IEEE
floating-point format.


@itemize *

@item 
"The result of a floating point arithmetic operation in
overflow situations, when the @cite{Machine_Overflows} attribute of the
result type is @cite{False}.  See G.2.1(13)."
@end itemize

Infinite and NaN values are produced as dictated by the IEEE
floating-point standard.
Note that on machines that are not fully compliant with the IEEE
floating-point standard, such as Alpha, the @emph{-mieee} compiler flag
must be used for achieving IEEE conforming behavior (although at the cost
of a significant performance penalty), so infinite and NaN values are
properly generated.


@itemize *

@item 
"The result interval for division (or exponentiation by a
negative exponent), when the floating point hardware implements division
as multiplication by a reciprocal.  See G.2.1(16)."
@end itemize

Not relevant, division is IEEE exact.


@itemize *

@item 
"The definition of close result set, which determines the
accuracy of certain fixed point multiplications and divisions.  See
G.2.3(5)."
@end itemize

Operations in the close result set are performed using IEEE long format
floating-point arithmetic.  The input operands are converted to
floating-point, the operation is done in floating-point, and the result
is converted to the target type.


@itemize *

@item 
"Conditions on a @cite{universal_real} operand of a fixed
point multiplication or division for which the result shall be in the
perfect result set.  See G.2.3(22)."
@end itemize

The result is only defined to be in the perfect result set if the result
can be computed by a single scaling operation involving a scale factor
representable in 64-bits.


@itemize *

@item 
"The result of a fixed point arithmetic operation in
overflow situations, when the @cite{Machine_Overflows} attribute of the
result type is @cite{False}.  See G.2.3(27)."
@end itemize

Not relevant, @cite{Machine_Overflows} is @cite{True} for fixed-point
types.


@itemize *

@item 
"The result of an elementary function reference in
overflow situations, when the @cite{Machine_Overflows} attribute of the
result type is @cite{False}.  See G.2.4(4)."
@end itemize

IEEE infinite and Nan values are produced as appropriate.


@itemize *

@item 
"The value of the angle threshold, within which certain
elementary functions, complex arithmetic operations, and complex
elementary functions yield results conforming to a maximum relative
error bound.  See G.2.4(10)."
@end itemize

Information on this subject is not yet available.


@itemize *

@item 
"The accuracy of certain elementary functions for
parameters beyond the angle threshold.  See G.2.4(10)."
@end itemize

Information on this subject is not yet available.


@itemize *

@item 
"The result of a complex arithmetic operation or complex
elementary function reference in overflow situations, when the
@cite{Machine_Overflows} attribute of the corresponding real type is
@cite{False}.  See G.2.6(5)."
@end itemize

IEEE infinite and Nan values are produced as appropriate.


@itemize *

@item 
"The accuracy of certain complex arithmetic operations and
certain complex elementary functions for parameters (or components
thereof) beyond the angle threshold.  See G.2.6(8)."
@end itemize

Information on those subjects is not yet available.


@itemize *

@item 
"Information regarding bounded errors and erroneous
execution.  See H.2(1)."
@end itemize

Information on this subject is not yet available.


@itemize *

@item 
"Implementation-defined aspects of pragma
@cite{Inspection_Point}.  See H.3.2(8)."
@end itemize

Pragma @cite{Inspection_Point} ensures that the variable is live and can
be examined by the debugger at the inspection point.


@itemize *

@item 
"Implementation-defined aspects of pragma
@cite{Restrictions}.  See H.4(25)."
@end itemize

There are no implementation-defined aspects of pragma @cite{Restrictions}.  The
use of pragma @cite{Restrictions [No_Exceptions]} has no effect on the
generated code.  Checks must suppressed by use of pragma @cite{Suppress}.


@itemize *

@item 
"Any restrictions on pragma @cite{Restrictions}.  See
H.4(27)."
@end itemize

There are no restrictions on pragma @cite{Restrictions}.

@node Intrinsic Subprograms,Representation Clauses and Pragmas,Implementation Defined Characteristics,Top
@anchor{gnat_rm/intrinsic_subprograms doc}@anchor{200}@anchor{gnat_rm/intrinsic_subprograms intrinsic-subprograms}@anchor{c}@anchor{gnat_rm/intrinsic_subprograms id1}@anchor{201}
@chapter Intrinsic Subprograms


@geindex Intrinsic Subprograms

GNAT allows a user application program to write the declaration:

@example
pragma Import (Intrinsic, name);
@end example

providing that the name corresponds to one of the implemented intrinsic
subprograms in GNAT, and that the parameter profile of the referenced
subprogram meets the requirements.  This chapter describes the set of
implemented intrinsic subprograms, and the requirements on parameter profiles.
Note that no body is supplied; as with other uses of pragma Import, the
body is supplied elsewhere (in this case by the compiler itself).  Note
that any use of this feature is potentially non-portable, since the
Ada standard does not require Ada compilers to implement this feature.

@menu
* Intrinsic Operators:: 
* Compilation_Date:: 
* Compilation_Time:: 
* Enclosing_Entity:: 
* Exception_Information:: 
* Exception_Message:: 
* Exception_Name:: 
* File:: 
* Line:: 
* Shifts and Rotates:: 
* Source_Location:: 

@end menu

@node Intrinsic Operators,Compilation_Date,,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id2}@anchor{202}@anchor{gnat_rm/intrinsic_subprograms intrinsic-operators}@anchor{203}
@section Intrinsic Operators


@geindex Intrinsic operator

All the predefined numeric operators in package Standard
in @cite{pragma Import (Intrinsic@comma{}..)}
declarations.  In the binary operator case, the operands must have the same
size.  The operand or operands must also be appropriate for
the operator.  For example, for addition, the operands must
both be floating-point or both be fixed-point, and the
right operand for @cite{"**"} must have a root type of
@cite{Standard.Integer'Base}.
You can use an intrinsic operator declaration as in the following example:

@example
type Int1 is new Integer;
type Int2 is new Integer;

function "+" (X1 : Int1; X2 : Int2) return Int1;
function "+" (X1 : Int1; X2 : Int2) return Int2;
pragma Import (Intrinsic, "+");
@end example

This declaration would permit 'mixed mode' arithmetic on items
of the differing types @cite{Int1} and @cite{Int2}.
It is also possible to specify such operators for private types, if the
full views are appropriate arithmetic types.

@node Compilation_Date,Compilation_Time,Intrinsic Operators,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms compilation-date}@anchor{204}@anchor{gnat_rm/intrinsic_subprograms id3}@anchor{205}
@section Compilation_Date


@geindex Compilation_Date

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@cite{GNAT.Source_Info.Compilation_Date} to obtain the date of
the current compilation (in local time format MMM DD YYYY).

@node Compilation_Time,Enclosing_Entity,Compilation_Date,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms compilation-time}@anchor{206}@anchor{gnat_rm/intrinsic_subprograms id4}@anchor{207}
@section Compilation_Time


@geindex Compilation_Time

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@cite{GNAT.Source_Info.Compilation_Time} to obtain the time of
the current compilation (in local time format HH:MM:SS).

@node Enclosing_Entity,Exception_Information,Compilation_Time,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id5}@anchor{208}@anchor{gnat_rm/intrinsic_subprograms enclosing-entity}@anchor{209}
@section Enclosing_Entity


@geindex Enclosing_Entity

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@cite{GNAT.Source_Info.Enclosing_Entity} to obtain the name of
the current subprogram, package, task, entry, or protected subprogram.

@node Exception_Information,Exception_Message,Enclosing_Entity,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id6}@anchor{20a}@anchor{gnat_rm/intrinsic_subprograms exception-information}@anchor{20b}
@section Exception_Information


@geindex Exception_Information'

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Current_Exception}.  The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@cite{GNAT.Current_Exception.Exception_Information} to obtain
the exception information associated with the current exception.

@node Exception_Message,Exception_Name,Exception_Information,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms exception-message}@anchor{20c}@anchor{gnat_rm/intrinsic_subprograms id7}@anchor{20d}
@section Exception_Message


@geindex Exception_Message

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Current_Exception}.  The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@cite{GNAT.Current_Exception.Exception_Message} to obtain
the message associated with the current exception.

@node Exception_Name,File,Exception_Message,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms exception-name}@anchor{20e}@anchor{gnat_rm/intrinsic_subprograms id8}@anchor{20f}
@section Exception_Name


@geindex Exception_Name

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Current_Exception}.  The only useful
use of the intrinsic import in this case is the one in this unit,
so an application program should simply call the function
@cite{GNAT.Current_Exception.Exception_Name} to obtain
the name of the current exception.

@node File,Line,Exception_Name,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms file}@anchor{210}@anchor{gnat_rm/intrinsic_subprograms id9}@anchor{211}
@section File


@geindex File

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@cite{GNAT.Source_Info.File} to obtain the name of the current
file.

@node Line,Shifts and Rotates,File,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id10}@anchor{212}@anchor{gnat_rm/intrinsic_subprograms line}@anchor{213}
@section Line


@geindex Line

This intrinsic subprogram is used in the implementation of the
library package @cite{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@cite{GNAT.Source_Info.Line} to obtain the number of the current
source line.

@node Shifts and Rotates,Source_Location,Line,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms id11}@anchor{214}@anchor{gnat_rm/intrinsic_subprograms shifts-and-rotates}@anchor{215}
@section Shifts and Rotates


@geindex Shift_Left

@geindex Shift_Right

@geindex Shift_Right_Arithmetic

@geindex Rotate_Left

@geindex Rotate_Right

In standard Ada, the shift and rotate functions are available only
for the predefined modular types in package @cite{Interfaces}.  However, in
GNAT it is possible to define these functions for any integer
type (signed or modular), as in this example:

@example
function Shift_Left
  (Value  : T;
   Amount : Natural) return T;
@end example

The function name must be one of
Shift_Left, Shift_Right, Shift_Right_Arithmetic, Rotate_Left, or
Rotate_Right. T must be an integer type. T'Size must be
8, 16, 32 or 64 bits; if T is modular, the modulus
must be 2**8, 2**16, 2**32 or 2**64.
The result type must be the same as the type of @cite{Value}.
The shift amount must be Natural.
The formal parameter names can be anything.

A more convenient way of providing these shift operators is to use
the Provide_Shift_Operators pragma, which provides the function declarations
and corresponding pragma Import's for all five shift functions.

@node Source_Location,,Shifts and Rotates,Intrinsic Subprograms
@anchor{gnat_rm/intrinsic_subprograms source-location}@anchor{216}@anchor{gnat_rm/intrinsic_subprograms id12}@anchor{217}
@section Source_Location


@geindex Source_Location

This intrinsic subprogram is used in the implementation of the
library routine @cite{GNAT.Source_Info}.  The only useful use of the
intrinsic import in this case is the one in this unit, so an
application program should simply call the function
@cite{GNAT.Source_Info.Source_Location} to obtain the current
source file location.

@node Representation Clauses and Pragmas,Standard Library Routines,Intrinsic Subprograms,Top
@anchor{gnat_rm/representation_clauses_and_pragmas representation-clauses-and-pragmas}@anchor{d}@anchor{gnat_rm/representation_clauses_and_pragmas doc}@anchor{218}@anchor{gnat_rm/representation_clauses_and_pragmas id1}@anchor{219}
@chapter Representation Clauses and Pragmas


@geindex Representation Clauses

@geindex Representation Clause

@geindex Representation Pragma

@geindex Pragma
@geindex representation

This section describes the representation clauses accepted by GNAT, and
their effect on the representation of corresponding data objects.

GNAT fully implements Annex C (Systems Programming).  This means that all
the implementation advice sections in chapter 13 are fully implemented.
However, these sections only require a minimal level of support for
representation clauses.  GNAT provides much more extensive capabilities,
and this section describes the additional capabilities provided.

@menu
* Alignment Clauses:: 
* Size Clauses:: 
* Storage_Size Clauses:: 
* Size of Variant Record Objects:: 
* Biased Representation:: 
* Value_Size and Object_Size Clauses:: 
* Component_Size Clauses:: 
* Bit_Order Clauses:: 
* Effect of Bit_Order on Byte Ordering:: 
* Pragma Pack for Arrays:: 
* Pragma Pack for Records:: 
* Record Representation Clauses:: 
* Handling of Records with Holes:: 
* Enumeration Clauses:: 
* Address Clauses:: 
* Use of Address Clauses for Memory-Mapped I/O:: 
* Effect of Convention on Representation:: 
* Conventions and Anonymous Access Types:: 
* Determining the Representations chosen by GNAT:: 

@end menu

@node Alignment Clauses,Size Clauses,,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id2}@anchor{21a}@anchor{gnat_rm/representation_clauses_and_pragmas alignment-clauses}@anchor{21b}
@section Alignment Clauses


@geindex Alignment Clause

GNAT requires that all alignment clauses specify a power of 2, and all
default alignments are always a power of 2.  The default alignment
values are as follows:


@itemize *

@item 
@emph{Primitive Types}.

For primitive types, the alignment is the minimum of the actual size of
objects of the type divided by @cite{Storage_Unit},
and the maximum alignment supported by the target.
(This maximum alignment is given by the GNAT-specific attribute
@cite{Standard'Maximum_Alignment}; see @ref{13e,,Attribute Maximum_Alignment}.)

@geindex Maximum_Alignment attribute

For example, for type @cite{Long_Float}, the object size is 8 bytes, and the
default alignment will be 8 on any target that supports alignments
this large, but on some targets, the maximum alignment may be smaller
than 8, in which case objects of type @cite{Long_Float} will be maximally
aligned.

@item 
@emph{Arrays}.

For arrays, the alignment is equal to the alignment of the component type
for the normal case where no packing or component size is given.  If the
array is packed, and the packing is effective (see separate section on
packed arrays), then the alignment will be one for long packed arrays,
or arrays whose length is not known at compile time.  For short packed
arrays, which are handled internally as modular types, the alignment
will be as described for primitive types, e.g., a packed array of length
31 bits will have an object size of four bytes, and an alignment of 4.

@item 
@emph{Records}.

For the normal non-packed case, the alignment of a record is equal to
the maximum alignment of any of its components.  For tagged records, this
includes the implicit access type used for the tag.  If a pragma @cite{Pack}
is used and all components are packable (see separate section on pragma
@cite{Pack}), then the resulting alignment is 1, unless the layout of the
record makes it profitable to increase it.

A special case is when:


@itemize *

@item 
the size of the record is given explicitly, or a
full record representation clause is given, and

@item 
the size of the record is 2, 4, or 8 bytes.
@end itemize

In this case, an alignment is chosen to match the
size of the record. For example, if we have:

@example
type Small is record
   A, B : Character;
end record;
for Small'Size use 16;
@end example

then the default alignment of the record type @cite{Small} is 2, not 1. This
leads to more efficient code when the record is treated as a unit, and also
allows the type to specified as @cite{Atomic} on architectures requiring
strict alignment.
@end itemize

An alignment clause may specify a larger alignment than the default value
up to some maximum value dependent on the target (obtainable by using the
attribute reference @cite{Standard'Maximum_Alignment}). It may also specify
a smaller alignment than the default value for enumeration, integer and
fixed point types, as well as for record types, for example

@example
type V is record
   A : Integer;
end record;

for V'alignment use 1;
@end example

@geindex Alignment
@geindex default

The default alignment for the type @cite{V} is 4, as a result of the
Integer field in the record, but it is permissible, as shown, to
override the default alignment of the record with a smaller value.

@geindex Alignment
@geindex subtypes

Note that according to the Ada standard, an alignment clause applies only
to the first named subtype. If additional subtypes are declared, then the
compiler is allowed to choose any alignment it likes, and there is no way
to control this choice. Consider:

@example
type R is range 1 .. 10_000;
for R'Alignment use 1;
subtype RS is R range 1 .. 1000;
@end example

The alignment clause specifies an alignment of 1 for the first named subtype
@cite{R} but this does not necessarily apply to @cite{RS}. When writing
portable Ada code, you should avoid writing code that explicitly or
implicitly relies on the alignment of such subtypes.

For the GNAT compiler, if an explicit alignment clause is given, this
value is also used for any subsequent subtypes. So for GNAT, in the
above example, you can count on the alignment of @cite{RS} being 1. But this
assumption is non-portable, and other compilers may choose different
alignments for the subtype @cite{RS}.

@node Size Clauses,Storage_Size Clauses,Alignment Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id3}@anchor{21c}@anchor{gnat_rm/representation_clauses_and_pragmas size-clauses}@anchor{21d}
@section Size Clauses


@geindex Size Clause

The default size for a type @cite{T} is obtainable through the
language-defined attribute @cite{T'Size} and also through the
equivalent GNAT-defined attribute @cite{T'Value_Size}.
For objects of type @cite{T}, GNAT will generally increase the type size
so that the object size (obtainable through the GNAT-defined attribute
@cite{T'Object_Size})
is a multiple of @cite{T'Alignment * Storage_Unit}.

For example:

@example
type Smallint is range 1 .. 6;

type Rec is record
   Y1 : integer;
   Y2 : boolean;
end record;
@end example

In this example, @cite{Smallint'Size} = @cite{Smallint'Value_Size} = 3,
as specified by the RM rules,
but objects of this type will have a size of 8
(@cite{Smallint'Object_Size} = 8),
since objects by default occupy an integral number
of storage units.  On some targets, notably older
versions of the Digital Alpha, the size of stand
alone objects of this type may be 32, reflecting
the inability of the hardware to do byte load/stores.

Similarly, the size of type @cite{Rec} is 40 bits
(@cite{Rec'Size} = @cite{Rec'Value_Size} = 40), but
the alignment is 4, so objects of this type will have
their size increased to 64 bits so that it is a multiple
of the alignment (in bits).  This decision is
in accordance with the specific Implementation Advice in RM 13.3(43):

@quotation

"A @cite{Size} clause should be supported for an object if the specified
@cite{Size} is at least as large as its subtype's @cite{Size}, and corresponds
to a size in storage elements that is a multiple of the object's
@cite{Alignment} (if the @cite{Alignment} is nonzero)."
@end quotation

An explicit size clause may be used to override the default size by
increasing it.  For example, if we have:

@example
type My_Boolean is new Boolean;
for My_Boolean'Size use 32;
@end example

then values of this type will always be 32 bits long.  In the case of
discrete types, the size can be increased up to 64 bits, with the effect
that the entire specified field is used to hold the value, sign- or
zero-extended as appropriate.  If more than 64 bits is specified, then
padding space is allocated after the value, and a warning is issued that
there are unused bits.

Similarly the size of records and arrays may be increased, and the effect
is to add padding bits after the value.  This also causes a warning message
to be generated.

The largest Size value permitted in GNAT is 2**31-1.  Since this is a
Size in bits, this corresponds to an object of size 256 megabytes (minus
one).  This limitation is true on all targets.  The reason for this
limitation is that it improves the quality of the code in many cases
if it is known that a Size value can be accommodated in an object of
type Integer.

@node Storage_Size Clauses,Size of Variant Record Objects,Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas storage-size-clauses}@anchor{21e}@anchor{gnat_rm/representation_clauses_and_pragmas id4}@anchor{21f}
@section Storage_Size Clauses


@geindex Storage_Size Clause

For tasks, the @cite{Storage_Size} clause specifies the amount of space
to be allocated for the task stack.  This cannot be extended, and if the
stack is exhausted, then @cite{Storage_Error} will be raised (if stack
checking is enabled).  Use a @cite{Storage_Size} attribute definition clause,
or a @cite{Storage_Size} pragma in the task definition to set the
appropriate required size.  A useful technique is to include in every
task definition a pragma of the form:

@example
pragma Storage_Size (Default_Stack_Size);
@end example

Then @cite{Default_Stack_Size} can be defined in a global package, and
modified as required. Any tasks requiring stack sizes different from the
default can have an appropriate alternative reference in the pragma.

You can also use the @emph{-d} binder switch to modify the default stack
size.

For access types, the @cite{Storage_Size} clause specifies the maximum
space available for allocation of objects of the type.  If this space is
exceeded then @cite{Storage_Error} will be raised by an allocation attempt.
In the case where the access type is declared local to a subprogram, the
use of a @cite{Storage_Size} clause triggers automatic use of a special
predefined storage pool (@cite{System.Pool_Size}) that ensures that all
space for the pool is automatically reclaimed on exit from the scope in
which the type is declared.

A special case recognized by the compiler is the specification of a
@cite{Storage_Size} of zero for an access type.  This means that no
items can be allocated from the pool, and this is recognized at compile
time, and all the overhead normally associated with maintaining a fixed
size storage pool is eliminated.  Consider the following example:

@example
procedure p is
   type R is array (Natural) of Character;
   type P is access all R;
   for P'Storage_Size use 0;
   --  Above access type intended only for interfacing purposes

   y : P;

   procedure g (m : P);
   pragma Import (C, g);

   --  ...

begin
   --  ...
   y := new R;
end;
@end example

As indicated in this example, these dummy storage pools are often useful in
connection with interfacing where no object will ever be allocated.  If you
compile the above example, you get the warning:

@example
p.adb:16:09: warning: allocation from empty storage pool
p.adb:16:09: warning: Storage_Error will be raised at run time
@end example

Of course in practice, there will not be any explicit allocators in the
case of such an access declaration.

@node Size of Variant Record Objects,Biased Representation,Storage_Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id5}@anchor{220}@anchor{gnat_rm/representation_clauses_and_pragmas size-of-variant-record-objects}@anchor{221}
@section Size of Variant Record Objects


@geindex Size
@geindex variant record objects

@geindex Variant record objects
@geindex size

In the case of variant record objects, there is a question whether Size gives
information about a particular variant, or the maximum size required
for any variant.  Consider the following program

@example
with Text_IO; use Text_IO;
procedure q is
   type R1 (A : Boolean := False) is record
     case A is
       when True  => X : Character;
       when False => null;
     end case;
   end record;

   V1 : R1 (False);
   V2 : R1;

begin
   Put_Line (Integer'Image (V1'Size));
   Put_Line (Integer'Image (V2'Size));
end q;
@end example

Here we are dealing with a variant record, where the True variant
requires 16 bits, and the False variant requires 8 bits.
In the above example, both V1 and V2 contain the False variant,
which is only 8 bits long.  However, the result of running the
program is:

@example
8
16
@end example

The reason for the difference here is that the discriminant value of
V1 is fixed, and will always be False.  It is not possible to assign
a True variant value to V1, therefore 8 bits is sufficient.  On the
other hand, in the case of V2, the initial discriminant value is
False (from the default), but it is possible to assign a True
variant value to V2, therefore 16 bits must be allocated for V2
in the general case, even fewer bits may be needed at any particular
point during the program execution.

As can be seen from the output of this program, the @cite{'Size}
attribute applied to such an object in GNAT gives the actual allocated
size of the variable, which is the largest size of any of the variants.
The Ada Reference Manual is not completely clear on what choice should
be made here, but the GNAT behavior seems most consistent with the
language in the RM.

In some cases, it may be desirable to obtain the size of the current
variant, rather than the size of the largest variant.  This can be
achieved in GNAT by making use of the fact that in the case of a
subprogram parameter, GNAT does indeed return the size of the current
variant (because a subprogram has no way of knowing how much space
is actually allocated for the actual).

Consider the following modified version of the above program:

@example
with Text_IO; use Text_IO;
procedure q is
   type R1 (A : Boolean := False) is record
     case A is
       when True  => X : Character;
       when False => null;
     end case;
   end record;

   V2 : R1;

   function Size (V : R1) return Integer is
   begin
      return V'Size;
   end Size;

begin
   Put_Line (Integer'Image (V2'Size));
   Put_Line (Integer'IMage (Size (V2)));
   V2 := (True, 'x');
   Put_Line (Integer'Image (V2'Size));
   Put_Line (Integer'IMage (Size (V2)));
end q;
@end example

The output from this program is

@example
16
8
16
16
@end example

Here we see that while the @cite{'Size} attribute always returns
the maximum size, regardless of the current variant value, the
@cite{Size} function does indeed return the size of the current
variant value.

@node Biased Representation,Value_Size and Object_Size Clauses,Size of Variant Record Objects,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id6}@anchor{222}@anchor{gnat_rm/representation_clauses_and_pragmas biased-representation}@anchor{223}
@section Biased Representation


@geindex Size for biased representation

@geindex Biased representation

In the case of scalars with a range starting at other than zero, it is
possible in some cases to specify a size smaller than the default minimum
value, and in such cases, GNAT uses an unsigned biased representation,
in which zero is used to represent the lower bound, and successive values
represent successive values of the type.

For example, suppose we have the declaration:

@example
type Small is range -7 .. -4;
for Small'Size use 2;
@end example

Although the default size of type @cite{Small} is 4, the @cite{Size}
clause is accepted by GNAT and results in the following representation
scheme:

@example
-7 is represented as 2#00#
-6 is represented as 2#01#
-5 is represented as 2#10#
-4 is represented as 2#11#
@end example

Biased representation is only used if the specified @cite{Size} clause
cannot be accepted in any other manner.  These reduced sizes that force
biased representation can be used for all discrete types except for
enumeration types for which a representation clause is given.

@node Value_Size and Object_Size Clauses,Component_Size Clauses,Biased Representation,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id7}@anchor{224}@anchor{gnat_rm/representation_clauses_and_pragmas value-size-and-object-size-clauses}@anchor{225}
@section Value_Size and Object_Size Clauses


@geindex Value_Size

@geindex Object_Size

@geindex Size
@geindex of objects

In Ada 95 and Ada 2005, @cite{T'Size} for a type @cite{T} is the minimum
number of bits required to hold values of type @cite{T}.
Although this interpretation was allowed in Ada 83, it was not required,
and this requirement in practice can cause some significant difficulties.
For example, in most Ada 83 compilers, @cite{Natural'Size} was 32.
However, in Ada 95 and Ada 2005,
@cite{Natural'Size} is
typically 31.  This means that code may change in behavior when moving
from Ada 83 to Ada 95 or Ada 2005.  For example, consider:

@example
type Rec is record;
   A : Natural;
   B : Natural;
end record;

for Rec use record
   at 0  range 0 .. Natural'Size - 1;
   at 0  range Natural'Size .. 2 * Natural'Size - 1;
end record;
@end example

In the above code, since the typical size of @cite{Natural} objects
is 32 bits and @cite{Natural'Size} is 31, the above code can cause
unexpected inefficient packing in Ada 95 and Ada 2005, and in general
there are cases where the fact that the object size can exceed the
size of the type causes surprises.

To help get around this problem GNAT provides two implementation
defined attributes, @cite{Value_Size} and @cite{Object_Size}.  When
applied to a type, these attributes yield the size of the type
(corresponding to the RM defined size attribute), and the size of
objects of the type respectively.

The @cite{Object_Size} is used for determining the default size of
objects and components.  This size value can be referred to using the
@cite{Object_Size} attribute.  The phrase 'is used' here means that it is
the basis of the determination of the size.  The backend is free to
pad this up if necessary for efficiency, e.g., an 8-bit stand-alone
character might be stored in 32 bits on a machine with no efficient
byte access instructions such as the Alpha.

The default rules for the value of @cite{Object_Size} for
discrete types are as follows:


@itemize *

@item 
The @cite{Object_Size} for base subtypes reflect the natural hardware
size in bits (run the compiler with @emph{-gnatS} to find those values
for numeric types). Enumeration types and fixed-point base subtypes have
8, 16, 32 or 64 bits for this size, depending on the range of values
to be stored.

@item 
The @cite{Object_Size} of a subtype is the same as the
@cite{Object_Size} of
the type from which it is obtained.

@item 
The @cite{Object_Size} of a derived base type is copied from the parent
base type, and the @cite{Object_Size} of a derived first subtype is copied
from the parent first subtype.
@end itemize

The @cite{Value_Size} attribute
is the (minimum) number of bits required to store a value
of the type.
This value is used to determine how tightly to pack
records or arrays with components of this type, and also affects
the semantics of unchecked conversion (unchecked conversions where
the @cite{Value_Size} values differ generate a warning, and are potentially
target dependent).

The default rules for the value of @cite{Value_Size} are as follows:


@itemize *

@item 
The @cite{Value_Size} for a base subtype is the minimum number of bits
required to store all values of the type (including the sign bit
only if negative values are possible).

@item 
If a subtype statically matches the first subtype of a given type, then it has
by default the same @cite{Value_Size} as the first subtype.  This is a
consequence of RM 13.1(14): "if two subtypes statically match,
then their subtype-specific aspects are the same".)

@item 
All other subtypes have a @cite{Value_Size} corresponding to the minimum
number of bits required to store all values of the subtype.  For
dynamic bounds, it is assumed that the value can range down or up
to the corresponding bound of the ancestor
@end itemize

The RM defined attribute @cite{Size} corresponds to the
@cite{Value_Size} attribute.

The @cite{Size} attribute may be defined for a first-named subtype.  This sets
the @cite{Value_Size} of
the first-named subtype to the given value, and the
@cite{Object_Size} of this first-named subtype to the given value padded up
to an appropriate boundary.  It is a consequence of the default rules
above that this @cite{Object_Size} will apply to all further subtypes.  On the
other hand, @cite{Value_Size} is affected only for the first subtype, any
dynamic subtypes obtained from it directly, and any statically matching
subtypes.  The @cite{Value_Size} of any other static subtypes is not affected.

@cite{Value_Size} and
@cite{Object_Size} may be explicitly set for any subtype using
an attribute definition clause.  Note that the use of these attributes
can cause the RM 13.1(14) rule to be violated.  If two access types
reference aliased objects whose subtypes have differing @cite{Object_Size}
values as a result of explicit attribute definition clauses, then it
is illegal to convert from one access subtype to the other. For a more
complete description of this additional legality rule, see the
description of the @cite{Object_Size} attribute.

At the implementation level, Esize stores the Object_Size and the
RM_Size field stores the @cite{Value_Size} (and hence the value of the
@cite{Size} attribute,
which, as noted above, is equivalent to @cite{Value_Size}).

To get a feel for the difference, consider the following examples (note
that in each case the base is @cite{Short_Short_Integer} with a size of 8):


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxx} 
@headitem

Type or subtype declaration

@tab

Object_Size

@tab

Value_Size

@item

@code{type x1 is range 0 .. 5;}

@tab

8

@tab

3

@item

@code{type x2 is range 0 .. 5;}
@code{for x2'size use 12;}

@tab

16

@tab

12

@item

@code{subtype x3 is x2 range 0 .. 3;}

@tab

16

@tab

2

@item

@code{subtype x4 is x2'base range 0 .. 10;}

@tab

8

@tab

4

@item

@code{subtype x5 is x2 range 0 .. dynamic;}

@tab

16

@tab

3*

@item

@code{subtype x6 is x2'base range 0 .. dynamic;}

@tab

8

@tab

3*

@end multitable


Note: the entries marked '3*' are not actually specified by the Ada
Reference Manual, but it seems in the spirit of the RM rules to allocate
the minimum number of bits (here 3, given the range for @cite{x2})
known to be large enough to hold the given range of values.

So far, so good, but GNAT has to obey the RM rules, so the question is
under what conditions must the RM @cite{Size} be used.
The following is a list
of the occasions on which the RM @cite{Size} must be used:


@itemize *

@item 
Component size for packed arrays or records

@item 
Value of the attribute @cite{Size} for a type

@item 
Warning about sizes not matching for unchecked conversion
@end itemize

For record types, the @cite{Object_Size} is always a multiple of the
alignment of the type (this is true for all types). In some cases the
@cite{Value_Size} can be smaller. Consider:

@example
type R is record
  X : Integer;
  Y : Character;
end record;
@end example

On a typical 32-bit architecture, the X component will be four bytes, and
require four-byte alignment, and the Y component will be one byte. In this
case @cite{R'Value_Size} will be 40 (bits) since this is the minimum size
required to store a value of this type, and for example, it is permissible
to have a component of type R in an outer array whose component size is
specified to be 48 bits. However, @cite{R'Object_Size} will be 64 (bits),
since it must be rounded up so that this value is a multiple of the
alignment (4 bytes = 32 bits).

For all other types, the @cite{Object_Size}
and Value_Size are the same (and equivalent to the RM attribute @cite{Size}).
Only @cite{Size} may be specified for such types.

Note that @cite{Value_Size} can be used to force biased representation
for a particular subtype. Consider this example:

@example
type R is (A, B, C, D, E, F);
subtype RAB is R range A .. B;
subtype REF is R range E .. F;
@end example

By default, @cite{RAB}
has a size of 1 (sufficient to accommodate the representation
of @cite{A} and @cite{B}, 0 and 1), and @cite{REF}
has a size of 3 (sufficient to accommodate the representation
of @cite{E} and @cite{F}, 4 and 5). But if we add the
following @cite{Value_Size} attribute definition clause:

@example
for REF'Value_Size use 1;
@end example

then biased representation is forced for @cite{REF},
and 0 will represent @cite{E} and 1 will represent @cite{F}.
A warning is issued when a @cite{Value_Size} attribute
definition clause forces biased representation. This
warning can be turned off using @cite{-gnatw.B}.

@node Component_Size Clauses,Bit_Order Clauses,Value_Size and Object_Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id8}@anchor{226}@anchor{gnat_rm/representation_clauses_and_pragmas component-size-clauses}@anchor{227}
@section Component_Size Clauses


@geindex Component_Size Clause

Normally, the value specified in a component size clause must be consistent
with the subtype of the array component with regard to size and alignment.
In other words, the value specified must be at least equal to the size
of this subtype, and must be a multiple of the alignment value.

In addition, component size clauses are allowed which cause the array
to be packed, by specifying a smaller value.  A first case is for
component size values in the range 1 through 63.  The value specified
must not be smaller than the Size of the subtype.  GNAT will accurately
honor all packing requests in this range.  For example, if we have:

@example
type r is array (1 .. 8) of Natural;
for r'Component_Size use 31;
@end example

then the resulting array has a length of 31 bytes (248 bits = 8 * 31).
Of course access to the components of such an array is considerably
less efficient than if the natural component size of 32 is used.
A second case is when the subtype of the component is a record type
padded because of its default alignment.  For example, if we have:

@example
type r is record
  i : Integer;
  j : Integer;
  b : Boolean;
end record;

type a is array (1 .. 8) of r;
for a'Component_Size use 72;
@end example

then the resulting array has a length of 72 bytes, instead of 96 bytes
if the alignment of the record (4) was obeyed.

Note that there is no point in giving both a component size clause
and a pragma Pack for the same array type. if such duplicate
clauses are given, the pragma Pack will be ignored.

@node Bit_Order Clauses,Effect of Bit_Order on Byte Ordering,Component_Size Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas bit-order-clauses}@anchor{228}@anchor{gnat_rm/representation_clauses_and_pragmas id9}@anchor{229}
@section Bit_Order Clauses


@geindex Bit_Order Clause

@geindex bit ordering

@geindex ordering
@geindex of bits

For record subtypes, GNAT permits the specification of the @cite{Bit_Order}
attribute.  The specification may either correspond to the default bit
order for the target, in which case the specification has no effect and
places no additional restrictions, or it may be for the non-standard
setting (that is the opposite of the default).

In the case where the non-standard value is specified, the effect is
to renumber bits within each byte, but the ordering of bytes is not
affected.  There are certain
restrictions placed on component clauses as follows:


@itemize *

@item 
Components fitting within a single storage unit.

These are unrestricted, and the effect is merely to renumber bits.  For
example if we are on a little-endian machine with @cite{Low_Order_First}
being the default, then the following two declarations have exactly
the same effect:

@example
type R1 is record
   A : Boolean;
   B : Integer range 1 .. 120;
end record;

for R1 use record
   A at 0 range 0 .. 0;
   B at 0 range 1 .. 7;
end record;

type R2 is record
   A : Boolean;
   B : Integer range 1 .. 120;
end record;

for R2'Bit_Order use High_Order_First;

for R2 use record
   A at 0 range 7 .. 7;
   B at 0 range 0 .. 6;
end record;
@end example

The useful application here is to write the second declaration with the
@cite{Bit_Order} attribute definition clause, and know that it will be treated
the same, regardless of whether the target is little-endian or big-endian.

@item 
Components occupying an integral number of bytes.

These are components that exactly fit in two or more bytes.  Such component
declarations are allowed, but have no effect, since it is important to realize
that the @cite{Bit_Order} specification does not affect the ordering of bytes.
In particular, the following attempt at getting an endian-independent integer
does not work:

@example
type R2 is record
   A : Integer;
end record;

for R2'Bit_Order use High_Order_First;

for R2 use record
   A at 0 range 0 .. 31;
end record;
@end example

This declaration will result in a little-endian integer on a
little-endian machine, and a big-endian integer on a big-endian machine.
If byte flipping is required for interoperability between big- and
little-endian machines, this must be explicitly programmed.  This capability
is not provided by @cite{Bit_Order}.

@item 
Components that are positioned across byte boundaries

but do not occupy an integral number of bytes.  Given that bytes are not
reordered, such fields would occupy a non-contiguous sequence of bits
in memory, requiring non-trivial code to reassemble.  They are for this
reason not permitted, and any component clause specifying such a layout
will be flagged as illegal by GNAT.
@end itemize

Since the misconception that Bit_Order automatically deals with all
endian-related incompatibilities is a common one, the specification of
a component field that is an integral number of bytes will always
generate a warning.  This warning may be suppressed using @cite{pragma Warnings (Off)}
if desired.  The following section contains additional
details regarding the issue of byte ordering.

@node Effect of Bit_Order on Byte Ordering,Pragma Pack for Arrays,Bit_Order Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id10}@anchor{22a}@anchor{gnat_rm/representation_clauses_and_pragmas effect-of-bit-order-on-byte-ordering}@anchor{22b}
@section Effect of Bit_Order on Byte Ordering


@geindex byte ordering

@geindex ordering
@geindex of bytes

In this section we will review the effect of the @cite{Bit_Order} attribute
definition clause on byte ordering.  Briefly, it has no effect at all, but
a detailed example will be helpful.  Before giving this
example, let us review the precise
definition of the effect of defining @cite{Bit_Order}.  The effect of a
non-standard bit order is described in section 15.5.3 of the Ada
Reference Manual:

@quotation

"2   A bit ordering is a method of interpreting the meaning of
the storage place attributes."
@end quotation

To understand the precise definition of storage place attributes in
this context, we visit section 13.5.1 of the manual:

@quotation

"13   A record_representation_clause (without the mod_clause)
specifies the layout.  The storage place attributes (see 13.5.2)
are taken from the values of the position, first_bit, and last_bit
expressions after normalizing those values so that first_bit is
less than Storage_Unit."
@end quotation

The critical point here is that storage places are taken from
the values after normalization, not before.  So the @cite{Bit_Order}
interpretation applies to normalized values.  The interpretation
is described in the later part of the 15.5.3 paragraph:

@quotation

"2   A bit ordering is a method of interpreting the meaning of
the storage place attributes.  High_Order_First (known in the
vernacular as 'big endian') means that the first bit of a
storage element (bit 0) is the most significant bit (interpreting
the sequence of bits that represent a component as an unsigned
integer value).  Low_Order_First (known in the vernacular as
'little endian') means the opposite: the first bit is the
least significant."
@end quotation

Note that the numbering is with respect to the bits of a storage
unit.  In other words, the specification affects only the numbering
of bits within a single storage unit.

We can make the effect clearer by giving an example.

Suppose that we have an external device which presents two bytes, the first
byte presented, which is the first (low addressed byte) of the two byte
record is called Master, and the second byte is called Slave.

The left most (most significant bit is called Control for each byte, and
the remaining 7 bits are called V1, V2, ... V7, where V7 is the rightmost
(least significant) bit.

On a big-endian machine, we can write the following representation clause

@example
type Data is record
   Master_Control : Bit;
   Master_V1      : Bit;
   Master_V2      : Bit;
   Master_V3      : Bit;
   Master_V4      : Bit;
   Master_V5      : Bit;
   Master_V6      : Bit;
   Master_V7      : Bit;
   Slave_Control  : Bit;
   Slave_V1       : Bit;
   Slave_V2       : Bit;
   Slave_V3       : Bit;
   Slave_V4       : Bit;
   Slave_V5       : Bit;
   Slave_V6       : Bit;
   Slave_V7       : Bit;
end record;

for Data use record
   Master_Control at 0 range 0 .. 0;
   Master_V1      at 0 range 1 .. 1;
   Master_V2      at 0 range 2 .. 2;
   Master_V3      at 0 range 3 .. 3;
   Master_V4      at 0 range 4 .. 4;
   Master_V5      at 0 range 5 .. 5;
   Master_V6      at 0 range 6 .. 6;
   Master_V7      at 0 range 7 .. 7;
   Slave_Control  at 1 range 0 .. 0;
   Slave_V1       at 1 range 1 .. 1;
   Slave_V2       at 1 range 2 .. 2;
   Slave_V3       at 1 range 3 .. 3;
   Slave_V4       at 1 range 4 .. 4;
   Slave_V5       at 1 range 5 .. 5;
   Slave_V6       at 1 range 6 .. 6;
   Slave_V7       at 1 range 7 .. 7;
end record;
@end example

Now if we move this to a little endian machine, then the bit ordering within
the byte is backwards, so we have to rewrite the record rep clause as:

@example
for Data use record
   Master_Control at 0 range 7 .. 7;
   Master_V1      at 0 range 6 .. 6;
   Master_V2      at 0 range 5 .. 5;
   Master_V3      at 0 range 4 .. 4;
   Master_V4      at 0 range 3 .. 3;
   Master_V5      at 0 range 2 .. 2;
   Master_V6      at 0 range 1 .. 1;
   Master_V7      at 0 range 0 .. 0;
   Slave_Control  at 1 range 7 .. 7;
   Slave_V1       at 1 range 6 .. 6;
   Slave_V2       at 1 range 5 .. 5;
   Slave_V3       at 1 range 4 .. 4;
   Slave_V4       at 1 range 3 .. 3;
   Slave_V5       at 1 range 2 .. 2;
   Slave_V6       at 1 range 1 .. 1;
   Slave_V7       at 1 range 0 .. 0;
end record;
@end example

It is a nuisance to have to rewrite the clause, especially if
the code has to be maintained on both machines.  However,
this is a case that we can handle with the
@cite{Bit_Order} attribute if it is implemented.
Note that the implementation is not required on byte addressed
machines, but it is indeed implemented in GNAT.
This means that we can simply use the
first record clause, together with the declaration

@example
for Data'Bit_Order use High_Order_First;
@end example

and the effect is what is desired, namely the layout is exactly the same,
independent of whether the code is compiled on a big-endian or little-endian
machine.

The important point to understand is that byte ordering is not affected.
A @cite{Bit_Order} attribute definition never affects which byte a field
ends up in, only where it ends up in that byte.
To make this clear, let us rewrite the record rep clause of the previous
example as:

@example
for Data'Bit_Order use High_Order_First;
for Data use record
   Master_Control at 0 range  0 .. 0;
   Master_V1      at 0 range  1 .. 1;
   Master_V2      at 0 range  2 .. 2;
   Master_V3      at 0 range  3 .. 3;
   Master_V4      at 0 range  4 .. 4;
   Master_V5      at 0 range  5 .. 5;
   Master_V6      at 0 range  6 .. 6;
   Master_V7      at 0 range  7 .. 7;
   Slave_Control  at 0 range  8 .. 8;
   Slave_V1       at 0 range  9 .. 9;
   Slave_V2       at 0 range 10 .. 10;
   Slave_V3       at 0 range 11 .. 11;
   Slave_V4       at 0 range 12 .. 12;
   Slave_V5       at 0 range 13 .. 13;
   Slave_V6       at 0 range 14 .. 14;
   Slave_V7       at 0 range 15 .. 15;
end record;
@end example

This is exactly equivalent to saying (a repeat of the first example):

@example
for Data'Bit_Order use High_Order_First;
for Data use record
   Master_Control at 0 range 0 .. 0;
   Master_V1      at 0 range 1 .. 1;
   Master_V2      at 0 range 2 .. 2;
   Master_V3      at 0 range 3 .. 3;
   Master_V4      at 0 range 4 .. 4;
   Master_V5      at 0 range 5 .. 5;
   Master_V6      at 0 range 6 .. 6;
   Master_V7      at 0 range 7 .. 7;
   Slave_Control  at 1 range 0 .. 0;
   Slave_V1       at 1 range 1 .. 1;
   Slave_V2       at 1 range 2 .. 2;
   Slave_V3       at 1 range 3 .. 3;
   Slave_V4       at 1 range 4 .. 4;
   Slave_V5       at 1 range 5 .. 5;
   Slave_V6       at 1 range 6 .. 6;
   Slave_V7       at 1 range 7 .. 7;
end record;
@end example

Why are they equivalent? Well take a specific field, the @cite{Slave_V2}
field.  The storage place attributes are obtained by normalizing the
values given so that the @cite{First_Bit} value is less than 8.  After
normalizing the values (0,10,10) we get (1,2,2) which is exactly what
we specified in the other case.

Now one might expect that the @cite{Bit_Order} attribute might affect
bit numbering within the entire record component (two bytes in this
case, thus affecting which byte fields end up in), but that is not
the way this feature is defined, it only affects numbering of bits,
not which byte they end up in.

Consequently it never makes sense to specify a starting bit number
greater than 7 (for a byte addressable field) if an attribute
definition for @cite{Bit_Order} has been given, and indeed it
may be actively confusing to specify such a value, so the compiler
generates a warning for such usage.

If you do need to control byte ordering then appropriate conditional
values must be used.  If in our example, the slave byte came first on
some machines we might write:

@example
Master_Byte_First constant Boolean := ...;

Master_Byte : constant Natural :=
                1 - Boolean'Pos (Master_Byte_First);
Slave_Byte  : constant Natural :=
                Boolean'Pos (Master_Byte_First);

for Data'Bit_Order use High_Order_First;
for Data use record
   Master_Control at Master_Byte range 0 .. 0;
   Master_V1      at Master_Byte range 1 .. 1;
   Master_V2      at Master_Byte range 2 .. 2;
   Master_V3      at Master_Byte range 3 .. 3;
   Master_V4      at Master_Byte range 4 .. 4;
   Master_V5      at Master_Byte range 5 .. 5;
   Master_V6      at Master_Byte range 6 .. 6;
   Master_V7      at Master_Byte range 7 .. 7;
   Slave_Control  at Slave_Byte  range 0 .. 0;
   Slave_V1       at Slave_Byte  range 1 .. 1;
   Slave_V2       at Slave_Byte  range 2 .. 2;
   Slave_V3       at Slave_Byte  range 3 .. 3;
   Slave_V4       at Slave_Byte  range 4 .. 4;
   Slave_V5       at Slave_Byte  range 5 .. 5;
   Slave_V6       at Slave_Byte  range 6 .. 6;
   Slave_V7       at Slave_Byte  range 7 .. 7;
end record;
@end example

Now to switch between machines, all that is necessary is
to set the boolean constant @cite{Master_Byte_First} in
an appropriate manner.

@node Pragma Pack for Arrays,Pragma Pack for Records,Effect of Bit_Order on Byte Ordering,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas pragma-pack-for-arrays}@anchor{22c}@anchor{gnat_rm/representation_clauses_and_pragmas id11}@anchor{22d}
@section Pragma Pack for Arrays


@geindex Pragma Pack (for arrays)

Pragma @cite{Pack} applied to an array has no effect unless the component type
is packable.  For a component type to be packable, it must be one of the
following cases:


@itemize *

@item 
Any scalar type

@item 
Any type whose size is specified with a size clause

@item 
Any packed array type with a static size

@item 
Any record type padded because of its default alignment
@end itemize

For all these cases, if the component subtype size is in the range
1 through 63, then the effect of the pragma @cite{Pack} is exactly as though a
component size were specified giving the component subtype size.
For example if we have:

@example
type r is range 0 .. 17;

type ar is array (1 .. 8) of r;
pragma Pack (ar);
@end example

Then the component size of @cite{ar} will be set to 5 (i.e., to @cite{r'size},
and the size of the array @cite{ar} will be exactly 40 bits.

Note that in some cases this rather fierce approach to packing can produce
unexpected effects.  For example, in Ada 95 and Ada 2005,
subtype @cite{Natural} typically has a size of 31, meaning that if you
pack an array of @cite{Natural}, you get 31-bit
close packing, which saves a few bits, but results in far less efficient
access.  Since many other Ada compilers will ignore such a packing request,
GNAT will generate a warning on some uses of pragma @cite{Pack} that it guesses
might not be what is intended.  You can easily remove this warning by
using an explicit @cite{Component_Size} setting instead, which never generates
a warning, since the intention of the programmer is clear in this case.

GNAT treats packed arrays in one of two ways.  If the size of the array is
known at compile time and is less than 64 bits, then internally the array
is represented as a single modular type, of exactly the appropriate number
of bits.  If the length is greater than 63 bits, or is not known at compile
time, then the packed array is represented as an array of bytes, and the
length is always a multiple of 8 bits.

Note that to represent a packed array as a modular type, the alignment must
be suitable for the modular type involved. For example, on typical machines
a 32-bit packed array will be represented by a 32-bit modular integer with
an alignment of four bytes. If you explicitly override the default alignment
with an alignment clause that is too small, the modular representation
cannot be used. For example, consider the following set of declarations:

@example
type R is range 1 .. 3;
type S is array (1 .. 31) of R;
for S'Component_Size use 2;
for S'Size use 62;
for S'Alignment use 1;
@end example

If the alignment clause were not present, then a 62-bit modular
representation would be chosen (typically with an alignment of 4 or 8
bytes depending on the target). But the default alignment is overridden
with the explicit alignment clause. This means that the modular
representation cannot be used, and instead the array of bytes
representation must be used, meaning that the length must be a multiple
of 8. Thus the above set of declarations will result in a diagnostic
rejecting the size clause and noting that the minimum size allowed is 64.

@geindex Pragma Pack (for type Natural)

@geindex Pragma Pack warning

One special case that is worth noting occurs when the base type of the
component size is 8/16/32 and the subtype is one bit less. Notably this
occurs with subtype @cite{Natural}. Consider:

@example
type Arr is array (1 .. 32) of Natural;
pragma Pack (Arr);
@end example

In all commonly used Ada 83 compilers, this pragma Pack would be ignored,
since typically @cite{Natural'Size} is 32 in Ada 83, and in any case most
Ada 83 compilers did not attempt 31 bit packing.

In Ada 95 and Ada 2005, @cite{Natural'Size} is required to be 31. Furthermore,
GNAT really does pack 31-bit subtype to 31 bits. This may result in a
substantial unintended performance penalty when porting legacy Ada 83 code.
To help prevent this, GNAT generates a warning in such cases. If you really
want 31 bit packing in a case like this, you can set the component size
explicitly:

@example
type Arr is array (1 .. 32) of Natural;
for Arr'Component_Size use 31;
@end example

Here 31-bit packing is achieved as required, and no warning is generated,
since in this case the programmer intention is clear.

@node Pragma Pack for Records,Record Representation Clauses,Pragma Pack for Arrays,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas pragma-pack-for-records}@anchor{22e}@anchor{gnat_rm/representation_clauses_and_pragmas id12}@anchor{22f}
@section Pragma Pack for Records


@geindex Pragma Pack (for records)

Pragma @cite{Pack} applied to a record will pack the components to reduce
wasted space from alignment gaps and by reducing the amount of space
taken by components.  We distinguish between @emph{packable} components and
@emph{non-packable} components.
Components of the following types are considered packable:


@itemize *

@item 
Components of a primitive type are packable unless they are aliased
or of an atomic type.

@item 
Small packed arrays, whose size does not exceed 64 bits, and where the
size is statically known at compile time, are represented internally
as modular integers, and so they are also packable.
@end itemize

All packable components occupy the exact number of bits corresponding to
their @cite{Size} value, and are packed with no padding bits, i.e., they
can start on an arbitrary bit boundary.

All other types are non-packable, they occupy an integral number of
storage units, and
are placed at a boundary corresponding to their alignment requirements.

For example, consider the record

@example
type Rb1 is array (1 .. 13) of Boolean;
pragma Pack (Rb1);

type Rb2 is array (1 .. 65) of Boolean;
pragma Pack (Rb2);

type AF is new Float with Atomic;

type X2 is record
   L1 : Boolean;
   L2 : Duration;
   L3 : AF;
   L4 : Boolean;
   L5 : Rb1;
   L6 : Rb2;
end record;
pragma Pack (X2);
@end example

The representation for the record X2 is as follows:

@example
for X2'Size use 224;
for X2 use record
   L1 at  0 range  0 .. 0;
   L2 at  0 range  1 .. 64;
   L3 at 12 range  0 .. 31;
   L4 at 16 range  0 .. 0;
   L5 at 16 range  1 .. 13;
   L6 at 18 range  0 .. 71;
end record;
@end example

Studying this example, we see that the packable fields @cite{L1}
and @cite{L2} are
of length equal to their sizes, and placed at specific bit boundaries (and
not byte boundaries) to
eliminate padding.  But @cite{L3} is of a non-packable float type (because
it is aliased), so it is on the next appropriate alignment boundary.

The next two fields are fully packable, so @cite{L4} and @cite{L5} are
minimally packed with no gaps.  However, type @cite{Rb2} is a packed
array that is longer than 64 bits, so it is itself non-packable.  Thus
the @cite{L6} field is aligned to the next byte boundary, and takes an
integral number of bytes, i.e., 72 bits.

@node Record Representation Clauses,Handling of Records with Holes,Pragma Pack for Records,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id13}@anchor{230}@anchor{gnat_rm/representation_clauses_and_pragmas record-representation-clauses}@anchor{231}
@section Record Representation Clauses


@geindex Record Representation Clause

Record representation clauses may be given for all record types, including
types obtained by record extension.  Component clauses are allowed for any
static component.  The restrictions on component clauses depend on the type
of the component.

@geindex Component Clause

For all components of an elementary type, the only restriction on component
clauses is that the size must be at least the 'Size value of the type
(actually the Value_Size).  There are no restrictions due to alignment,
and such components may freely cross storage boundaries.

Packed arrays with a size up to and including 64 bits are represented
internally using a modular type with the appropriate number of bits, and
thus the same lack of restriction applies.  For example, if you declare:

@example
type R is array (1 .. 49) of Boolean;
pragma Pack (R);
for R'Size use 49;
@end example

then a component clause for a component of type R may start on any
specified bit boundary, and may specify a value of 49 bits or greater.

For packed bit arrays that are longer than 64 bits, there are two
cases. If the component size is a power of 2 (1,2,4,8,16,32 bits),
including the important case of single bits or boolean values, then
there are no limitations on placement of such components, and they
may start and end at arbitrary bit boundaries.

If the component size is not a power of 2 (e.g., 3 or 5), then
an array of this type longer than 64 bits must always be placed on
on a storage unit (byte) boundary and occupy an integral number
of storage units (bytes). Any component clause that does not
meet this requirement will be rejected.

Any aliased component, or component of an aliased type, must
have its normal alignment and size. A component clause that
does not meet this requirement will be rejected.

The tag field of a tagged type always occupies an address sized field at
the start of the record.  No component clause may attempt to overlay this
tag. When a tagged type appears as a component, the tag field must have
proper alignment

In the case of a record extension T1, of a type T, no component clause applied
to the type T1 can specify a storage location that would overlap the first
T'Size bytes of the record.

For all other component types, including non-bit-packed arrays,
the component can be placed at an arbitrary bit boundary,
so for example, the following is permitted:

@example
type R is array (1 .. 10) of Boolean;
for R'Size use 80;

type Q is record
   G, H : Boolean;
   L, M : R;
end record;

for Q use record
   G at 0 range  0 ..   0;
   H at 0 range  1 ..   1;
   L at 0 range  2 ..  81;
   R at 0 range 82 .. 161;
end record;
@end example

Note: the above rules apply to recent releases of GNAT 5.
In GNAT 3, there are more severe restrictions on larger components.
For non-primitive types, including packed arrays with a size greater than
64 bits, component clauses must respect the alignment requirement of the
type, in particular, always starting on a byte boundary, and the length
must be a multiple of the storage unit.

@node Handling of Records with Holes,Enumeration Clauses,Record Representation Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas handling-of-records-with-holes}@anchor{232}@anchor{gnat_rm/representation_clauses_and_pragmas id14}@anchor{233}
@section Handling of Records with Holes


@geindex Handling of Records with Holes

As a result of alignment considerations, records may contain "holes"
or gaps
which do not correspond to the data bits of any of the components.
Record representation clauses can also result in holes in records.

GNAT does not attempt to clear these holes, so in record objects,
they should be considered to hold undefined rubbish. The generated
equality routine just tests components so does not access these
undefined bits, and assignment and copy operations may or may not
preserve the contents of these holes (for assignments, the holes
in the target will in practice contain either the bits that are
present in the holes in the source, or the bits that were present
in the target before the assignment).

If it is necessary to ensure that holes in records have all zero
bits, then record objects for which this initialization is desired
should be explicitly set to all zero values using Unchecked_Conversion
or address overlays. For example

@example
type HRec is record
   C : Character;
   I : Integer;
end record;
@end example

On typical machines, integers need to be aligned on a four-byte
boundary, resulting in three bytes of undefined rubbish following
the 8-bit field for C. To ensure that the hole in a variable of
type HRec is set to all zero bits,
you could for example do:

@example
type Base is record
   Dummy1, Dummy2 : Integer := 0;
end record;

BaseVar : Base;
RealVar : Hrec;
for RealVar'Address use BaseVar'Address;
@end example

Now the 8-bytes of the value of RealVar start out containing all zero
bits. A safer approach is to just define dummy fields, avoiding the
holes, as in:

@example
type HRec is record
   C      : Character;
   Dummy1 : Short_Short_Integer := 0;
   Dummy2 : Short_Short_Integer := 0;
   Dummy3 : Short_Short_Integer := 0;
   I      : Integer;
end record;
@end example

And to make absolutely sure that the intent of this is followed, you
can use representation clauses:

@example
for Hrec use record
   C      at 0 range 0 .. 7;
   Dummy1 at 1 range 0 .. 7;
   Dummy2 at 2 range 0 .. 7;
   Dummy3 at 3 range 0 .. 7;
   I      at 4 range 0 .. 31;
end record;
for Hrec'Size use 64;
@end example

@node Enumeration Clauses,Address Clauses,Handling of Records with Holes,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas enumeration-clauses}@anchor{234}@anchor{gnat_rm/representation_clauses_and_pragmas id15}@anchor{235}
@section Enumeration Clauses


The only restriction on enumeration clauses is that the range of values
must be representable.  For the signed case, if one or more of the
representation values are negative, all values must be in the range:

@example
System.Min_Int .. System.Max_Int
@end example

For the unsigned case, where all values are nonnegative, the values must
be in the range:

@example
0 .. System.Max_Binary_Modulus;
@end example

A @emph{confirming} representation clause is one in which the values range
from 0 in sequence, i.e., a clause that confirms the default representation
for an enumeration type.
Such a confirming representation
is permitted by these rules, and is specially recognized by the compiler so
that no extra overhead results from the use of such a clause.

If an array has an index type which is an enumeration type to which an
enumeration clause has been applied, then the array is stored in a compact
manner.  Consider the declarations:

@example
type r is (A, B, C);
for r use (A => 1, B => 5, C => 10);
type t is array (r) of Character;
@end example

The array type t corresponds to a vector with exactly three elements and
has a default size equal to @cite{3*Character'Size}.  This ensures efficient
use of space, but means that accesses to elements of the array will incur
the overhead of converting representation values to the corresponding
positional values, (i.e., the value delivered by the @cite{Pos} attribute).

@node Address Clauses,Use of Address Clauses for Memory-Mapped I/O,Enumeration Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id16}@anchor{236}@anchor{gnat_rm/representation_clauses_and_pragmas address-clauses}@anchor{237}
@section Address Clauses


@geindex Address Clause

The reference manual allows a general restriction on representation clauses,
as found in RM 13.1(22):

@quotation

"An implementation need not support representation
items containing nonstatic expressions, except that
an implementation should support a representation item
for a given entity if each nonstatic expression in the
representation item is a name that statically denotes
a constant declared before the entity."
@end quotation

In practice this is applicable only to address clauses, since this is the
only case in which a non-static expression is permitted by the syntax.  As
the AARM notes in sections 13.1 (22.a-22.h):

@quotation

22.a   Reason: This is to avoid the following sort of thing:

22.b        X : Integer := F(...);
Y : Address := G(...);
for X'Address use Y;

22.c   In the above, we have to evaluate the
initialization expression for X before we
know where to put the result.  This seems
like an unreasonable implementation burden.

22.d   The above code should instead be written
like this:

22.e        Y : constant Address := G(...);
X : Integer := F(...);
for X'Address use Y;

22.f   This allows the expression 'Y' to be safely
evaluated before X is created.

22.g   The constant could be a formal parameter of mode in.

22.h   An implementation can support other nonstatic
expressions if it wants to.  Expressions of type
Address are hardly ever static, but their value
might be known at compile time anyway in many
cases.
@end quotation

GNAT does indeed permit many additional cases of non-static expressions.  In
particular, if the type involved is elementary there are no restrictions
(since in this case, holding a temporary copy of the initialization value,
if one is present, is inexpensive).  In addition, if there is no implicit or
explicit initialization, then there are no restrictions.  GNAT will reject
only the case where all three of these conditions hold:


@itemize *

@item 
The type of the item is non-elementary (e.g., a record or array).

@item 
There is explicit or implicit initialization required for the object.
Note that access values are always implicitly initialized.

@item 
The address value is non-static.  Here GNAT is more permissive than the
RM, and allows the address value to be the address of a previously declared
stand-alone variable, as long as it does not itself have an address clause.

@example
Anchor  : Some_Initialized_Type;
Overlay : Some_Initialized_Type;
for Overlay'Address use Anchor'Address;
@end example

However, the prefix of the address clause cannot be an array component, or
a component of a discriminated record.
@end itemize

As noted above in section 22.h, address values are typically non-static.  In
particular the To_Address function, even if applied to a literal value, is
a non-static function call.  To avoid this minor annoyance, GNAT provides
the implementation defined attribute 'To_Address.  The following two
expressions have identical values:

@geindex Attribute

@geindex To_Address

@example
To_Address (16#1234_0000#)
System'To_Address (16#1234_0000#);
@end example

except that the second form is considered to be a static expression, and
thus when used as an address clause value is always permitted.

Additionally, GNAT treats as static an address clause that is an
unchecked_conversion of a static integer value.  This simplifies the porting
of legacy code, and provides a portable equivalent to the GNAT attribute
@cite{To_Address}.

Another issue with address clauses is the interaction with alignment
requirements.  When an address clause is given for an object, the address
value must be consistent with the alignment of the object (which is usually
the same as the alignment of the type of the object).  If an address clause
is given that specifies an inappropriately aligned address value, then the
program execution is erroneous.

Since this source of erroneous behavior can have unfortunate effects on
machines with strict alignment requirements, GNAT
checks (at compile time if possible, generating a warning, or at execution
time with a run-time check) that the alignment is appropriate.  If the
run-time check fails, then @cite{Program_Error} is raised.  This run-time
check is suppressed if range checks are suppressed, or if the special GNAT
check Alignment_Check is suppressed, or if
@cite{pragma Restrictions (No_Elaboration_Code)} is in effect. It is also
suppressed by default on non-strict alignment machines (such as the x86).

Finally, GNAT does not permit overlaying of objects of controlled types or
composite types containing a controlled component. In most cases, the compiler
can detect an attempt at such overlays and will generate a warning at compile
time and a Program_Error exception at run time.

@geindex Export

An address clause cannot be given for an exported object.  More
understandably the real restriction is that objects with an address
clause cannot be exported.  This is because such variables are not
defined by the Ada program, so there is no external object to export.

@geindex Import

It is permissible to give an address clause and a pragma Import for the
same object.  In this case, the variable is not really defined by the
Ada program, so there is no external symbol to be linked.  The link name
and the external name are ignored in this case.  The reason that we allow this
combination is that it provides a useful idiom to avoid unwanted
initializations on objects with address clauses.

When an address clause is given for an object that has implicit or
explicit initialization, then by default initialization takes place.  This
means that the effect of the object declaration is to overwrite the
memory at the specified address.  This is almost always not what the
programmer wants, so GNAT will output a warning:

@example
with System;
package G is
   type R is record
      M : Integer := 0;
   end record;

   Ext : R;
   for Ext'Address use System'To_Address (16#1234_1234#);
       |
>>> warning: implicit initialization of "Ext" may
    modify overlaid storage
>>> warning: use pragma Import for "Ext" to suppress
    initialization (RM B(24))

end G;
@end example

As indicated by the warning message, the solution is to use a (dummy) pragma
Import to suppress this initialization.  The pragma tell the compiler that the
object is declared and initialized elsewhere.  The following package compiles
without warnings (and the initialization is suppressed):

@example
with System;
package G is
   type R is record
      M : Integer := 0;
   end record;

   Ext : R;
   for Ext'Address use System'To_Address (16#1234_1234#);
   pragma Import (Ada, Ext);
end G;
@end example

A final issue with address clauses involves their use for overlaying
variables, as in the following example:

@geindex Overlaying of objects

@example
A : Integer;
B : Integer;
for B'Address use A'Address;
@end example

or alternatively, using the form recommended by the RM:

@example
A    : Integer;
Addr : constant Address := A'Address;
B    : Integer;
for B'Address use Addr;
@end example

In both of these cases, @cite{A}
and @cite{B} become aliased to one another via the
address clause. This use of address clauses to overlay
variables, achieving an effect similar to unchecked
conversion was erroneous in Ada 83, but in Ada 95 and Ada 2005
the effect is implementation defined. Furthermore, the
Ada RM specifically recommends that in a situation
like this, @cite{B} should be subject to the following
implementation advice (RM 13.3(19)):

@quotation

"19  If the Address of an object is specified, or it is imported
or exported, then the implementation should not perform
optimizations based on assumptions of no aliases."
@end quotation

GNAT follows this recommendation, and goes further by also applying
this recommendation to the overlaid variable (@cite{A}
in the above example) in this case. This means that the overlay
works "as expected", in that a modification to one of the variables
will affect the value of the other.

Note that when address clause overlays are used in this way, there is an
issue of unintentional initialization, as shown by this example:

@example
package Overwrite_Record is
   type R is record
      A : Character := 'C';
      B : Character := 'A';
   end record;
   X : Short_Integer := 3;
   Y : R;
   for Y'Address use X'Address;
       |
>>> warning: default initialization of "Y" may
    modify "X", use pragma Import for "Y" to
    suppress initialization (RM B.1(24))

end Overwrite_Record;
@end example

Here the default initialization of @cite{Y} will clobber the value
of @cite{X}, which justifies the warning. The warning notes that
this effect can be eliminated by adding a @cite{pragma Import}
which suppresses the initialization:

@example
package Overwrite_Record is
   type R is record
      A : Character := 'C';
      B : Character := 'A';
   end record;
   X : Short_Integer := 3;
   Y : R;
   for Y'Address use X'Address;
   pragma Import (Ada, Y);
end Overwrite_Record;
@end example

Note that the use of @cite{pragma Initialize_Scalars} may cause variables to
be initialized when they would not otherwise have been in the absence
of the use of this pragma. This may cause an overlay to have this
unintended clobbering effect. The compiler avoids this for scalar
types, but not for composite objects (where in general the effect
of @cite{Initialize_Scalars} is part of the initialization routine
for the composite object:

@example
pragma Initialize_Scalars;
with Ada.Text_IO;  use Ada.Text_IO;
procedure Overwrite_Array is
   type Arr is array (1 .. 5) of Integer;
   X : Arr := (others => 1);
   A : Arr;
   for A'Address use X'Address;
       |
>>> warning: default initialization of "A" may
    modify "X", use pragma Import for "A" to
    suppress initialization (RM B.1(24))

begin
   if X /= Arr'(others => 1) then
      Put_Line ("X was clobbered");
   else
      Put_Line ("X was not clobbered");
   end if;
end Overwrite_Array;
@end example

The above program generates the warning as shown, and at execution
time, prints @cite{X was clobbered}. If the @cite{pragma Import} is
added as suggested:

@example
pragma Initialize_Scalars;
with Ada.Text_IO;  use Ada.Text_IO;
procedure Overwrite_Array is
   type Arr is array (1 .. 5) of Integer;
   X : Arr := (others => 1);
   A : Arr;
   for A'Address use X'Address;
   pragma Import (Ada, A);
begin
   if X /= Arr'(others => 1) then
      Put_Line ("X was clobbered");
   else
      Put_Line ("X was not clobbered");
   end if;
end Overwrite_Array;
@end example

then the program compiles without the warning and when run will generate
the output @cite{X was not clobbered}.

@node Use of Address Clauses for Memory-Mapped I/O,Effect of Convention on Representation,Address Clauses,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id17}@anchor{238}@anchor{gnat_rm/representation_clauses_and_pragmas use-of-address-clauses-for-memory-mapped-i-o}@anchor{239}
@section Use of Address Clauses for Memory-Mapped I/O


@geindex Memory-mapped I/O

A common pattern is to use an address clause to map an atomic variable to
a location in memory that corresponds to a memory-mapped I/O operation or
operations, for example:

@example
type Mem_Word is record
   A,B,C,D : Byte;
end record;
pragma Atomic (Mem_Word);
for Mem_Word_Size use 32;

Mem : Mem_Word;
for Mem'Address use some-address;
...
Temp := Mem;
Temp.A := 32;
Mem := Temp;
@end example

For a full access (reference or modification) of the variable (Mem) in
this case, as in the above examples, GNAT guarantees that the entire atomic
word will be accessed. It is not clear whether the RM requires this. For
example in the above, can the compiler reference only the Mem.A field as
an optimization? Whatever the answer to this question is, GNAT makes the
guarantee that for such a reference, the entire word is read or written.

A problem arises with a component access such as:

@example
Mem.A := 32;
@end example

Note that the component A is not declared as atomic. This means that it is
not clear what this assignment means. It could correspond to full word read
and write as given in the first example, or on architectures that supported
such an operation it might be a single byte store instruction. The RM does
not have anything to say in this situation, and GNAT does not make any
guarantee. The code generated may vary from target to target. GNAT will issue
a warning in such a case:

@example
Mem.A := 32;
|
>>> warning: access to non-atomic component of atomic array,
    may cause unexpected accesses to atomic object
@end example

It is best to be explicit in this situation, by either declaring the
components to be atomic if you want the byte store, or explicitly writing
the full word access sequence if that is what the hardware requires.

@node Effect of Convention on Representation,Conventions and Anonymous Access Types,Use of Address Clauses for Memory-Mapped I/O,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id18}@anchor{23a}@anchor{gnat_rm/representation_clauses_and_pragmas effect-of-convention-on-representation}@anchor{23b}
@section Effect of Convention on Representation


@geindex Convention
@geindex effect on representation

Normally the specification of a foreign language convention for a type or
an object has no effect on the chosen representation.  In particular, the
representation chosen for data in GNAT generally meets the standard system
conventions, and for example records are laid out in a manner that is
consistent with C.  This means that specifying convention C (for example)
has no effect.

There are four exceptions to this general rule:


@itemize *

@item 
@emph{Convention Fortran and array subtypes}.

If pragma Convention Fortran is specified for an array subtype, then in
accordance with the implementation advice in section 3.6.2(11) of the
Ada Reference Manual, the array will be stored in a Fortran-compatible
column-major manner, instead of the normal default row-major order.

@item 
@emph{Convention C and enumeration types}

GNAT normally stores enumeration types in 8, 16, or 32 bits as required
to accommodate all values of the type.  For example, for the enumeration
type declared by:

@example
type Color is (Red, Green, Blue);
@end example

8 bits is sufficient to store all values of the type, so by default, objects
of type @cite{Color} will be represented using 8 bits.  However, normal C
convention is to use 32 bits for all enum values in C, since enum values
are essentially of type int.  If pragma @cite{Convention C} is specified for an
Ada enumeration type, then the size is modified as necessary (usually to
32 bits) to be consistent with the C convention for enum values.

Note that this treatment applies only to types. If Convention C is given for
an enumeration object, where the enumeration type is not Convention C, then
Object_Size bits are allocated. For example, for a normal enumeration type,
with less than 256 elements, only 8 bits will be allocated for the object.
Since this may be a surprise in terms of what C expects, GNAT will issue a
warning in this situation. The warning can be suppressed by giving an explicit
size clause specifying the desired size.

@item 
@emph{Convention C/Fortran and Boolean types}

In C, the usual convention for boolean values, that is values used for
conditions, is that zero represents false, and nonzero values represent
true.  In Ada, the normal convention is that two specific values, typically
0/1, are used to represent false/true respectively.

Fortran has a similar convention for @cite{LOGICAL} values (any nonzero
value represents true).

To accommodate the Fortran and C conventions, if a pragma Convention specifies
C or Fortran convention for a derived Boolean, as in the following example:

@example
type C_Switch is new Boolean;
pragma Convention (C, C_Switch);
@end example

then the GNAT generated code will treat any nonzero value as true.  For truth
values generated by GNAT, the conventional value 1 will be used for True, but
when one of these values is read, any nonzero value is treated as True.
@end itemize

@node Conventions and Anonymous Access Types,Determining the Representations chosen by GNAT,Effect of Convention on Representation,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas conventions-and-anonymous-access-types}@anchor{23c}@anchor{gnat_rm/representation_clauses_and_pragmas id19}@anchor{23d}
@section Conventions and Anonymous Access Types


@geindex Anonymous access types

@geindex Convention for anonymous access types

The RM is not entirely clear on convention handling in a number of cases,
and in particular, it is not clear on the convention to be given to
anonymous access types in general, and in particular what is to be
done for the case of anonymous access-to-subprogram.

In GNAT, we decide that if an explicit Convention is applied
to an object or component, and its type is such an anonymous type,
then the convention will apply to this anonymous type as well. This
seems to make sense since it is anomolous in any case to have a
different convention for an object and its type, and there is clearly
no way to explicitly specify a convention for an anonymous type, since
it doesn't have a name to specify!

Furthermore, we decide that if a convention is applied to a record type,
then this convention is inherited by any of its components that are of an
anonymous access type which do not have an explicitly specified convention.

The following program shows these conventions in action:

@example
package ConvComp is
   type Foo is range 1 .. 10;
   type T1 is record
      A : access function (X : Foo) return Integer;
      B : Integer;
   end record;
   pragma Convention (C, T1);

   type T2 is record
      A : access function (X : Foo) return Integer;
      pragma Convention  (C, A);
      B : Integer;
   end record;
   pragma Convention (COBOL, T2);

   type T3 is record
      A : access function (X : Foo) return Integer;
      pragma Convention  (COBOL, A);
      B : Integer;
   end record;
   pragma Convention (C, T3);

   type T4 is record
      A : access function (X : Foo) return Integer;
      B : Integer;
   end record;
   pragma Convention (COBOL, T4);

   function F (X : Foo) return Integer;
   pragma Convention (C, F);

   function F (X : Foo) return Integer is (13);

   TV1 : T1 := (F'Access, 12);  -- OK
   TV2 : T2 := (F'Access, 13);  -- OK

   TV3 : T3 := (F'Access, 13);  -- ERROR
                |
>>> subprogram "F" has wrong convention
>>> does not match access to subprogram declared at line 17
     38.    TV4 : T4 := (F'Access, 13);  -- ERROR
                |
>>> subprogram "F" has wrong convention
>>> does not match access to subprogram declared at line 24
     39. end ConvComp;
@end example

@node Determining the Representations chosen by GNAT,,Conventions and Anonymous Access Types,Representation Clauses and Pragmas
@anchor{gnat_rm/representation_clauses_and_pragmas id20}@anchor{23e}@anchor{gnat_rm/representation_clauses_and_pragmas determining-the-representations-chosen-by-gnat}@anchor{23f}
@section Determining the Representations chosen by GNAT


@geindex Representation
@geindex determination of

@geindex -gnatR (gcc)

Although the descriptions in this section are intended to be complete, it is
often easier to simply experiment to see what GNAT accepts and what the
effect is on the layout of types and objects.

As required by the Ada RM, if a representation clause is not accepted, then
it must be rejected as illegal by the compiler.  However, when a
representation clause or pragma is accepted, there can still be questions
of what the compiler actually does.  For example, if a partial record
representation clause specifies the location of some components and not
others, then where are the non-specified components placed? Or if pragma
@cite{Pack} is used on a record, then exactly where are the resulting
fields placed? The section on pragma @cite{Pack} in this chapter can be
used to answer the second question, but it is often easier to just see
what the compiler does.

For this purpose, GNAT provides the option @emph{-gnatR}.  If you compile
with this option, then the compiler will output information on the actual
representations chosen, in a format similar to source representation
clauses.  For example, if we compile the package:

@example
package q is
   type r (x : boolean) is tagged record
      case x is
         when True => S : String (1 .. 100);
         when False => null;
      end case;
   end record;

   type r2 is new r (false) with record
      y2 : integer;
   end record;

   for r2 use record
      y2 at 16 range 0 .. 31;
   end record;

   type x is record
      y : character;
   end record;

   type x1 is array (1 .. 10) of x;
   for x1'component_size use 11;

   type ia is access integer;

   type Rb1 is array (1 .. 13) of Boolean;
   pragma Pack (rb1);

   type Rb2 is array (1 .. 65) of Boolean;
   pragma Pack (rb2);

   type x2 is record
      l1 : Boolean;
      l2 : Duration;
      l3 : Float;
      l4 : Boolean;
      l5 : Rb1;
      l6 : Rb2;
   end record;
   pragma Pack (x2);
end q;
@end example

using the switch @emph{-gnatR} we obtain the following output:

@example
Representation information for unit q
-------------------------------------

for r'Size use ??;
for r'Alignment use 4;
for r use record
   x    at 4 range  0 .. 7;
   _tag at 0 range  0 .. 31;
   s    at 5 range  0 .. 799;
end record;

for r2'Size use 160;
for r2'Alignment use 4;
for r2 use record
   x       at  4 range  0 .. 7;
   _tag    at  0 range  0 .. 31;
   _parent at  0 range  0 .. 63;
   y2      at 16 range  0 .. 31;
end record;

for x'Size use 8;
for x'Alignment use 1;
for x use record
   y at 0 range  0 .. 7;
end record;

for x1'Size use 112;
for x1'Alignment use 1;
for x1'Component_Size use 11;

for rb1'Size use 13;
for rb1'Alignment use 2;
for rb1'Component_Size use 1;

for rb2'Size use 72;
for rb2'Alignment use 1;
for rb2'Component_Size use 1;

for x2'Size use 224;
for x2'Alignment use 4;
for x2 use record
   l1 at  0 range  0 .. 0;
   l2 at  0 range  1 .. 64;
   l3 at 12 range  0 .. 31;
   l4 at 16 range  0 .. 0;
   l5 at 16 range  1 .. 13;
   l6 at 18 range  0 .. 71;
end record;
@end example

The Size values are actually the Object_Size, i.e., the default size that
will be allocated for objects of the type.
The @code{??} size for type r indicates that we have a variant record, and the
actual size of objects will depend on the discriminant value.

The Alignment values show the actual alignment chosen by the compiler
for each record or array type.

The record representation clause for type r shows where all fields
are placed, including the compiler generated tag field (whose location
cannot be controlled by the programmer).

The record representation clause for the type extension r2 shows all the
fields present, including the parent field, which is a copy of the fields
of the parent type of r2, i.e., r1.

The component size and size clauses for types rb1 and rb2 show
the exact effect of pragma @cite{Pack} on these arrays, and the record
representation clause for type x2 shows how pragma @cite{Pack} affects
this record type.

In some cases, it may be useful to cut and paste the representation clauses
generated by the compiler into the original source to fix and guarantee
the actual representation to be used.

@node Standard Library Routines,The Implementation of Standard I/O,Representation Clauses and Pragmas,Top
@anchor{gnat_rm/standard_library_routines standard-library-routines}@anchor{e}@anchor{gnat_rm/standard_library_routines doc}@anchor{240}@anchor{gnat_rm/standard_library_routines id1}@anchor{241}
@chapter Standard Library Routines


The Ada Reference Manual contains in Annex A a full description of an
extensive set of standard library routines that can be used in any Ada
program, and which must be provided by all Ada compilers.  They are
analogous to the standard C library used by C programs.

GNAT implements all of the facilities described in annex A, and for most
purposes the description in the Ada Reference Manual, or appropriate Ada
text book, will be sufficient for making use of these facilities.

In the case of the input-output facilities,
@ref{f,,The Implementation of Standard I/O},
gives details on exactly how GNAT interfaces to the
file system.  For the remaining packages, the Ada Reference Manual
should be sufficient.  The following is a list of the packages included,
together with a brief description of the functionality that is provided.

For completeness, references are included to other predefined library
routines defined in other sections of the Ada Reference Manual (these are
cross-indexed from Annex A). For further details see the relevant
package declarations in the run-time library. In particular, a few units
are not implemented, as marked by the presence of pragma Unimplemented_Unit,
and in this case the package declaration contains comments explaining why
the unit is not implemented.


@table @asis

@item @code{Ada} @emph{(A.2)}

This is a parent package for all the standard library packages.  It is
usually included implicitly in your program, and itself contains no
useful data or routines.

@item @code{Ada.Assertions} @emph{(11.4.2)}

@cite{Assertions} provides the @cite{Assert} subprograms, and also
the declaration of the @cite{Assertion_Error} exception.

@item @code{Ada.Asynchronous_Task_Control} @emph{(D.11)}

@cite{Asynchronous_Task_Control} provides low level facilities for task
synchronization. It is typically not implemented. See package spec for details.

@item @code{Ada.Calendar} @emph{(9.6)}

@cite{Calendar} provides time of day access, and routines for
manipulating times and durations.

@item @code{Ada.Calendar.Arithmetic} @emph{(9.6.1)}

This package provides additional arithmetic
operations for @cite{Calendar}.

@item @code{Ada.Calendar.Formatting} @emph{(9.6.1)}

This package provides formatting operations for @cite{Calendar}.

@item @code{Ada.Calendar.Time_Zones} @emph{(9.6.1)}

This package provides additional @cite{Calendar} facilities
for handling time zones.

@item @code{Ada.Characters} @emph{(A.3.1)}

This is a dummy parent package that contains no useful entities

@item @code{Ada.Characters.Conversions} @emph{(A.3.2)}

This package provides character conversion functions.

@item @code{Ada.Characters.Handling} @emph{(A.3.2)}

This package provides some basic character handling capabilities,
including classification functions for classes of characters (e.g., test
for letters, or digits).

@item @code{Ada.Characters.Latin_1} @emph{(A.3.3)}

This package includes a complete set of definitions of the characters
that appear in type CHARACTER.  It is useful for writing programs that
will run in international environments.  For example, if you want an
upper case E with an acute accent in a string, it is often better to use
the definition of @cite{UC_E_Acute} in this package.  Then your program
will print in an understandable manner even if your environment does not
support these extended characters.

@item @code{Ada.Command_Line} @emph{(A.15)}

This package provides access to the command line parameters and the name
of the current program (analogous to the use of @cite{argc} and @cite{argv}
in C), and also allows the exit status for the program to be set in a
system-independent manner.

@item @code{Ada.Complex_Text_IO} @emph{(G.1.3)}

This package provides text input and output of complex numbers.

@item @code{Ada.Containers} @emph{(A.18.1)}

A top level package providing a few basic definitions used by all the
following specific child packages that provide specific kinds of
containers.
@end table

@code{Ada.Containers.Bounded_Priority_Queues} @emph{(A.18.31)}

@code{Ada.Containers.Bounded_Synchronized_Queues} @emph{(A.18.29)}

@code{Ada.Containers.Doubly_Linked_Lists} @emph{(A.18.3)}

@code{Ada.Containers.Generic_Array_Sort} @emph{(A.18.26)}

@code{Ada.Containers.Generic_Constrained_Array_Sort} @emph{(A.18.26)}

@code{Ada.Containers.Generic_Sort} @emph{(A.18.26)}

@code{Ada.Containers.Hashed_Maps} @emph{(A.18.5)}

@code{Ada.Containers.Hashed_Sets} @emph{(A.18.8)}

@code{Ada.Containers.Indefinite_Doubly_Linked_Lists} @emph{(A.18.12)}

@code{Ada.Containers.Indefinite_Hashed_Maps} @emph{(A.18.13)}

@code{Ada.Containers.Indefinite_Hashed_Sets} @emph{(A.18.15)}

@code{Ada.Containers.Indefinite_Holders} @emph{(A.18.18)}

@code{Ada.Containers.Indefinite_Multiway_Trees} @emph{(A.18.17)}

@code{Ada.Containers.Indefinite_Ordered_Maps} @emph{(A.18.14)}

@code{Ada.Containers.Indefinite_Ordered_Sets} @emph{(A.18.16)}

@code{Ada.Containers.Indefinite_Vectors} @emph{(A.18.11)}

@code{Ada.Containers.Multiway_Trees} @emph{(A.18.10)}

@code{Ada.Containers.Ordered_Maps} @emph{(A.18.6)}

@code{Ada.Containers.Ordered_Sets} @emph{(A.18.9)}

@code{Ada.Containers.Synchronized_Queue_Interfaces} @emph{(A.18.27)}

@code{Ada.Containers.Unbounded_Priority_Queues} @emph{(A.18.30)}

@code{Ada.Containers.Unbounded_Synchronized_Queues} @emph{(A.18.28)}

@code{Ada.Containers.Vectors} @emph{(A.18.2)}


@table @asis

@item @code{Ada.Directories} @emph{(A.16)}

This package provides operations on directories.

@item @code{Ada.Directories.Hierarchical_File_Names} @emph{(A.16.1)}

This package provides additional directory operations handling
hiearchical file names.

@item @code{Ada.Directories.Information} @emph{(A.16)}

This is an implementation defined package for additional directory
operations, which is not implemented in GNAT.

@item @code{Ada.Decimal} @emph{(F.2)}

This package provides constants describing the range of decimal numbers
implemented, and also a decimal divide routine (analogous to the COBOL
verb DIVIDE ... GIVING ... REMAINDER ...)

@item @code{Ada.Direct_IO} @emph{(A.8.4)}

This package provides input-output using a model of a set of records of
fixed-length, containing an arbitrary definite Ada type, indexed by an
integer record number.

@item @code{Ada.Dispatching} @emph{(D.2.1)}

A parent package containing definitions for task dispatching operations.

@item @code{Ada.Dispatching.EDF} @emph{(D.2.6)}

Not implemented in GNAT.

@item @code{Ada.Dispatching.Non_Preemptive} @emph{(D.2.4)}

Not implemented in GNAT.

@item @code{Ada.Dispatching.Round_Robin} @emph{(D.2.5)}

Not implemented in GNAT.

@item @code{Ada.Dynamic_Priorities} @emph{(D.5)}

This package allows the priorities of a task to be adjusted dynamically
as the task is running.

@item @code{Ada.Environment_Variables} @emph{(A.17)}

This package provides facilities for accessing environment variables.

@item @code{Ada.Exceptions} @emph{(11.4.1)}

This package provides additional information on exceptions, and also
contains facilities for treating exceptions as data objects, and raising
exceptions with associated messages.

@item @code{Ada.Execution_Time} @emph{(D.14)}

Not implemented in GNAT.

@item @code{Ada.Execution_Time.Group_Budgets} @emph{(D.14.2)}

Not implemented in GNAT.

@item @code{Ada.Execution_Time.Timers} @emph{(D.14.1)'}

Not implemented in GNAT.

@item @code{Ada.Finalization} @emph{(7.6)}

This package contains the declarations and subprograms to support the
use of controlled types, providing for automatic initialization and
finalization (analogous to the constructors and destructors of C++).

@item @code{Ada.Float_Text_IO} @emph{(A.10.9)}

A library level instantiation of Text_IO.Float_IO for type Float.

@item @code{Ada.Float_Wide_Text_IO} @emph{(A.10.9)}

A library level instantiation of Wide_Text_IO.Float_IO for type Float.

@item @code{Ada.Float_Wide_Wide_Text_IO} @emph{(A.10.9)}

A library level instantiation of Wide_Wide_Text_IO.Float_IO for type Float.

@item @code{Ada.Integer_Text_IO} @emph{(A.10.9)}

A library level instantiation of Text_IO.Integer_IO for type Integer.

@item @code{Ada.Integer_Wide_Text_IO} @emph{(A.10.9)}

A library level instantiation of Wide_Text_IO.Integer_IO for type Integer.

@item @code{Ada.Integer_Wide_Wide_Text_IO} @emph{(A.10.9)}

A library level instantiation of Wide_Wide_Text_IO.Integer_IO for type Integer.

@item @code{Ada.Interrupts} @emph{(C.3.2)}

This package provides facilities for interfacing to interrupts, which
includes the set of signals or conditions that can be raised and
recognized as interrupts.

@item @code{Ada.Interrupts.Names} @emph{(C.3.2)}

This package provides the set of interrupt names (actually signal
or condition names) that can be handled by GNAT.

@item @code{Ada.IO_Exceptions} @emph{(A.13)}

This package defines the set of exceptions that can be raised by use of
the standard IO packages.

@item @code{Ada.Iterator_Interfaces} @emph{(5.5.1)}

This package provides a generic interface to generalized iterators.

@item @code{Ada.Locales} @emph{(A.19)}

This package provides declarations providing information (Language
and Country) about the current locale.

@item @code{Ada.Numerics}

This package contains some standard constants and exceptions used
throughout the numerics packages.  Note that the constants pi and e are
defined here, and it is better to use these definitions than rolling
your own.

@item @code{Ada.Numerics.Complex_Arrays} @emph{(G.3.2)}

Provides operations on arrays of complex numbers.

@item @code{Ada.Numerics.Complex_Elementary_Functions}

Provides the implementation of standard elementary functions (such as
log and trigonometric functions) operating on complex numbers using the
standard @cite{Float} and the @cite{Complex} and @cite{Imaginary} types
created by the package @cite{Numerics.Complex_Types}.

@item @code{Ada.Numerics.Complex_Types}

This is a predefined instantiation of
@cite{Numerics.Generic_Complex_Types} using @cite{Standard.Float} to
build the type @cite{Complex} and @cite{Imaginary}.

@item @code{Ada.Numerics.Discrete_Random}

This generic package provides a random number generator suitable for generating
uniformly distributed values of a specified discrete subtype.

@item @code{Ada.Numerics.Float_Random}

This package provides a random number generator suitable for generating
uniformly distributed floating point values in the unit interval.

@item @code{Ada.Numerics.Generic_Complex_Elementary_Functions}

This is a generic version of the package that provides the
implementation of standard elementary functions (such as log and
trigonometric functions) for an arbitrary complex type.

The following predefined instantiations of this package are provided:


@itemize *

@item 
@code{Short_Float}

@cite{Ada.Numerics.Short_Complex_Elementary_Functions}

@item 
@code{Float}

@cite{Ada.Numerics.Complex_Elementary_Functions}

@item 
@code{Long_Float}

@cite{Ada.Numerics.Long_Complex_Elementary_Functions}
@end itemize

@item @code{Ada.Numerics.Generic_Complex_Types}

This is a generic package that allows the creation of complex types,
with associated complex arithmetic operations.

The following predefined instantiations of this package exist


@itemize *

@item 
@code{Short_Float}

@cite{Ada.Numerics.Short_Complex_Complex_Types}

@item 
@code{Float}

@cite{Ada.Numerics.Complex_Complex_Types}

@item 
@code{Long_Float}

@cite{Ada.Numerics.Long_Complex_Complex_Types}
@end itemize

@item @code{Ada.Numerics.Generic_Elementary_Functions}

This is a generic package that provides the implementation of standard
elementary functions (such as log an trigonometric functions) for an
arbitrary float type.

The following predefined instantiations of this package exist


@itemize *

@item 
@code{Short_Float}

@cite{Ada.Numerics.Short_Elementary_Functions}

@item 
@code{Float}

@cite{Ada.Numerics.Elementary_Functions}

@item 
@code{Long_Float}

@cite{Ada.Numerics.Long_Elementary_Functions}
@end itemize

@item @code{Ada.Numerics.Generic_Real_Arrays} @emph{(G.3.1)}

Generic operations on arrays of reals

@item @code{Ada.Numerics.Real_Arrays} @emph{(G.3.1)}

Preinstantiation of Ada.Numerics.Generic_Real_Arrays (Float).

@item @code{Ada.Real_Time} @emph{(D.8)}

This package provides facilities similar to those of @cite{Calendar}, but
operating with a finer clock suitable for real time control. Note that
annex D requires that there be no backward clock jumps, and GNAT generally
guarantees this behavior, but of course if the external clock on which
the GNAT runtime depends is deliberately reset by some external event,
then such a backward jump may occur.

@item @code{Ada.Real_Time.Timing_Events} @emph{(D.15)}

Not implemented in GNAT.

@item @code{Ada.Sequential_IO} @emph{(A.8.1)}

This package provides input-output facilities for sequential files,
which can contain a sequence of values of a single type, which can be
any Ada type, including indefinite (unconstrained) types.

@item @code{Ada.Storage_IO} @emph{(A.9)}

This package provides a facility for mapping arbitrary Ada types to and
from a storage buffer.  It is primarily intended for the creation of new
IO packages.

@item @code{Ada.Streams} @emph{(13.13.1)}

This is a generic package that provides the basic support for the
concept of streams as used by the stream attributes (@cite{Input},
@cite{Output}, @cite{Read} and @cite{Write}).

@item @code{Ada.Streams.Stream_IO} @emph{(A.12.1)}

This package is a specialization of the type @cite{Streams} defined in
package @cite{Streams} together with a set of operations providing
Stream_IO capability.  The Stream_IO model permits both random and
sequential access to a file which can contain an arbitrary set of values
of one or more Ada types.

@item @code{Ada.Strings} @emph{(A.4.1)}

This package provides some basic constants used by the string handling
packages.

@item @code{Ada.Strings.Bounded} @emph{(A.4.4)}

This package provides facilities for handling variable length
strings.  The bounded model requires a maximum length.  It is thus
somewhat more limited than the unbounded model, but avoids the use of
dynamic allocation or finalization.

@item @code{Ada.Strings.Bounded.Equal_Case_Insensitive} @emph{(A.4.10)}

Provides case-insensitive comparisons of bounded strings

@item @code{Ada.Strings.Bounded.Hash} @emph{(A.4.9)}

This package provides a generic hash function for bounded strings

@item @code{Ada.Strings.Bounded.Hash_Case_Insensitive} @emph{(A.4.9)}

This package provides a generic hash function for bounded strings that
converts the string to be hashed to lower case.

@item @code{Ada.Strings.Bounded.Less_Case_Insensitive} @emph{(A.4.10)}

This package provides a comparison function for bounded strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.Fixed} @emph{(A.4.3)}

This package provides facilities for handling fixed length strings.

@item @code{Ada.Strings.Fixed.Equal_Case_Insensitive} @emph{(A.4.10)}

This package provides an equality function for fixed strings that compares
the strings after converting both to lower case.

@item @code{Ada.Strings.Fixed.Hash_Case_Insensitive} @emph{(A.4.9)}

This package provides a case insensitive hash function for fixed strings that
converts the string to lower case before computing the hash.

@item @code{Ada.Strings.Fixed.Less_Case_Insensitive} @emph{(A.4.10)}

This package provides a comparison function for fixed strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.Hash} @emph{(A.4.9)}

This package provides a hash function for strings.

@item @code{Ada.Strings.Hash_Case_Insensitive} @emph{(A.4.9)}

This package provides a hash function for strings that is case insensitive.
The string is converted to lower case before computing the hash.

@item @code{Ada.Strings.Less_Case_Insensitive} @emph{(A.4.10)}

This package provides a comparison function for\strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.Maps} @emph{(A.4.2)}

This package provides facilities for handling character mappings and
arbitrarily defined subsets of characters.  For instance it is useful in
defining specialized translation tables.

@item @code{Ada.Strings.Maps.Constants} @emph{(A.4.6)}

This package provides a standard set of predefined mappings and
predefined character sets.  For example, the standard upper to lower case
conversion table is found in this package.  Note that upper to lower case
conversion is non-trivial if you want to take the entire set of
characters, including extended characters like E with an acute accent,
into account.  You should use the mappings in this package (rather than
adding 32 yourself) to do case mappings.

@item @code{Ada.Strings.Unbounded} @emph{(A.4.5)}

This package provides facilities for handling variable length
strings.  The unbounded model allows arbitrary length strings, but
requires the use of dynamic allocation and finalization.

@item @code{Ada.Strings.Unbounded.Equal_Case_Insensitive} @emph{(A.4.10)}

Provides case-insensitive comparisons of unbounded strings

@item @code{Ada.Strings.Unbounded.Hash} @emph{(A.4.9)}

This package provides a generic hash function for unbounded strings

@item @code{Ada.Strings.Unbounded.Hash_Case_Insensitive} @emph{(A.4.9)}

This package provides a generic hash function for unbounded strings that
converts the string to be hashed to lower case.

@item @code{Ada.Strings.Unbounded.Less_Case_Insensitive} @emph{(A.4.10)}

This package provides a comparison function for unbounded strings that works
in a case insensitive manner by converting to lower case before the comparison.

@item @code{Ada.Strings.UTF_Encoding} @emph{(A.4.11)}

This package provides basic definitions for dealing with UTF-encoded strings.

@item @code{Ada.Strings.UTF_Encoding.Conversions} @emph{(A.4.11)}

This package provides conversion functions for UTF-encoded strings.
@end table

@code{Ada.Strings.UTF_Encoding.Strings} @emph{(A.4.11)}

@code{Ada.Strings.UTF_Encoding.Wide_Strings} @emph{(A.4.11)}


@table @asis

@item @code{Ada.Strings.UTF_Encoding.Wide_Wide_Strings} @emph{(A.4.11)}

These packages provide facilities for handling UTF encodings for
Strings, Wide_Strings and Wide_Wide_Strings.
@end table

@code{Ada.Strings.Wide_Bounded} @emph{(A.4.7)}

@code{Ada.Strings.Wide_Fixed} @emph{(A.4.7)}

@code{Ada.Strings.Wide_Maps} @emph{(A.4.7)}


@table @asis

@item @code{Ada.Strings.Wide_Unbounded} @emph{(A.4.7)}

These packages provide analogous capabilities to the corresponding
packages without @code{Wide_} in the name, but operate with the types
@cite{Wide_String} and @cite{Wide_Character} instead of @cite{String}
and @cite{Character}. Versions of all the child packages are available.
@end table

@code{Ada.Strings.Wide_Wide_Bounded} @emph{(A.4.7)}

@code{Ada.Strings.Wide_Wide_Fixed} @emph{(A.4.7)}

@code{Ada.Strings.Wide_Wide_Maps} @emph{(A.4.7)}


@table @asis

@item @code{Ada.Strings.Wide_Wide_Unbounded} @emph{(A.4.7)}

These packages provide analogous capabilities to the corresponding
packages without @code{Wide_} in the name, but operate with the types
@cite{Wide_Wide_String} and @cite{Wide_Wide_Character} instead
of @cite{String} and @cite{Character}.

@item @code{Ada.Synchronous_Barriers} @emph{(D.10.1)}

This package provides facilities for synchronizing tasks at a low level
with barriers.

@item @code{Ada.Synchronous_Task_Control} @emph{(D.10)}

This package provides some standard facilities for controlling task
communication in a synchronous manner.

@item @code{Ada.Synchronous_Task_Control.EDF} @emph{(D.10)}

Not implemented in GNAT.

@item @code{Ada.Tags}

This package contains definitions for manipulation of the tags of tagged
values.

@item @code{Ada.Tags.Generic_Dispatching_Constructor} @emph{(3.9)}

This package provides a way of constructing tagged class-wide values given
only the tag value.

@item @code{Ada.Task_Attributes} @emph{(C.7.2)}

This package provides the capability of associating arbitrary
task-specific data with separate tasks.

@item @code{Ada.Task_Identifification} @emph{(C.7.1)}

This package provides capabilities for task identification.

@item @code{Ada.Task_Termination} @emph{(C.7.3)}

This package provides control over task termination.

@item @code{Ada.Text_IO}

This package provides basic text input-output capabilities for
character, string and numeric data.  The subpackages of this
package are listed next. Note that although these are defined
as subpackages in the RM, they are actually transparently
implemented as child packages in GNAT, meaning that they
are only loaded if needed.

@item @code{Ada.Text_IO.Decimal_IO}

Provides input-output facilities for decimal fixed-point types

@item @code{Ada.Text_IO.Enumeration_IO}

Provides input-output facilities for enumeration types.

@item @code{Ada.Text_IO.Fixed_IO}

Provides input-output facilities for ordinary fixed-point types.

@item @code{Ada.Text_IO.Float_IO}

Provides input-output facilities for float types.  The following
predefined instantiations of this generic package are available:


@itemize *

@item 
@code{Short_Float}

@cite{Short_Float_Text_IO}

@item 
@code{Float}

@cite{Float_Text_IO}

@item 
@code{Long_Float}

@cite{Long_Float_Text_IO}
@end itemize

@item @code{Ada.Text_IO.Integer_IO}

Provides input-output facilities for integer types.  The following
predefined instantiations of this generic package are available:


@itemize *

@item 
@code{Short_Short_Integer}

@cite{Ada.Short_Short_Integer_Text_IO}

@item 
@code{Short_Integer}

@cite{Ada.Short_Integer_Text_IO}

@item 
@code{Integer}

@cite{Ada.Integer_Text_IO}

@item 
@code{Long_Integer}

@cite{Ada.Long_Integer_Text_IO}

@item 
@code{Long_Long_Integer}

@cite{Ada.Long_Long_Integer_Text_IO}
@end itemize

@item @code{Ada.Text_IO.Modular_IO}

Provides input-output facilities for modular (unsigned) types.

@item @code{Ada.Text_IO.Bounded_IO (A.10.11)}

Provides input-output facilities for bounded strings.

@item @code{Ada.Text_IO.Complex_IO (G.1.3)}

This package provides basic text input-output capabilities for complex
data.

@item @code{Ada.Text_IO.Editing (F.3.3)}

This package contains routines for edited output, analogous to the use
of pictures in COBOL.  The picture formats used by this package are a
close copy of the facility in COBOL.

@item @code{Ada.Text_IO.Text_Streams (A.12.2)}

This package provides a facility that allows Text_IO files to be treated
as streams, so that the stream attributes can be used for writing
arbitrary data, including binary data, to Text_IO files.

@item @code{Ada.Text_IO.Unbounded_IO (A.10.12)}

This package provides input-output facilities for unbounded strings.

@item @code{Ada.Unchecked_Conversion (13.9)}

This generic package allows arbitrary conversion from one type to
another of the same size, providing for breaking the type safety in
special circumstances.

If the types have the same Size (more accurately the same Value_Size),
then the effect is simply to transfer the bits from the source to the
target type without any modification.  This usage is well defined, and
for simple types whose representation is typically the same across
all implementations, gives a portable method of performing such
conversions.

If the types do not have the same size, then the result is implementation
defined, and thus may be non-portable.  The following describes how GNAT
handles such unchecked conversion cases.

If the types are of different sizes, and are both discrete types, then
the effect is of a normal type conversion without any constraint checking.
In particular if the result type has a larger size, the result will be
zero or sign extended.  If the result type has a smaller size, the result
will be truncated by ignoring high order bits.

If the types are of different sizes, and are not both discrete types,
then the conversion works as though pointers were created to the source
and target, and the pointer value is converted.  The effect is that bits
are copied from successive low order storage units and bits of the source
up to the length of the target type.

A warning is issued if the lengths differ, since the effect in this
case is implementation dependent, and the above behavior may not match
that of some other compiler.

A pointer to one type may be converted to a pointer to another type using
unchecked conversion.  The only case in which the effect is undefined is
when one or both pointers are pointers to unconstrained array types.  In
this case, the bounds information may get incorrectly transferred, and in
particular, GNAT uses double size pointers for such types, and it is
meaningless to convert between such pointer types.  GNAT will issue a
warning if the alignment of the target designated type is more strict
than the alignment of the source designated type (since the result may
be unaligned in this case).

A pointer other than a pointer to an unconstrained array type may be
converted to and from System.Address.  Such usage is common in Ada 83
programs, but note that Ada.Address_To_Access_Conversions is the
preferred method of performing such conversions in Ada 95 and Ada 2005.
Neither
unchecked conversion nor Ada.Address_To_Access_Conversions should be
used in conjunction with pointers to unconstrained objects, since
the bounds information cannot be handled correctly in this case.

@item @code{Ada.Unchecked_Deallocation} @emph{(13.11.2)}

This generic package allows explicit freeing of storage previously
allocated by use of an allocator.

@item @code{Ada.Wide_Text_IO} @emph{(A.11)}

This package is similar to @cite{Ada.Text_IO}, except that the external
file supports wide character representations, and the internal types are
@cite{Wide_Character} and @cite{Wide_String} instead of @cite{Character}
and @cite{String}. The corresponding set of nested packages and child
packages are defined.

@item @code{Ada.Wide_Wide_Text_IO} @emph{(A.11)}

This package is similar to @cite{Ada.Text_IO}, except that the external
file supports wide character representations, and the internal types are
@cite{Wide_Character} and @cite{Wide_String} instead of @cite{Character}
and @cite{String}. The corresponding set of nested packages and child
packages are defined.
@end table

For packages in Interfaces and System, all the RM defined packages are
available in GNAT, see the Ada 2012 RM for full details.

@node The Implementation of Standard I/O,The GNAT Library,Standard Library Routines,Top
@anchor{gnat_rm/the_implementation_of_standard_i_o the-implementation-of-standard-i-o}@anchor{f}@anchor{gnat_rm/the_implementation_of_standard_i_o doc}@anchor{242}@anchor{gnat_rm/the_implementation_of_standard_i_o id1}@anchor{243}
@chapter The Implementation of Standard I/O


GNAT implements all the required input-output facilities described in
A.6 through A.14.  These sections of the Ada Reference Manual describe the
required behavior of these packages from the Ada point of view, and if
you are writing a portable Ada program that does not need to know the
exact manner in which Ada maps to the outside world when it comes to
reading or writing external files, then you do not need to read this
chapter.  As long as your files are all regular files (not pipes or
devices), and as long as you write and read the files only from Ada, the
description in the Ada Reference Manual is sufficient.

However, if you want to do input-output to pipes or other devices, such
as the keyboard or screen, or if the files you are dealing with are
either generated by some other language, or to be read by some other
language, then you need to know more about the details of how the GNAT
implementation of these input-output facilities behaves.

In this chapter we give a detailed description of exactly how GNAT
interfaces to the file system.  As always, the sources of the system are
available to you for answering questions at an even more detailed level,
but for most purposes the information in this chapter will suffice.

Another reason that you may need to know more about how input-output is
implemented arises when you have a program written in mixed languages
where, for example, files are shared between the C and Ada sections of
the same program.  GNAT provides some additional facilities, in the form
of additional child library packages, that facilitate this sharing, and
these additional facilities are also described in this chapter.

@menu
* Standard I/O Packages:: 
* FORM Strings:: 
* Direct_IO:: 
* Sequential_IO:: 
* Text_IO:: 
* Wide_Text_IO:: 
* Wide_Wide_Text_IO:: 
* Stream_IO:: 
* Text Translation:: 
* Shared Files:: 
* Filenames encoding:: 
* File content encoding:: 
* Open Modes:: 
* Operations on C Streams:: 
* Interfacing to C Streams:: 

@end menu

@node Standard I/O Packages,FORM Strings,,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o standard-i-o-packages}@anchor{244}@anchor{gnat_rm/the_implementation_of_standard_i_o id2}@anchor{245}
@section Standard I/O Packages


The Standard I/O packages described in Annex A for


@itemize *

@item 
Ada.Text_IO

@item 
Ada.Text_IO.Complex_IO

@item 
Ada.Text_IO.Text_Streams

@item 
Ada.Wide_Text_IO

@item 
Ada.Wide_Text_IO.Complex_IO

@item 
Ada.Wide_Text_IO.Text_Streams

@item 
Ada.Wide_Wide_Text_IO

@item 
Ada.Wide_Wide_Text_IO.Complex_IO

@item 
Ada.Wide_Wide_Text_IO.Text_Streams

@item 
Ada.Stream_IO

@item 
Ada.Sequential_IO

@item 
Ada.Direct_IO
@end itemize

are implemented using the C
library streams facility; where


@itemize *

@item 
All files are opened using @cite{fopen}.

@item 
All input/output operations use @cite{fread}/@cite{fwrite}.
@end itemize

There is no internal buffering of any kind at the Ada library level. The only
buffering is that provided at the system level in the implementation of the
library routines that support streams. This facilitates shared use of these
streams by mixed language programs. Note though that system level buffering is
explicitly enabled at elaboration of the standard I/O packages and that can
have an impact on mixed language programs, in particular those using I/O before
calling the Ada elaboration routine (e.g., adainit). It is recommended to call
the Ada elaboration routine before performing any I/O or when impractical,
flush the common I/O streams and in particular Standard_Output before
elaborating the Ada code.

@node FORM Strings,Direct_IO,Standard I/O Packages,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o form-strings}@anchor{246}@anchor{gnat_rm/the_implementation_of_standard_i_o id3}@anchor{247}
@section FORM Strings


The format of a FORM string in GNAT is:

@example
"keyword=value,keyword=value,...,keyword=value"
@end example

where letters may be in upper or lower case, and there are no spaces
between values.  The order of the entries is not important.  Currently
the following keywords defined.

@example
TEXT_TRANSLATION=[YES|NO|TEXT|BINARY|U8TEXT|WTEXT|U16TEXT]
SHARED=[YES|NO]
WCEM=[n|h|u|s|e|8|b]
ENCODING=[UTF8|8BITS]
@end example

The use of these parameters is described later in this section. If an
unrecognized keyword appears in a form string, it is silently ignored
and not considered invalid.

@node Direct_IO,Sequential_IO,FORM Strings,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o direct-io}@anchor{248}@anchor{gnat_rm/the_implementation_of_standard_i_o id4}@anchor{249}
@section Direct_IO


Direct_IO can only be instantiated for definite types.  This is a
restriction of the Ada language, which means that the records are fixed
length (the length being determined by @code{type'Size}, rounded
up to the next storage unit boundary if necessary).

The records of a Direct_IO file are simply written to the file in index
sequence, with the first record starting at offset zero, and subsequent
records following.  There is no control information of any kind.  For
example, if 32-bit integers are being written, each record takes
4-bytes, so the record at index @cite{K} starts at offset
(@cite{K}-1)*4.

There is no limit on the size of Direct_IO files, they are expanded as
necessary to accommodate whatever records are written to the file.

@node Sequential_IO,Text_IO,Direct_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o sequential-io}@anchor{24a}@anchor{gnat_rm/the_implementation_of_standard_i_o id5}@anchor{24b}
@section Sequential_IO


Sequential_IO may be instantiated with either a definite (constrained)
or indefinite (unconstrained) type.

For the definite type case, the elements written to the file are simply
the memory images of the data values with no control information of any
kind.  The resulting file should be read using the same type, no validity
checking is performed on input.

For the indefinite type case, the elements written consist of two
parts.  First is the size of the data item, written as the memory image
of a @cite{Interfaces.C.size_t} value, followed by the memory image of
the data value.  The resulting file can only be read using the same
(unconstrained) type.  Normal assignment checks are performed on these
read operations, and if these checks fail, @cite{Data_Error} is
raised.  In particular, in the array case, the lengths must match, and in
the variant record case, if the variable for a particular read operation
is constrained, the discriminants must match.

Note that it is not possible to use Sequential_IO to write variable
length array items, and then read the data back into different length
arrays.  For example, the following will raise @cite{Data_Error}:

@example
package IO is new Sequential_IO (String);
F : IO.File_Type;
S : String (1..4);
...
IO.Create (F)
IO.Write (F, "hello!")
IO.Reset (F, Mode=>In_File);
IO.Read (F, S);
Put_Line (S);
@end example

On some Ada implementations, this will print @cite{hell}, but the program is
clearly incorrect, since there is only one element in the file, and that
element is the string @cite{hello!}.

In Ada 95 and Ada 2005, this kind of behavior can be legitimately achieved
using Stream_IO, and this is the preferred mechanism.  In particular, the
above program fragment rewritten to use Stream_IO will work correctly.

@node Text_IO,Wide_Text_IO,Sequential_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id6}@anchor{24c}@anchor{gnat_rm/the_implementation_of_standard_i_o text-io}@anchor{24d}
@section Text_IO


Text_IO files consist of a stream of characters containing the following
special control characters:

@example
LF (line feed, 16#0A#) Line Mark
FF (form feed, 16#0C#) Page Mark
@end example

A canonical Text_IO file is defined as one in which the following
conditions are met:


@itemize *

@item 
The character @cite{LF} is used only as a line mark, i.e., to mark the end
of the line.

@item 
The character @cite{FF} is used only as a page mark, i.e., to mark the
end of a page and consequently can appear only immediately following a
@cite{LF} (line mark) character.

@item 
The file ends with either @cite{LF} (line mark) or @cite{LF}-@cite{FF}
(line mark, page mark).  In the former case, the page mark is implicitly
assumed to be present.
@end itemize

A file written using Text_IO will be in canonical form provided that no
explicit @cite{LF} or @cite{FF} characters are written using @cite{Put}
or @cite{Put_Line}.  There will be no @cite{FF} character at the end of
the file unless an explicit @cite{New_Page} operation was performed
before closing the file.

A canonical Text_IO file that is a regular file (i.e., not a device or a
pipe) can be read using any of the routines in Text_IO.  The
semantics in this case will be exactly as defined in the Ada Reference
Manual, and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO
file has one of the following:


@itemize *

@item 
The file contains @cite{FF} characters not immediately following a
@cite{LF} character.

@item 
The file contains @cite{LF} or @cite{FF} characters written by
@cite{Put} or @cite{Put_Line}, which are not logically considered to be
line marks or page marks.

@item 
The file ends in a character other than @cite{LF} or @cite{FF},
i.e., there is no explicit line mark or page mark at the end of the file.
@end itemize

Text_IO can be used to read such non-standard text files but subprograms
to do with line or page numbers do not have defined meanings.  In
particular, a @cite{FF} character that does not follow a @cite{LF}
character may or may not be treated as a page mark from the point of
view of page and line numbering.  Every @cite{LF} character is considered
to end a line, and there is an implied @cite{LF} character at the end of
the file.

@menu
* Stream Pointer Positioning:: 
* Reading and Writing Non-Regular Files:: 
* Get_Immediate:: 
* Treating Text_IO Files as Streams:: 
* Text_IO Extensions:: 
* Text_IO Facilities for Unbounded Strings:: 

@end menu

@node Stream Pointer Positioning,Reading and Writing Non-Regular Files,,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id7}@anchor{24e}@anchor{gnat_rm/the_implementation_of_standard_i_o stream-pointer-positioning}@anchor{24f}
@subsection Stream Pointer Positioning


@cite{Ada.Text_IO} has a definition of current position for a file that
is being read.  No internal buffering occurs in Text_IO, and usually the
physical position in the stream used to implement the file corresponds
to this logical position defined by Text_IO.  There are two exceptions:


@itemize *

@item 
After a call to @cite{End_Of_Page} that returns @cite{True}, the stream
is positioned past the @cite{LF} (line mark) that precedes the page
mark.  Text_IO maintains an internal flag so that subsequent read
operations properly handle the logical position which is unchanged by
the @cite{End_Of_Page} call.

@item 
After a call to @cite{End_Of_File} that returns @cite{True}, if the
Text_IO file was positioned before the line mark at the end of file
before the call, then the logical position is unchanged, but the stream
is physically positioned right at the end of file (past the line mark,
and past a possible page mark following the line mark.  Again Text_IO
maintains internal flags so that subsequent read operations properly
handle the logical position.
@end itemize

These discrepancies have no effect on the observable behavior of
Text_IO, but if a single Ada stream is shared between a C program and
Ada program, or shared (using @code{shared=yes} in the form string)
between two Ada files, then the difference may be observable in some
situations.

@node Reading and Writing Non-Regular Files,Get_Immediate,Stream Pointer Positioning,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o reading-and-writing-non-regular-files}@anchor{250}@anchor{gnat_rm/the_implementation_of_standard_i_o id8}@anchor{251}
@subsection Reading and Writing Non-Regular Files


A non-regular file is a device (such as a keyboard), or a pipe.  Text_IO
can be used for reading and writing.  Writing is not affected and the
sequence of characters output is identical to the normal file case, but
for reading, the behavior of Text_IO is modified to avoid undesirable
look-ahead as follows:

An input file that is not a regular file is considered to have no page
marks.  Any @cite{Ascii.FF} characters (the character normally used for a
page mark) appearing in the file are considered to be data
characters.  In particular:


@itemize *

@item 
@cite{Get_Line} and @cite{Skip_Line} do not test for a page mark
following a line mark.  If a page mark appears, it will be treated as a
data character.

@item 
This avoids the need to wait for an extra character to be typed or
entered from the pipe to complete one of these operations.

@item 
@cite{End_Of_Page} always returns @cite{False}

@item 
@cite{End_Of_File} will return @cite{False} if there is a page mark at
the end of the file.
@end itemize

Output to non-regular files is the same as for regular files.  Page marks
may be written to non-regular files using @cite{New_Page}, but as noted
above they will not be treated as page marks on input if the output is
piped to another Ada program.

Another important discrepancy when reading non-regular files is that the end
of file indication is not 'sticky'.  If an end of file is entered, e.g., by
pressing the @code{EOT} key,
then end of file
is signaled once (i.e., the test @cite{End_Of_File}
will yield @cite{True}, or a read will
raise @cite{End_Error}), but then reading can resume
to read data past that end of
file indication, until another end of file indication is entered.

@node Get_Immediate,Treating Text_IO Files as Streams,Reading and Writing Non-Regular Files,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o get-immediate}@anchor{252}@anchor{gnat_rm/the_implementation_of_standard_i_o id9}@anchor{253}
@subsection Get_Immediate


@geindex Get_Immediate

Get_Immediate returns the next character (including control characters)
from the input file.  In particular, Get_Immediate will return LF or FF
characters used as line marks or page marks.  Such operations leave the
file positioned past the control character, and it is thus not treated
as having its normal function.  This means that page, line and column
counts after this kind of Get_Immediate call are set as though the mark
did not occur.  In the case where a Get_Immediate leaves the file
positioned between the line mark and page mark (which is not normally
possible), it is undefined whether the FF character will be treated as a
page mark.

@node Treating Text_IO Files as Streams,Text_IO Extensions,Get_Immediate,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id10}@anchor{254}@anchor{gnat_rm/the_implementation_of_standard_i_o treating-text-io-files-as-streams}@anchor{255}
@subsection Treating Text_IO Files as Streams


@geindex Stream files

The package @cite{Text_IO.Streams} allows a Text_IO file to be treated
as a stream.  Data written to a Text_IO file in this stream mode is
binary data.  If this binary data contains bytes 16#0A# (@cite{LF}) or
16#0C# (@cite{FF}), the resulting file may have non-standard
format.  Similarly if read operations are used to read from a Text_IO
file treated as a stream, then @cite{LF} and @cite{FF} characters may be
skipped and the effect is similar to that described above for
@cite{Get_Immediate}.

@node Text_IO Extensions,Text_IO Facilities for Unbounded Strings,Treating Text_IO Files as Streams,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id11}@anchor{256}@anchor{gnat_rm/the_implementation_of_standard_i_o text-io-extensions}@anchor{257}
@subsection Text_IO Extensions


@geindex Text_IO extensions

A package GNAT.IO_Aux in the GNAT library provides some useful extensions
to the standard @cite{Text_IO} package:


@itemize *

@item 
function File_Exists (Name : String) return Boolean;
Determines if a file of the given name exists.

@item 
function Get_Line return String;
Reads a string from the standard input file.  The value returned is exactly
the length of the line that was read.

@item 
function Get_Line (File : Ada.Text_IO.File_Type) return String;
Similar, except that the parameter File specifies the file from which
the string is to be read.
@end itemize

@node Text_IO Facilities for Unbounded Strings,,Text_IO Extensions,Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o text-io-facilities-for-unbounded-strings}@anchor{258}@anchor{gnat_rm/the_implementation_of_standard_i_o id12}@anchor{259}
@subsection Text_IO Facilities for Unbounded Strings


@geindex Text_IO for unbounded strings

@geindex Unbounded_String
@geindex Text_IO operations

The package @cite{Ada.Strings.Unbounded.Text_IO}
in library files @cite{a-suteio.ads/adb} contains some GNAT-specific
subprograms useful for Text_IO operations on unbounded strings:


@itemize *

@item 
function Get_Line (File : File_Type) return Unbounded_String;
Reads a line from the specified file
and returns the result as an unbounded string.

@item 
procedure Put (File : File_Type; U : Unbounded_String);
Writes the value of the given unbounded string to the specified file
Similar to the effect of
@cite{Put (To_String (U))} except that an extra copy is avoided.

@item 
procedure Put_Line (File : File_Type; U : Unbounded_String);
Writes the value of the given unbounded string to the specified file,
followed by a @cite{New_Line}.
Similar to the effect of @cite{Put_Line (To_String (U))} except
that an extra copy is avoided.
@end itemize

In the above procedures, @cite{File} is of type @cite{Ada.Text_IO.File_Type}
and is optional.  If the parameter is omitted, then the standard input or
output file is referenced as appropriate.

The package @cite{Ada.Strings.Wide_Unbounded.Wide_Text_IO} in library
files @code{a-swuwti.ads} and @code{a-swuwti.adb} provides similar extended
@cite{Wide_Text_IO} functionality for unbounded wide strings.

The package @cite{Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO} in library
files @code{a-szuzti.ads} and @code{a-szuzti.adb} provides similar extended
@cite{Wide_Wide_Text_IO} functionality for unbounded wide wide strings.

@node Wide_Text_IO,Wide_Wide_Text_IO,Text_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o wide-text-io}@anchor{25a}@anchor{gnat_rm/the_implementation_of_standard_i_o id13}@anchor{25b}
@section Wide_Text_IO


@cite{Wide_Text_IO} is similar in most respects to Text_IO, except that
both input and output files may contain special sequences that represent
wide character values.  The encoding scheme for a given file may be
specified using a FORM parameter:

@example
WCEM=`x`
@end example

as part of the FORM string (WCEM = wide character encoding method),
where @cite{x} is one of the following characters


@multitable {xxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Character

@tab

Encoding

@item

@emph{h}

@tab

Hex ESC encoding

@item

@emph{u}

@tab

Upper half encoding

@item

@emph{s}

@tab

Shift-JIS encoding

@item

@emph{e}

@tab

EUC Encoding

@item

@emph{8}

@tab

UTF-8 encoding

@item

@emph{b}

@tab

Brackets encoding

@end multitable


The encoding methods match those that
can be used in a source
program, but there is no requirement that the encoding method used for
the source program be the same as the encoding method used for files,
and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files
for which no WCEM parameter is given in the FORM string matches the
wide character encoding specified for the main program (the default
being brackets encoding if no coding method was specified with -gnatW).


@table @asis

@item @emph{Hex Coding}

In this encoding, a wide character is represented by a five character
sequence:
@end table

@example
ESC a b c d
@end example


@quotation

where @cite{a}, @cite{b}, @cite{c}, @cite{d} are the four hexadecimal
characters (using upper case letters) of the wide character code.  For
example, ESC A345 is used to represent the wide character with code
16#A345#.  This scheme is compatible with use of the full
@cite{Wide_Character} set.
@end quotation


@table @asis

@item @emph{Upper Half Coding}

The wide character with encoding 16#abcd#, where the upper bit is on
(i.e., a is in the range 8-F) is represented as two bytes 16#ab# and
16#cd#.  The second byte may never be a format control character, but is
not required to be in the upper half.  This method can be also used for
shift-JIS or EUC where the internal coding matches the external coding.

@item @emph{Shift JIS Coding}

A wide character is represented by a two character sequence 16#ab# and
16#cd#, with the restrictions described for upper half encoding as
described above.  The internal character code is the corresponding JIS
character according to the standard algorithm for Shift-JIS
conversion.  Only characters defined in the JIS code set table can be
used with this encoding method.

@item @emph{EUC Coding}

A wide character is represented by a two character sequence 16#ab# and
16#cd#, with both characters being in the upper half.  The internal
character code is the corresponding JIS character according to the EUC
encoding algorithm.  Only characters defined in the JIS code set table
can be used with this encoding method.

@item @emph{UTF-8 Coding}

A wide character is represented using
UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO
10646-1/Am.2.  Depending on the character value, the representation
is a one, two, or three byte sequence:
@end table

@example
16#0000#-16#007f#: 2#0xxxxxxx#
16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
@end example


@quotation

where the @cite{xxx} bits correspond to the left-padded bits of the
16-bit character value.  Note that all lower half ASCII characters
are represented as ASCII bytes and all upper half characters and
other wide characters are represented as sequences of upper-half
(The full UTF-8 scheme allows for encoding 31-bit characters as
6-byte sequences, but in this implementation, all UTF-8 sequences
of four or more bytes length will raise a Constraint_Error, as
will all invalid UTF-8 sequences.)
@end quotation


@table @asis

@item @emph{Brackets Coding}

In this encoding, a wide character is represented by the following eight
character sequence:
@end table

@example
[ " a b c d " ]
@end example


@quotation

where @cite{a}, @cite{b}, @cite{c}, @cite{d} are the four hexadecimal
characters (using uppercase letters) of the wide character code.  For
example, @cite{["A345"]} is used to represent the wide character with code
@cite{16#A345#}.
This scheme is compatible with use of the full Wide_Character set.
On input, brackets coding can also be used for upper half characters,
e.g., @cite{["C1"]} for lower case a.  However, on output, brackets notation
is only used for wide characters with a code greater than @cite{16#FF#}.

Note that brackets coding is not normally used in the context of
Wide_Text_IO or Wide_Wide_Text_IO, since it is really just designed as
a portable way of encoding source files. In the context of Wide_Text_IO
or Wide_Wide_Text_IO, it can only be used if the file does not contain
any instance of the left bracket character other than to encode wide
character values using the brackets encoding method. In practice it is
expected that some standard wide character encoding method such
as UTF-8 will be used for text input output.

If brackets notation is used, then any occurrence of a left bracket
in the input file which is not the start of a valid wide character
sequence will cause Constraint_Error to be raised. It is possible to
encode a left bracket as ["5B"] and Wide_Text_IO and Wide_Wide_Text_IO
input will interpret this as a left bracket.

However, when a left bracket is output, it will be output as a left bracket
and not as ["5B"]. We make this decision because for normal use of
Wide_Text_IO for outputting messages, it is unpleasant to clobber left
brackets. For example, if we write:

@example
Put_Line ("Start of output [first run]");
@end example

we really do not want to have the left bracket in this message clobbered so
that the output reads:
@end quotation

@example
Start of output ["5B"]first run]
@end example


@quotation

In practice brackets encoding is reasonably useful for normal Put_Line use
since we won't get confused between left brackets and wide character
sequences in the output. But for input, or when files are written out
and read back in, it really makes better sense to use one of the standard
encoding methods such as UTF-8.
@end quotation

For the coding schemes other than UTF-8, Hex, or Brackets encoding,
not all wide character
values can be represented.  An attempt to output a character that cannot
be represented using the encoding scheme for the file causes
Constraint_Error to be raised.  An invalid wide character sequence on
input also causes Constraint_Error to be raised.

@menu
* Stream Pointer Positioning: Stream Pointer Positioning<2>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<2>. 

@end menu

@node Stream Pointer Positioning<2>,Reading and Writing Non-Regular Files<2>,,Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o stream-pointer-positioning-1}@anchor{25c}@anchor{gnat_rm/the_implementation_of_standard_i_o id14}@anchor{25d}
@subsection Stream Pointer Positioning


@cite{Ada.Wide_Text_IO} is similar to @cite{Ada.Text_IO} in its handling
of stream pointer positioning (@ref{24d,,Text_IO}).  There is one additional
case:

If @cite{Ada.Wide_Text_IO.Look_Ahead} reads a character outside the
normal lower ASCII set (i.e., a character in the range:

@example
Wide_Character'Val (16#0080#) .. Wide_Character'Val (16#FFFF#)
@end example

then although the logical position of the file pointer is unchanged by
the @cite{Look_Ahead} call, the stream is physically positioned past the
wide character sequence.  Again this is to avoid the need for buffering
or backup, and all @cite{Wide_Text_IO} routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using @cite{Wide_Text_IO}.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.

@node Reading and Writing Non-Regular Files<2>,,Stream Pointer Positioning<2>,Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o reading-and-writing-non-regular-files-1}@anchor{25e}@anchor{gnat_rm/the_implementation_of_standard_i_o id15}@anchor{25f}
@subsection Reading and Writing Non-Regular Files


As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and @cite{End_Of_Page} always returns
@cite{False}.  Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

@node Wide_Wide_Text_IO,Stream_IO,Wide_Text_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id16}@anchor{260}@anchor{gnat_rm/the_implementation_of_standard_i_o wide-wide-text-io}@anchor{261}
@section Wide_Wide_Text_IO


@cite{Wide_Wide_Text_IO} is similar in most respects to Text_IO, except that
both input and output files may contain special sequences that represent
wide wide character values.  The encoding scheme for a given file may be
specified using a FORM parameter:

@example
WCEM=`x`
@end example

as part of the FORM string (WCEM = wide character encoding method),
where @cite{x} is one of the following characters


@multitable {xxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Character

@tab

Encoding

@item

@emph{h}

@tab

Hex ESC encoding

@item

@emph{u}

@tab

Upper half encoding

@item

@emph{s}

@tab

Shift-JIS encoding

@item

@emph{e}

@tab

EUC Encoding

@item

@emph{8}

@tab

UTF-8 encoding

@item

@emph{b}

@tab

Brackets encoding

@end multitable


The encoding methods match those that
can be used in a source
program, but there is no requirement that the encoding method used for
the source program be the same as the encoding method used for files,
and different files may use different encoding methods.

The default encoding method for the standard files, and for opened files
for which no WCEM parameter is given in the FORM string matches the
wide character encoding specified for the main program (the default
being brackets encoding if no coding method was specified with -gnatW).


@table @asis

@item @emph{UTF-8 Coding}

A wide character is represented using
UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO
10646-1/Am.2.  Depending on the character value, the representation
is a one, two, three, or four byte sequence:
@end table

@example
16#000000#-16#00007f#: 2#0xxxxxxx#
16#000080#-16#0007ff#: 2#110xxxxx# 2#10xxxxxx#
16#000800#-16#00ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
16#010000#-16#10ffff#: 2#11110xxx# 2#10xxxxxx# 2#10xxxxxx# 2#10xxxxxx#
@end example


@quotation

where the @cite{xxx} bits correspond to the left-padded bits of the
21-bit character value.  Note that all lower half ASCII characters
are represented as ASCII bytes and all upper half characters and
other wide characters are represented as sequences of upper-half
characters.
@end quotation


@table @asis

@item @emph{Brackets Coding}

In this encoding, a wide wide character is represented by the following eight
character sequence if is in wide character range
@end table

@example
[ " a b c d " ]
@end example


@quotation

and by the following ten character sequence if not
@end quotation

@example
[ " a b c d e f " ]
@end example


@quotation

where @cite{a}, @cite{b}, @cite{c}, @cite{d}, @cite{e}, and @cite{f}
are the four or six hexadecimal
characters (using uppercase letters) of the wide wide character code.  For
example, @cite{["01A345"]} is used to represent the wide wide character
with code @cite{16#01A345#}.

This scheme is compatible with use of the full Wide_Wide_Character set.
On input, brackets coding can also be used for upper half characters,
e.g., @cite{["C1"]} for lower case a.  However, on output, brackets notation
is only used for wide characters with a code greater than @cite{16#FF#}.
@end quotation

If is also possible to use the other Wide_Character encoding methods,
such as Shift-JIS, but the other schemes cannot support the full range
of wide wide characters.
An attempt to output a character that cannot
be represented using the encoding scheme for the file causes
Constraint_Error to be raised.  An invalid wide character sequence on
input also causes Constraint_Error to be raised.

@menu
* Stream Pointer Positioning: Stream Pointer Positioning<3>. 
* Reading and Writing Non-Regular Files: Reading and Writing Non-Regular Files<3>. 

@end menu

@node Stream Pointer Positioning<3>,Reading and Writing Non-Regular Files<3>,,Wide_Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o stream-pointer-positioning-2}@anchor{262}@anchor{gnat_rm/the_implementation_of_standard_i_o id17}@anchor{263}
@subsection Stream Pointer Positioning


@cite{Ada.Wide_Wide_Text_IO} is similar to @cite{Ada.Text_IO} in its handling
of stream pointer positioning (@ref{24d,,Text_IO}).  There is one additional
case:

If @cite{Ada.Wide_Wide_Text_IO.Look_Ahead} reads a character outside the
normal lower ASCII set (i.e., a character in the range:

@example
Wide_Wide_Character'Val (16#0080#) .. Wide_Wide_Character'Val (16#10FFFF#)
@end example

then although the logical position of the file pointer is unchanged by
the @cite{Look_Ahead} call, the stream is physically positioned past the
wide character sequence.  Again this is to avoid the need for buffering
or backup, and all @cite{Wide_Wide_Text_IO} routines check the internal
indication that this situation has occurred so that this is not visible
to a normal program using @cite{Wide_Wide_Text_IO}.  However, this discrepancy
can be observed if the wide text file shares a stream with another file.

@node Reading and Writing Non-Regular Files<3>,,Stream Pointer Positioning<3>,Wide_Wide_Text_IO
@anchor{gnat_rm/the_implementation_of_standard_i_o id18}@anchor{264}@anchor{gnat_rm/the_implementation_of_standard_i_o reading-and-writing-non-regular-files-2}@anchor{265}
@subsection Reading and Writing Non-Regular Files


As in the case of Text_IO, when a non-regular file is read, it is
assumed that the file contains no page marks (any form characters are
treated as data characters), and @cite{End_Of_Page} always returns
@cite{False}.  Similarly, the end of file indication is not sticky, so
it is possible to read beyond an end of file.

@node Stream_IO,Text Translation,Wide_Wide_Text_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id19}@anchor{266}@anchor{gnat_rm/the_implementation_of_standard_i_o stream-io}@anchor{267}
@section Stream_IO


A stream file is a sequence of bytes, where individual elements are
written to the file as described in the Ada Reference Manual.  The type
@cite{Stream_Element} is simply a byte.  There are two ways to read or
write a stream file.


@itemize *

@item 
The operations @cite{Read} and @cite{Write} directly read or write a
sequence of stream elements with no control information.

@item 
The stream attributes applied to a stream file transfer data in the
manner described for stream attributes.
@end itemize

@node Text Translation,Shared Files,Stream_IO,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id20}@anchor{268}@anchor{gnat_rm/the_implementation_of_standard_i_o text-translation}@anchor{269}
@section Text Translation


@code{Text_Translation=xxx} may be used as the Form parameter
passed to Text_IO.Create and Text_IO.Open. @code{Text_Translation=xxx}
has no effect on Unix systems. Possible values are:


@itemize *

@item 
@code{Yes} or @code{Text} is the default, which means to
translate LF to/from CR/LF on Windows systems.

@code{No} disables this translation; i.e. it
uses binary mode. For output files, @code{Text_Translation=No}
may be used to create Unix-style files on
Windows.

@item 
@code{wtext} translation enabled in Unicode mode.
(corresponds to _O_WTEXT).

@item 
@code{u8text} translation enabled in Unicode UTF-8 mode.
(corresponds to O_U8TEXT).

@item 
@code{u16text} translation enabled in Unicode UTF-16
mode. (corresponds to_O_U16TEXT).
@end itemize

@node Shared Files,Filenames encoding,Text Translation,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o id21}@anchor{26a}@anchor{gnat_rm/the_implementation_of_standard_i_o shared-files}@anchor{26b}
@section Shared Files


Section A.14 of the Ada Reference Manual allows implementations to
provide a wide variety of behavior if an attempt is made to access the
same external file with two or more internal files.

To provide a full range of functionality, while at the same time
minimizing the problems of portability caused by this implementation
dependence, GNAT handles file sharing as follows:


@itemize *

@item 
In the absence of a @code{shared=xxx} form parameter, an attempt
to open two or more files with the same full name is considered an error
and is not supported.  The exception @cite{Use_Error} will be
raised.  Note that a file that is not explicitly closed by the program
remains open until the program terminates.

@item 
If the form parameter @code{shared=no} appears in the form string, the
file can be opened or created with its own separate stream identifier,
regardless of whether other files sharing the same external file are
opened.  The exact effect depends on how the C stream routines handle
multiple accesses to the same external files using separate streams.

@item 
If the form parameter @code{shared=yes} appears in the form string for
each of two or more files opened using the same full name, the same
stream is shared between these files, and the semantics are as described
in Ada Reference Manual, Section A.14.
@end itemize

When a program that opens multiple files with the same name is ported
from another Ada compiler to GNAT, the effect will be that
@cite{Use_Error} is raised.

The documentation of the original compiler and the documentation of the
program should then be examined to determine if file sharing was
expected, and @code{shared=xxx} parameters added to @cite{Open}
and @cite{Create} calls as required.

When a program is ported from GNAT to some other Ada compiler, no
special attention is required unless the @code{shared=xxx} form
parameter is used in the program.  In this case, you must examine the
documentation of the new compiler to see if it supports the required
file sharing semantics, and form strings modified appropriately.  Of
course it may be the case that the program cannot be ported if the
target compiler does not support the required functionality.  The best
approach in writing portable code is to avoid file sharing (and hence
the use of the @code{shared=xxx} parameter in the form string)
completely.

One common use of file sharing in Ada 83 is the use of instantiations of
Sequential_IO on the same file with different types, to achieve
heterogeneous input-output.  Although this approach will work in GNAT if
@code{shared=yes} is specified, it is preferable in Ada to use Stream_IO
for this purpose (using the stream attributes)

@node Filenames encoding,File content encoding,Shared Files,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o filenames-encoding}@anchor{26c}@anchor{gnat_rm/the_implementation_of_standard_i_o id22}@anchor{26d}
@section Filenames encoding


An encoding form parameter can be used to specify the filename
encoding @code{encoding=xxx}.


@itemize *

@item 
If the form parameter @code{encoding=utf8} appears in the form string, the
filename must be encoded in UTF-8.

@item 
If the form parameter @code{encoding=8bits} appears in the form
string, the filename must be a standard 8bits string.
@end itemize

In the absence of a @code{encoding=xxx} form parameter, the
encoding is controlled by the @code{GNAT_CODE_PAGE} environment
variable. And if not set @code{utf8} is assumed.


@table @asis

@item @emph{CP_ACP}

The current system Windows ANSI code page.

@item @emph{CP_UTF8}

UTF-8 encoding
@end table

This encoding form parameter is only supported on the Windows
platform. On the other Operating Systems the run-time is supporting
UTF-8 natively.

@node File content encoding,Open Modes,Filenames encoding,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o file-content-encoding}@anchor{26e}@anchor{gnat_rm/the_implementation_of_standard_i_o id23}@anchor{26f}
@section File content encoding


For text files it is possible to specify the encoding to use. This is
controlled by the by the @code{GNAT_CCS_ENCODING} environment
variable. And if not set @code{TEXT} is assumed.

The possible values are those supported on Windows:


@table @asis

@item @emph{TEXT}

Translated text mode

@item @emph{WTEXT}

Translated unicode encoding

@item @emph{U16TEXT}

Unicode 16-bit encoding

@item @emph{U8TEXT}

Unicode 8-bit encoding
@end table

This encoding is only supported on the Windows platform.

@node Open Modes,Operations on C Streams,File content encoding,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o open-modes}@anchor{270}@anchor{gnat_rm/the_implementation_of_standard_i_o id24}@anchor{271}
@section Open Modes


@cite{Open} and @cite{Create} calls result in a call to @cite{fopen}
using the mode shown in the following table:


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxx} 
@headitem

@cite{Open} and @cite{Create} Call Modes

@tab

@tab

@item

@tab

@strong{OPEN}

@tab

@strong{CREATE}

@item

Append_File

@tab

"r+"

@tab

"w+"

@item

In_File

@tab

"r"

@tab

"w+"

@item

Out_File (Direct_IO)

@tab

"r+"

@tab

"w"

@item

Out_File (all other cases)

@tab

"w"

@tab

"w"

@item

Inout_File

@tab

"r+"

@tab

"w+"

@end multitable


If text file translation is required, then either @code{b} or @code{t}
is added to the mode, depending on the setting of Text.  Text file
translation refers to the mapping of CR/LF sequences in an external file
to LF characters internally.  This mapping only occurs in DOS and
DOS-like systems, and is not relevant to other systems.

A special case occurs with Stream_IO.  As shown in the above table, the
file is initially opened in @code{r} or @code{w} mode for the
@cite{In_File} and @cite{Out_File} cases.  If a @cite{Set_Mode} operation
subsequently requires switching from reading to writing or vice-versa,
then the file is reopened in @code{r+} mode to permit the required operation.

@node Operations on C Streams,Interfacing to C Streams,Open Modes,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o operations-on-c-streams}@anchor{272}@anchor{gnat_rm/the_implementation_of_standard_i_o id25}@anchor{273}
@section Operations on C Streams


The package @cite{Interfaces.C_Streams} provides an Ada program with direct
access to the C library functions for operations on C streams:

@example
package Interfaces.C_Streams is
  -- Note: the reason we do not use the types that are in
  -- Interfaces.C is that we want to avoid dragging in the
  -- code in this unit if possible.
  subtype chars is System.Address;
  -- Pointer to null-terminated array of characters
  subtype FILEs is System.Address;
  -- Corresponds to the C type FILE*
  subtype voids is System.Address;
  -- Corresponds to the C type void*
  subtype int is Integer;
  subtype long is Long_Integer;
  -- Note: the above types are subtypes deliberately, and it
  -- is part of this spec that the above correspondences are
  -- guaranteed.  This means that it is legitimate to, for
  -- example, use Integer instead of int.  We provide these
  -- synonyms for clarity, but in some cases it may be
  -- convenient to use the underlying types (for example to
  -- avoid an unnecessary dependency of a spec on the spec
  -- of this unit).
  type size_t is mod 2 ** Standard'Address_Size;
  NULL_Stream : constant FILEs;
  -- Value returned (NULL in C) to indicate an
  -- fdopen/fopen/tmpfile error
  ----------------------------------
  -- Constants Defined in stdio.h --
  ----------------------------------
  EOF : constant int;
  -- Used by a number of routines to indicate error or
  -- end of file
  IOFBF : constant int;
  IOLBF : constant int;
  IONBF : constant int;
  -- Used to indicate buffering mode for setvbuf call
  SEEK_CUR : constant int;
  SEEK_END : constant int;
  SEEK_SET : constant int;
  -- Used to indicate origin for fseek call
  function stdin return FILEs;
  function stdout return FILEs;
  function stderr return FILEs;
  -- Streams associated with standard files
  --------------------------
  -- Standard C functions --
  --------------------------
  -- The functions selected below are ones that are
  -- available in UNIX (but not necessarily in ANSI C).
  -- These are very thin interfaces
  -- which copy exactly the C headers.  For more
  -- documentation on these functions, see the Microsoft C
  -- "Run-Time Library Reference" (Microsoft Press, 1990,
  -- ISBN 1-55615-225-6), which includes useful information
  -- on system compatibility.
  procedure clearerr (stream : FILEs);
  function fclose (stream : FILEs) return int;
  function fdopen (handle : int; mode : chars) return FILEs;
  function feof (stream : FILEs) return int;
  function ferror (stream : FILEs) return int;
  function fflush (stream : FILEs) return int;
  function fgetc (stream : FILEs) return int;
  function fgets (strng : chars; n : int; stream : FILEs)
      return chars;
  function fileno (stream : FILEs) return int;
  function fopen (filename : chars; Mode : chars)
      return FILEs;
  -- Note: to maintain target independence, use
  -- text_translation_required, a boolean variable defined in
  -- a-sysdep.c to deal with the target dependent text
  -- translation requirement.  If this variable is set,
  -- then  b/t should be appended to the standard mode
  -- argument to set the text translation mode off or on
  -- as required.
  function fputc (C : int; stream : FILEs) return int;
  function fputs (Strng : chars; Stream : FILEs) return int;
  function fread
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t;
  function freopen
     (filename : chars;
      mode : chars;
      stream : FILEs)
      return FILEs;
  function fseek
     (stream : FILEs;
      offset : long;
      origin : int)
      return int;
  function ftell (stream : FILEs) return long;
  function fwrite
     (buffer : voids;
      size : size_t;
      count : size_t;
      stream : FILEs)
      return size_t;
  function isatty (handle : int) return int;
  procedure mktemp (template : chars);
  -- The return value (which is just a pointer to template)
  -- is discarded
  procedure rewind (stream : FILEs);
  function rmtmp return int;
  function setvbuf
     (stream : FILEs;
      buffer : chars;
      mode : int;
      size : size_t)
      return int;

  function tmpfile return FILEs;
  function ungetc (c : int; stream : FILEs) return int;
  function unlink (filename : chars) return int;
  ---------------------
  -- Extra functions --
  ---------------------
  -- These functions supply slightly thicker bindings than
  -- those above.  They are derived from functions in the
  -- C Run-Time Library, but may do a bit more work than
  -- just directly calling one of the Library functions.
  function is_regular_file (handle : int) return int;
  -- Tests if given handle is for a regular file (result 1)
  -- or for a non-regular file (pipe or device, result 0).
  ---------------------------------
  -- Control of Text/Binary Mode --
  ---------------------------------
  -- If text_translation_required is true, then the following
  -- functions may be used to dynamically switch a file from
  -- binary to text mode or vice versa.  These functions have
  -- no effect if text_translation_required is false (i.e., in
  -- normal UNIX mode).  Use fileno to get a stream handle.
  procedure set_binary_mode (handle : int);
  procedure set_text_mode (handle : int);
  ----------------------------
  -- Full Path Name support --
  ----------------------------
  procedure full_name (nam : chars; buffer : chars);
  -- Given a NUL terminated string representing a file
  -- name, returns in buffer a NUL terminated string
  -- representing the full path name for the file name.
  -- On systems where it is relevant the   drive is also
  -- part of the full path name.  It is the responsibility
  -- of the caller to pass an actual parameter for buffer
  -- that is big enough for any full path name.  Use
  -- max_path_len given below as the size of buffer.
  max_path_len : integer;
  -- Maximum length of an allowable full path name on the
  -- system, including a terminating NUL character.
end Interfaces.C_Streams;
@end example

@node Interfacing to C Streams,,Operations on C Streams,The Implementation of Standard I/O
@anchor{gnat_rm/the_implementation_of_standard_i_o interfacing-to-c-streams}@anchor{274}@anchor{gnat_rm/the_implementation_of_standard_i_o id26}@anchor{275}
@section Interfacing to C Streams


The packages in this section permit interfacing Ada files to C Stream
operations.

@example
with Interfaces.C_Streams;
package Ada.Sequential_IO.C_Streams is
   function C_Stream (F : File_Type)
      return Interfaces.C_Streams.FILEs;
   procedure Open
     (File : in out File_Type;
      Mode : in File_Mode;
      C_Stream : in Interfaces.C_Streams.FILEs;
      Form : in String := "");
end Ada.Sequential_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Direct_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Direct_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Text_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
 end Ada.Text_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Wide_Text_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
end Ada.Wide_Text_IO.C_Streams;

 with Interfaces.C_Streams;
 package Ada.Wide_Wide_Text_IO.C_Streams is
    function C_Stream (F : File_Type)
       return Interfaces.C_Streams.FILEs;
    procedure Open
      (File : in out File_Type;
       Mode : in File_Mode;
       C_Stream : in Interfaces.C_Streams.FILEs;
       Form : in String := "");
end Ada.Wide_Wide_Text_IO.C_Streams;

with Interfaces.C_Streams;
package Ada.Stream_IO.C_Streams is
   function C_Stream (F : File_Type)
      return Interfaces.C_Streams.FILEs;
   procedure Open
     (File : in out File_Type;
      Mode : in File_Mode;
      C_Stream : in Interfaces.C_Streams.FILEs;
      Form : in String := "");
end Ada.Stream_IO.C_Streams;
@end example

In each of these six packages, the @cite{C_Stream} function obtains the
@cite{FILE} pointer from a currently opened Ada file.  It is then
possible to use the @cite{Interfaces.C_Streams} package to operate on
this stream, or the stream can be passed to a C program which can
operate on it directly.  Of course the program is responsible for
ensuring that only appropriate sequences of operations are executed.

One particular use of relevance to an Ada program is that the
@cite{setvbuf} function can be used to control the buffering of the
stream used by an Ada file.  In the absence of such a call the standard
default buffering is used.

The @cite{Open} procedures in these packages open a file giving an
existing C Stream instead of a file name.  Typically this stream is
imported from a C program, allowing an Ada file to operate on an
existing C file.

@node The GNAT Library,Interfacing to Other Languages,The Implementation of Standard I/O,Top
@anchor{gnat_rm/the_gnat_library the-gnat-library}@anchor{10}@anchor{gnat_rm/the_gnat_library doc}@anchor{276}@anchor{gnat_rm/the_gnat_library id1}@anchor{277}
@chapter The GNAT Library


The GNAT library contains a number of general and special purpose packages.
It represents functionality that the GNAT developers have found useful, and
which is made available to GNAT users.  The packages described here are fully
supported, and upwards compatibility will be maintained in future releases,
so you can use these facilities with the confidence that the same functionality
will be available in future releases.

The chapter here simply gives a brief summary of the facilities available.
The full documentation is found in the spec file for the package.  The full
sources of these library packages, including both spec and body, are provided
with all GNAT releases.  For example, to find out the full specifications of
the SPITBOL pattern matching capability, including a full tutorial and
extensive examples, look in the @code{g-spipat.ads} file in the library.

For each entry here, the package name (as it would appear in a @cite{with}
clause) is given, followed by the name of the corresponding spec file in
parentheses.  The packages are children in four hierarchies, @cite{Ada},
@cite{Interfaces}, @cite{System}, and @cite{GNAT}, the latter being a
GNAT-specific hierarchy.

Note that an application program should only use packages in one of these
four hierarchies if the package is defined in the Ada Reference Manual,
or is listed in this section of the GNAT Programmers Reference Manual.
All other units should be considered internal implementation units and
should not be directly @cite{with}'ed by application code.  The use of
a @cite{with} statement that references one of these internal implementation
units makes an application potentially dependent on changes in versions
of GNAT, and will generate a warning message.

@menu
* Ada.Characters.Latin_9 (a-chlat9.ads): Ada Characters Latin_9 a-chlat9 ads. 
* Ada.Characters.Wide_Latin_1 (a-cwila1.ads): Ada Characters Wide_Latin_1 a-cwila1 ads. 
* Ada.Characters.Wide_Latin_9 (a-cwila1.ads): Ada Characters Wide_Latin_9 a-cwila1 ads. 
* Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads): Ada Characters Wide_Wide_Latin_1 a-chzla1 ads. 
* Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads): Ada Characters Wide_Wide_Latin_9 a-chzla9 ads. 
* Ada.Containers.Formal_Doubly_Linked_Lists (a-cfdlli.ads): Ada Containers Formal_Doubly_Linked_Lists a-cfdlli ads. 
* Ada.Containers.Formal_Hashed_Maps (a-cfhama.ads): Ada Containers Formal_Hashed_Maps a-cfhama ads. 
* Ada.Containers.Formal_Hashed_Sets (a-cfhase.ads): Ada Containers Formal_Hashed_Sets a-cfhase ads. 
* Ada.Containers.Formal_Ordered_Maps (a-cforma.ads): Ada Containers Formal_Ordered_Maps a-cforma ads. 
* Ada.Containers.Formal_Ordered_Sets (a-cforse.ads): Ada Containers Formal_Ordered_Sets a-cforse ads. 
* Ada.Containers.Formal_Vectors (a-cofove.ads): Ada Containers Formal_Vectors a-cofove ads. 
* Ada.Containers.Formal_Indefinite_Vectors (a-cfinve.ads): Ada Containers Formal_Indefinite_Vectors a-cfinve ads. 
* Ada.Containers.Bounded_Holders (a-coboho.ads): Ada Containers Bounded_Holders a-coboho ads. 
* Ada.Command_Line.Environment (a-colien.ads): Ada Command_Line Environment a-colien ads. 
* Ada.Command_Line.Remove (a-colire.ads): Ada Command_Line Remove a-colire ads. 
* Ada.Command_Line.Response_File (a-clrefi.ads): Ada Command_Line Response_File a-clrefi ads. 
* Ada.Direct_IO.C_Streams (a-diocst.ads): Ada Direct_IO C_Streams a-diocst ads. 
* Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads): Ada Exceptions Is_Null_Occurrence a-einuoc ads. 
* Ada.Exceptions.Last_Chance_Handler (a-elchha.ads): Ada Exceptions Last_Chance_Handler a-elchha ads. 
* Ada.Exceptions.Traceback (a-exctra.ads): Ada Exceptions Traceback a-exctra ads. 
* Ada.Sequential_IO.C_Streams (a-siocst.ads): Ada Sequential_IO C_Streams a-siocst ads. 
* Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads): Ada Streams Stream_IO C_Streams a-ssicst ads. 
* Ada.Strings.Unbounded.Text_IO (a-suteio.ads): Ada Strings Unbounded Text_IO a-suteio ads. 
* Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads): Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads. 
* Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads): Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads. 
* Ada.Text_IO.C_Streams (a-tiocst.ads): Ada Text_IO C_Streams a-tiocst ads. 
* Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads): Ada Text_IO Reset_Standard_Files a-tirsfi ads. 
* Ada.Wide_Characters.Unicode (a-wichun.ads): Ada Wide_Characters Unicode a-wichun ads. 
* Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads): Ada Wide_Text_IO C_Streams a-wtcstr ads. 
* Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads): Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads. 
* Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads): Ada Wide_Wide_Characters Unicode a-zchuni ads. 
* Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads): Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads. 
* Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads): Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads. 
* GNAT.Altivec (g-altive.ads): GNAT Altivec g-altive ads. 
* GNAT.Altivec.Conversions (g-altcon.ads): GNAT Altivec Conversions g-altcon ads. 
* GNAT.Altivec.Vector_Operations (g-alveop.ads): GNAT Altivec Vector_Operations g-alveop ads. 
* GNAT.Altivec.Vector_Types (g-alvety.ads): GNAT Altivec Vector_Types g-alvety ads. 
* GNAT.Altivec.Vector_Views (g-alvevi.ads): GNAT Altivec Vector_Views g-alvevi ads. 
* GNAT.Array_Split (g-arrspl.ads): GNAT Array_Split g-arrspl ads. 
* GNAT.AWK (g-awk.ads): GNAT AWK g-awk ads. 
* GNAT.Bounded_Buffers (g-boubuf.ads): GNAT Bounded_Buffers g-boubuf ads. 
* GNAT.Bounded_Mailboxes (g-boumai.ads): GNAT Bounded_Mailboxes g-boumai ads. 
* GNAT.Bubble_Sort (g-bubsor.ads): GNAT Bubble_Sort g-bubsor ads. 
* GNAT.Bubble_Sort_A (g-busora.ads): GNAT Bubble_Sort_A g-busora ads. 
* GNAT.Bubble_Sort_G (g-busorg.ads): GNAT Bubble_Sort_G g-busorg ads. 
* GNAT.Byte_Order_Mark (g-byorma.ads): GNAT Byte_Order_Mark g-byorma ads. 
* GNAT.Byte_Swapping (g-bytswa.ads): GNAT Byte_Swapping g-bytswa ads. 
* GNAT.Calendar (g-calend.ads): GNAT Calendar g-calend ads. 
* GNAT.Calendar.Time_IO (g-catiio.ads): GNAT Calendar Time_IO g-catiio ads. 
* GNAT.CRC32 (g-crc32.ads): GNAT CRC32 g-crc32 ads. 
* GNAT.Case_Util (g-casuti.ads): GNAT Case_Util g-casuti ads. 
* GNAT.CGI (g-cgi.ads): GNAT CGI g-cgi ads. 
* GNAT.CGI.Cookie (g-cgicoo.ads): GNAT CGI Cookie g-cgicoo ads. 
* GNAT.CGI.Debug (g-cgideb.ads): GNAT CGI Debug g-cgideb ads. 
* GNAT.Command_Line (g-comlin.ads): GNAT Command_Line g-comlin ads. 
* GNAT.Compiler_Version (g-comver.ads): GNAT Compiler_Version g-comver ads. 
* GNAT.Ctrl_C (g-ctrl_c.ads): GNAT Ctrl_C g-ctrl_c ads. 
* GNAT.Current_Exception (g-curexc.ads): GNAT Current_Exception g-curexc ads. 
* GNAT.Debug_Pools (g-debpoo.ads): GNAT Debug_Pools g-debpoo ads. 
* GNAT.Debug_Utilities (g-debuti.ads): GNAT Debug_Utilities g-debuti ads. 
* GNAT.Decode_String (g-decstr.ads): GNAT Decode_String g-decstr ads. 
* GNAT.Decode_UTF8_String (g-deutst.ads): GNAT Decode_UTF8_String g-deutst ads. 
* GNAT.Directory_Operations (g-dirope.ads): GNAT Directory_Operations g-dirope ads. 
* GNAT.Directory_Operations.Iteration (g-diopit.ads): GNAT Directory_Operations Iteration g-diopit ads. 
* GNAT.Dynamic_HTables (g-dynhta.ads): GNAT Dynamic_HTables g-dynhta ads. 
* GNAT.Dynamic_Tables (g-dyntab.ads): GNAT Dynamic_Tables g-dyntab ads. 
* GNAT.Encode_String (g-encstr.ads): GNAT Encode_String g-encstr ads. 
* GNAT.Encode_UTF8_String (g-enutst.ads): GNAT Encode_UTF8_String g-enutst ads. 
* GNAT.Exception_Actions (g-excact.ads): GNAT Exception_Actions g-excact ads. 
* GNAT.Exception_Traces (g-exctra.ads): GNAT Exception_Traces g-exctra ads. 
* GNAT.Exceptions (g-expect.ads): GNAT Exceptions g-expect ads. 
* GNAT.Expect (g-expect.ads): GNAT Expect g-expect ads. 
* GNAT.Expect.TTY (g-exptty.ads): GNAT Expect TTY g-exptty ads. 
* GNAT.Float_Control (g-flocon.ads): GNAT Float_Control g-flocon ads. 
* GNAT.Formatted_String (g-forstr.ads): GNAT Formatted_String g-forstr ads. 
* GNAT.Heap_Sort (g-heasor.ads): GNAT Heap_Sort g-heasor ads. 
* GNAT.Heap_Sort_A (g-hesora.ads): GNAT Heap_Sort_A g-hesora ads. 
* GNAT.Heap_Sort_G (g-hesorg.ads): GNAT Heap_Sort_G g-hesorg ads. 
* GNAT.HTable (g-htable.ads): GNAT HTable g-htable ads. 
* GNAT.IO (g-io.ads): GNAT IO g-io ads. 
* GNAT.IO_Aux (g-io_aux.ads): GNAT IO_Aux g-io_aux ads. 
* GNAT.Lock_Files (g-locfil.ads): GNAT Lock_Files g-locfil ads. 
* GNAT.MBBS_Discrete_Random (g-mbdira.ads): GNAT MBBS_Discrete_Random g-mbdira ads. 
* GNAT.MBBS_Float_Random (g-mbflra.ads): GNAT MBBS_Float_Random g-mbflra ads. 
* GNAT.MD5 (g-md5.ads): GNAT MD5 g-md5 ads. 
* GNAT.Memory_Dump (g-memdum.ads): GNAT Memory_Dump g-memdum ads. 
* GNAT.Most_Recent_Exception (g-moreex.ads): GNAT Most_Recent_Exception g-moreex ads. 
* GNAT.OS_Lib (g-os_lib.ads): GNAT OS_Lib g-os_lib ads. 
* GNAT.Perfect_Hash_Generators (g-pehage.ads): GNAT Perfect_Hash_Generators g-pehage ads. 
* GNAT.Random_Numbers (g-rannum.ads): GNAT Random_Numbers g-rannum ads. 
* GNAT.Regexp (g-regexp.ads): GNAT Regexp g-regexp ads. 
* GNAT.Registry (g-regist.ads): GNAT Registry g-regist ads. 
* GNAT.Regpat (g-regpat.ads): GNAT Regpat g-regpat ads. 
* GNAT.Rewrite_Data (g-rewdat.ads): GNAT Rewrite_Data g-rewdat ads. 
* GNAT.Secondary_Stack_Info (g-sestin.ads): GNAT Secondary_Stack_Info g-sestin ads. 
* GNAT.Semaphores (g-semaph.ads): GNAT Semaphores g-semaph ads. 
* GNAT.Serial_Communications (g-sercom.ads): GNAT Serial_Communications g-sercom ads. 
* GNAT.SHA1 (g-sha1.ads): GNAT SHA1 g-sha1 ads. 
* GNAT.SHA224 (g-sha224.ads): GNAT SHA224 g-sha224 ads. 
* GNAT.SHA256 (g-sha256.ads): GNAT SHA256 g-sha256 ads. 
* GNAT.SHA384 (g-sha384.ads): GNAT SHA384 g-sha384 ads. 
* GNAT.SHA512 (g-sha512.ads): GNAT SHA512 g-sha512 ads. 
* GNAT.Signals (g-signal.ads): GNAT Signals g-signal ads. 
* GNAT.Sockets (g-socket.ads): GNAT Sockets g-socket ads. 
* GNAT.Source_Info (g-souinf.ads): GNAT Source_Info g-souinf ads. 
* GNAT.Spelling_Checker (g-speche.ads): GNAT Spelling_Checker g-speche ads. 
* GNAT.Spelling_Checker_Generic (g-spchge.ads): GNAT Spelling_Checker_Generic g-spchge ads. 
* GNAT.Spitbol.Patterns (g-spipat.ads): GNAT Spitbol Patterns g-spipat ads. 
* GNAT.Spitbol (g-spitbo.ads): GNAT Spitbol g-spitbo ads. 
* GNAT.Spitbol.Table_Boolean (g-sptabo.ads): GNAT Spitbol Table_Boolean g-sptabo ads. 
* GNAT.Spitbol.Table_Integer (g-sptain.ads): GNAT Spitbol Table_Integer g-sptain ads. 
* GNAT.Spitbol.Table_VString (g-sptavs.ads): GNAT Spitbol Table_VString g-sptavs ads. 
* GNAT.SSE (g-sse.ads): GNAT SSE g-sse ads. 
* GNAT.SSE.Vector_Types (g-ssvety.ads): GNAT SSE Vector_Types g-ssvety ads. 
* GNAT.Strings (g-string.ads): GNAT Strings g-string ads. 
* GNAT.String_Split (g-strspl.ads): GNAT String_Split g-strspl ads. 
* GNAT.Table (g-table.ads): GNAT Table g-table ads. 
* GNAT.Task_Lock (g-tasloc.ads): GNAT Task_Lock g-tasloc ads. 
* GNAT.Time_Stamp (g-timsta.ads): GNAT Time_Stamp g-timsta ads. 
* GNAT.Threads (g-thread.ads): GNAT Threads g-thread ads. 
* GNAT.Traceback (g-traceb.ads): GNAT Traceback g-traceb ads. 
* GNAT.Traceback.Symbolic (g-trasym.ads): GNAT Traceback Symbolic g-trasym ads. 
* GNAT.UTF_32 (g-table.ads): GNAT UTF_32 g-table ads. 
* GNAT.Wide_Spelling_Checker (g-u3spch.ads): GNAT Wide_Spelling_Checker g-u3spch ads. 
* GNAT.Wide_Spelling_Checker (g-wispch.ads): GNAT Wide_Spelling_Checker g-wispch ads. 
* GNAT.Wide_String_Split (g-wistsp.ads): GNAT Wide_String_Split g-wistsp ads. 
* GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads): GNAT Wide_Wide_Spelling_Checker g-zspche ads. 
* GNAT.Wide_Wide_String_Split (g-zistsp.ads): GNAT Wide_Wide_String_Split g-zistsp ads. 
* Interfaces.C.Extensions (i-cexten.ads): Interfaces C Extensions i-cexten ads. 
* Interfaces.C.Streams (i-cstrea.ads): Interfaces C Streams i-cstrea ads. 
* Interfaces.Packed_Decimal (i-pacdec.ads): Interfaces Packed_Decimal i-pacdec ads. 
* Interfaces.VxWorks (i-vxwork.ads): Interfaces VxWorks i-vxwork ads. 
* Interfaces.VxWorks.IO (i-vxwoio.ads): Interfaces VxWorks IO i-vxwoio ads. 
* System.Address_Image (s-addima.ads): System Address_Image s-addima ads. 
* System.Assertions (s-assert.ads): System Assertions s-assert ads. 
* System.Atomic_Counters (s-atocou.ads): System Atomic_Counters s-atocou ads. 
* System.Memory (s-memory.ads): System Memory s-memory ads. 
* System.Multiprocessors (s-multip.ads): System Multiprocessors s-multip ads. 
* System.Multiprocessors.Dispatching_Domains (s-mudido.ads): System Multiprocessors Dispatching_Domains s-mudido ads. 
* System.Partition_Interface (s-parint.ads): System Partition_Interface s-parint ads. 
* System.Pool_Global (s-pooglo.ads): System Pool_Global s-pooglo ads. 
* System.Pool_Local (s-pooloc.ads): System Pool_Local s-pooloc ads. 
* System.Restrictions (s-restri.ads): System Restrictions s-restri ads. 
* System.Rident (s-rident.ads): System Rident s-rident ads. 
* System.Strings.Stream_Ops (s-ststop.ads): System Strings Stream_Ops s-ststop ads. 
* System.Unsigned_Types (s-unstyp.ads): System Unsigned_Types s-unstyp ads. 
* System.Wch_Cnv (s-wchcnv.ads): System Wch_Cnv s-wchcnv ads. 
* System.Wch_Con (s-wchcon.ads): System Wch_Con s-wchcon ads. 

@end menu

@node Ada Characters Latin_9 a-chlat9 ads,Ada Characters Wide_Latin_1 a-cwila1 ads,,The GNAT Library
@anchor{gnat_rm/the_gnat_library id2}@anchor{278}@anchor{gnat_rm/the_gnat_library ada-characters-latin-9-a-chlat9-ads}@anchor{279}
@section @cite{Ada.Characters.Latin_9} (@code{a-chlat9.ads})


@geindex Ada.Characters.Latin_9 (a-chlat9.ads)

@geindex Latin_9 constants for Character

This child of @cite{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @cite{Ada.Characters.Latin_1} but with the
few modifications required for @cite{Latin-9}
The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Latin_1 a-cwila1 ads,Ada Characters Wide_Latin_9 a-cwila1 ads,Ada Characters Latin_9 a-chlat9 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-latin-1-a-cwila1-ads}@anchor{27a}@anchor{gnat_rm/the_gnat_library id3}@anchor{27b}
@section @cite{Ada.Characters.Wide_Latin_1} (@code{a-cwila1.ads})


@geindex Ada.Characters.Wide_Latin_1 (a-cwila1.ads)

@geindex Latin_1 constants for Wide_Character

This child of @cite{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @cite{Ada.Characters.Latin_1} but with the
types of the constants being @cite{Wide_Character}
instead of @cite{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Latin_9 a-cwila1 ads,Ada Characters Wide_Wide_Latin_1 a-chzla1 ads,Ada Characters Wide_Latin_1 a-cwila1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id4}@anchor{27c}@anchor{gnat_rm/the_gnat_library ada-characters-wide-latin-9-a-cwila1-ads}@anchor{27d}
@section @cite{Ada.Characters.Wide_Latin_9} (@code{a-cwila1.ads})


@geindex Ada.Characters.Wide_Latin_9 (a-cwila1.ads)

@geindex Latin_9 constants for Wide_Character

This child of @cite{Ada.Characters}
provides a set of definitions corresponding to those in the
GNAT defined package @cite{Ada.Characters.Latin_9} but with the
types of the constants being @cite{Wide_Character}
instead of @cite{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Wide_Latin_1 a-chzla1 ads,Ada Characters Wide_Wide_Latin_9 a-chzla9 ads,Ada Characters Wide_Latin_9 a-cwila1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-wide-latin-1-a-chzla1-ads}@anchor{27e}@anchor{gnat_rm/the_gnat_library id5}@anchor{27f}
@section @cite{Ada.Characters.Wide_Wide_Latin_1} (@code{a-chzla1.ads})


@geindex Ada.Characters.Wide_Wide_Latin_1 (a-chzla1.ads)

@geindex Latin_1 constants for Wide_Wide_Character

This child of @cite{Ada.Characters}
provides a set of definitions corresponding to those in the
RM-defined package @cite{Ada.Characters.Latin_1} but with the
types of the constants being @cite{Wide_Wide_Character}
instead of @cite{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Characters Wide_Wide_Latin_9 a-chzla9 ads,Ada Containers Formal_Doubly_Linked_Lists a-cfdlli ads,Ada Characters Wide_Wide_Latin_1 a-chzla1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-characters-wide-wide-latin-9-a-chzla9-ads}@anchor{280}@anchor{gnat_rm/the_gnat_library id6}@anchor{281}
@section @cite{Ada.Characters.Wide_Wide_Latin_9} (@code{a-chzla9.ads})


@geindex Ada.Characters.Wide_Wide_Latin_9 (a-chzla9.ads)

@geindex Latin_9 constants for Wide_Wide_Character

This child of @cite{Ada.Characters}
provides a set of definitions corresponding to those in the
GNAT defined package @cite{Ada.Characters.Latin_9} but with the
types of the constants being @cite{Wide_Wide_Character}
instead of @cite{Character}.  The provision of such a package
is specifically authorized by the Ada Reference Manual
(RM A.3.3(27)).

@node Ada Containers Formal_Doubly_Linked_Lists a-cfdlli ads,Ada Containers Formal_Hashed_Maps a-cfhama ads,Ada Characters Wide_Wide_Latin_9 a-chzla9 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id7}@anchor{282}@anchor{gnat_rm/the_gnat_library ada-containers-formal-doubly-linked-lists-a-cfdlli-ads}@anchor{283}
@section @cite{Ada.Containers.Formal_Doubly_Linked_Lists} (@code{a-cfdlli.ads})


@geindex Ada.Containers.Formal_Doubly_Linked_Lists (a-cfdlli.ads)

@geindex Formal container for doubly linked lists

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for doubly linked lists, meant to facilitate formal
verification of code using such containers. The specification of this
unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Formal_Hashed_Maps a-cfhama ads,Ada Containers Formal_Hashed_Sets a-cfhase ads,Ada Containers Formal_Doubly_Linked_Lists a-cfdlli ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id8}@anchor{284}@anchor{gnat_rm/the_gnat_library ada-containers-formal-hashed-maps-a-cfhama-ads}@anchor{285}
@section @cite{Ada.Containers.Formal_Hashed_Maps} (@code{a-cfhama.ads})


@geindex Ada.Containers.Formal_Hashed_Maps (a-cfhama.ads)

@geindex Formal container for hashed maps

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for hashed maps, meant to facilitate formal
verification of code using such containers. The specification of this
unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Formal_Hashed_Sets a-cfhase ads,Ada Containers Formal_Ordered_Maps a-cforma ads,Ada Containers Formal_Hashed_Maps a-cfhama ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id9}@anchor{286}@anchor{gnat_rm/the_gnat_library ada-containers-formal-hashed-sets-a-cfhase-ads}@anchor{287}
@section @cite{Ada.Containers.Formal_Hashed_Sets} (@code{a-cfhase.ads})


@geindex Ada.Containers.Formal_Hashed_Sets (a-cfhase.ads)

@geindex Formal container for hashed sets

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for hashed sets, meant to facilitate formal
verification of code using such containers. The specification of this
unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Formal_Ordered_Maps a-cforma ads,Ada Containers Formal_Ordered_Sets a-cforse ads,Ada Containers Formal_Hashed_Sets a-cfhase ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id10}@anchor{288}@anchor{gnat_rm/the_gnat_library ada-containers-formal-ordered-maps-a-cforma-ads}@anchor{289}
@section @cite{Ada.Containers.Formal_Ordered_Maps} (@code{a-cforma.ads})


@geindex Ada.Containers.Formal_Ordered_Maps (a-cforma.ads)

@geindex Formal container for ordered maps

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for ordered maps, meant to facilitate formal
verification of code using such containers. The specification of this
unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Formal_Ordered_Sets a-cforse ads,Ada Containers Formal_Vectors a-cofove ads,Ada Containers Formal_Ordered_Maps a-cforma ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-containers-formal-ordered-sets-a-cforse-ads}@anchor{28a}@anchor{gnat_rm/the_gnat_library id11}@anchor{28b}
@section @cite{Ada.Containers.Formal_Ordered_Sets} (@code{a-cforse.ads})


@geindex Ada.Containers.Formal_Ordered_Sets (a-cforse.ads)

@geindex Formal container for ordered sets

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for ordered sets, meant to facilitate formal
verification of code using such containers. The specification of this
unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Formal_Vectors a-cofove ads,Ada Containers Formal_Indefinite_Vectors a-cfinve ads,Ada Containers Formal_Ordered_Sets a-cforse ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id12}@anchor{28c}@anchor{gnat_rm/the_gnat_library ada-containers-formal-vectors-a-cofove-ads}@anchor{28d}
@section @cite{Ada.Containers.Formal_Vectors} (@code{a-cofove.ads})


@geindex Ada.Containers.Formal_Vectors (a-cofove.ads)

@geindex Formal container for vectors

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for vectors, meant to facilitate formal
verification of code using such containers. The specification of this
unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Formal_Indefinite_Vectors a-cfinve ads,Ada Containers Bounded_Holders a-coboho ads,Ada Containers Formal_Vectors a-cofove ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id13}@anchor{28e}@anchor{gnat_rm/the_gnat_library ada-containers-formal-indefinite-vectors-a-cfinve-ads}@anchor{28f}
@section @cite{Ada.Containers.Formal_Indefinite_Vectors} (@code{a-cfinve.ads})


@geindex Ada.Containers.Formal_Indefinite_Vectors (a-cfinve.ads)

@geindex Formal container for vectors

This child of @cite{Ada.Containers} defines a modified version of the
Ada 2005 container for vectors of indefinite elements, meant to
facilitate formal verification of code using such containers. The
specification of this unit is compatible with SPARK 2014.

Note that although this container was designed with formal verification
in mind, it may well be generally useful in that it is a simplified more
efficient version than the one defined in the standard. In particular it
does not have the complex overhead required to detect cursor tampering.

@node Ada Containers Bounded_Holders a-coboho ads,Ada Command_Line Environment a-colien ads,Ada Containers Formal_Indefinite_Vectors a-cfinve ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id14}@anchor{290}@anchor{gnat_rm/the_gnat_library ada-containers-bounded-holders-a-coboho-ads}@anchor{291}
@section @cite{Ada.Containers.Bounded_Holders} (@code{a-coboho.ads})


@geindex Ada.Containers.Bounded_Holders (a-coboho.ads)

@geindex Formal container for vectors

This child of @cite{Ada.Containers} defines a modified version of
Indefinite_Holders that avoids heap allocation.

@node Ada Command_Line Environment a-colien ads,Ada Command_Line Remove a-colire ads,Ada Containers Bounded_Holders a-coboho ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-command-line-environment-a-colien-ads}@anchor{292}@anchor{gnat_rm/the_gnat_library id15}@anchor{293}
@section @cite{Ada.Command_Line.Environment} (@code{a-colien.ads})


@geindex Ada.Command_Line.Environment (a-colien.ads)

@geindex Environment entries

This child of @cite{Ada.Command_Line}
provides a mechanism for obtaining environment values on systems
where this concept makes sense.

@node Ada Command_Line Remove a-colire ads,Ada Command_Line Response_File a-clrefi ads,Ada Command_Line Environment a-colien ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id16}@anchor{294}@anchor{gnat_rm/the_gnat_library ada-command-line-remove-a-colire-ads}@anchor{295}
@section @cite{Ada.Command_Line.Remove} (@code{a-colire.ads})


@geindex Ada.Command_Line.Remove (a-colire.ads)

@geindex Removing command line arguments

@geindex Command line
@geindex argument removal

This child of @cite{Ada.Command_Line}
provides a mechanism for logically removing
arguments from the argument list.  Once removed, an argument is not visible
to further calls on the subprograms in @cite{Ada.Command_Line} will not
see the removed argument.

@node Ada Command_Line Response_File a-clrefi ads,Ada Direct_IO C_Streams a-diocst ads,Ada Command_Line Remove a-colire ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-command-line-response-file-a-clrefi-ads}@anchor{296}@anchor{gnat_rm/the_gnat_library id17}@anchor{297}
@section @cite{Ada.Command_Line.Response_File} (@code{a-clrefi.ads})


@geindex Ada.Command_Line.Response_File (a-clrefi.ads)

@geindex Response file for command line

@geindex Command line
@geindex response file

@geindex Command line
@geindex handling long command lines

This child of @cite{Ada.Command_Line} provides a mechanism facilities for
getting command line arguments from a text file, called a "response file".
Using a response file allow passing a set of arguments to an executable longer
than the maximum allowed by the system on the command line.

@node Ada Direct_IO C_Streams a-diocst ads,Ada Exceptions Is_Null_Occurrence a-einuoc ads,Ada Command_Line Response_File a-clrefi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id18}@anchor{298}@anchor{gnat_rm/the_gnat_library ada-direct-io-c-streams-a-diocst-ads}@anchor{299}
@section @cite{Ada.Direct_IO.C_Streams} (@code{a-diocst.ads})


@geindex Ada.Direct_IO.C_Streams (a-diocst.ads)

@geindex C Streams
@geindex Interfacing with Direct_IO

This package provides subprograms that allow interfacing between
C streams and @cite{Direct_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Exceptions Is_Null_Occurrence a-einuoc ads,Ada Exceptions Last_Chance_Handler a-elchha ads,Ada Direct_IO C_Streams a-diocst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id19}@anchor{29a}@anchor{gnat_rm/the_gnat_library ada-exceptions-is-null-occurrence-a-einuoc-ads}@anchor{29b}
@section @cite{Ada.Exceptions.Is_Null_Occurrence} (@code{a-einuoc.ads})


@geindex Ada.Exceptions.Is_Null_Occurrence (a-einuoc.ads)

@geindex Null_Occurrence
@geindex testing for

This child subprogram provides a way of testing for the null
exception occurrence (@cite{Null_Occurrence}) without raising
an exception.

@node Ada Exceptions Last_Chance_Handler a-elchha ads,Ada Exceptions Traceback a-exctra ads,Ada Exceptions Is_Null_Occurrence a-einuoc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id20}@anchor{29c}@anchor{gnat_rm/the_gnat_library ada-exceptions-last-chance-handler-a-elchha-ads}@anchor{29d}
@section @cite{Ada.Exceptions.Last_Chance_Handler} (@code{a-elchha.ads})


@geindex Ada.Exceptions.Last_Chance_Handler (a-elchha.ads)

@geindex Null_Occurrence
@geindex testing for

This child subprogram is used for handling otherwise unhandled
exceptions (hence the name last chance), and perform clean ups before
terminating the program. Note that this subprogram never returns.

@node Ada Exceptions Traceback a-exctra ads,Ada Sequential_IO C_Streams a-siocst ads,Ada Exceptions Last_Chance_Handler a-elchha ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-exceptions-traceback-a-exctra-ads}@anchor{29e}@anchor{gnat_rm/the_gnat_library id21}@anchor{29f}
@section @cite{Ada.Exceptions.Traceback} (@code{a-exctra.ads})


@geindex Ada.Exceptions.Traceback (a-exctra.ads)

@geindex Traceback for Exception Occurrence

This child package provides the subprogram (@cite{Tracebacks}) to
give a traceback array of addresses based on an exception
occurrence.

@node Ada Sequential_IO C_Streams a-siocst ads,Ada Streams Stream_IO C_Streams a-ssicst ads,Ada Exceptions Traceback a-exctra ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-sequential-io-c-streams-a-siocst-ads}@anchor{2a0}@anchor{gnat_rm/the_gnat_library id22}@anchor{2a1}
@section @cite{Ada.Sequential_IO.C_Streams} (@code{a-siocst.ads})


@geindex Ada.Sequential_IO.C_Streams (a-siocst.ads)

@geindex C Streams
@geindex Interfacing with Sequential_IO

This package provides subprograms that allow interfacing between
C streams and @cite{Sequential_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Streams Stream_IO C_Streams a-ssicst ads,Ada Strings Unbounded Text_IO a-suteio ads,Ada Sequential_IO C_Streams a-siocst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id23}@anchor{2a2}@anchor{gnat_rm/the_gnat_library ada-streams-stream-io-c-streams-a-ssicst-ads}@anchor{2a3}
@section @cite{Ada.Streams.Stream_IO.C_Streams} (@code{a-ssicst.ads})


@geindex Ada.Streams.Stream_IO.C_Streams (a-ssicst.ads)

@geindex C Streams
@geindex Interfacing with Stream_IO

This package provides subprograms that allow interfacing between
C streams and @cite{Stream_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Strings Unbounded Text_IO a-suteio ads,Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads,Ada Streams Stream_IO C_Streams a-ssicst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-strings-unbounded-text-io-a-suteio-ads}@anchor{2a4}@anchor{gnat_rm/the_gnat_library id24}@anchor{2a5}
@section @cite{Ada.Strings.Unbounded.Text_IO} (@code{a-suteio.ads})


@geindex Ada.Strings.Unbounded.Text_IO (a-suteio.ads)

@geindex Unbounded_String
@geindex IO support

@geindex Text_IO
@geindex extensions for unbounded strings

This package provides subprograms for Text_IO for unbounded
strings, avoiding the necessity for an intermediate operation
with ordinary strings.

@node Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads,Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads,Ada Strings Unbounded Text_IO a-suteio ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id25}@anchor{2a6}@anchor{gnat_rm/the_gnat_library ada-strings-wide-unbounded-wide-text-io-a-swuwti-ads}@anchor{2a7}
@section @cite{Ada.Strings.Wide_Unbounded.Wide_Text_IO} (@code{a-swuwti.ads})


@geindex Ada.Strings.Wide_Unbounded.Wide_Text_IO (a-swuwti.ads)

@geindex Unbounded_Wide_String
@geindex IO support

@geindex Text_IO
@geindex extensions for unbounded wide strings

This package provides subprograms for Text_IO for unbounded
wide strings, avoiding the necessity for an intermediate operation
with ordinary wide strings.

@node Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads,Ada Text_IO C_Streams a-tiocst ads,Ada Strings Wide_Unbounded Wide_Text_IO a-swuwti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-strings-wide-wide-unbounded-wide-wide-text-io-a-szuzti-ads}@anchor{2a8}@anchor{gnat_rm/the_gnat_library id26}@anchor{2a9}
@section @cite{Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO} (@code{a-szuzti.ads})


@geindex Ada.Strings.Wide_Wide_Unbounded.Wide_Wide_Text_IO (a-szuzti.ads)

@geindex Unbounded_Wide_Wide_String
@geindex IO support

@geindex Text_IO
@geindex extensions for unbounded wide wide strings

This package provides subprograms for Text_IO for unbounded
wide wide strings, avoiding the necessity for an intermediate operation
with ordinary wide wide strings.

@node Ada Text_IO C_Streams a-tiocst ads,Ada Text_IO Reset_Standard_Files a-tirsfi ads,Ada Strings Wide_Wide_Unbounded Wide_Wide_Text_IO a-szuzti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-text-io-c-streams-a-tiocst-ads}@anchor{2aa}@anchor{gnat_rm/the_gnat_library id27}@anchor{2ab}
@section @cite{Ada.Text_IO.C_Streams} (@code{a-tiocst.ads})


@geindex Ada.Text_IO.C_Streams (a-tiocst.ads)

@geindex C Streams
@geindex Interfacing with `Text_IO`

This package provides subprograms that allow interfacing between
C streams and @cite{Text_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Text_IO Reset_Standard_Files a-tirsfi ads,Ada Wide_Characters Unicode a-wichun ads,Ada Text_IO C_Streams a-tiocst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id28}@anchor{2ac}@anchor{gnat_rm/the_gnat_library ada-text-io-reset-standard-files-a-tirsfi-ads}@anchor{2ad}
@section @cite{Ada.Text_IO.Reset_Standard_Files} (@code{a-tirsfi.ads})


@geindex Ada.Text_IO.Reset_Standard_Files (a-tirsfi.ads)

@geindex Text_IO resetting standard files

This procedure is used to reset the status of the standard files used
by Ada.Text_IO.  This is useful in a situation (such as a restart in an
embedded application) where the status of the files may change during
execution (for example a standard input file may be redefined to be
interactive).

@node Ada Wide_Characters Unicode a-wichun ads,Ada Wide_Text_IO C_Streams a-wtcstr ads,Ada Text_IO Reset_Standard_Files a-tirsfi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id29}@anchor{2ae}@anchor{gnat_rm/the_gnat_library ada-wide-characters-unicode-a-wichun-ads}@anchor{2af}
@section @cite{Ada.Wide_Characters.Unicode} (@code{a-wichun.ads})


@geindex Ada.Wide_Characters.Unicode (a-wichun.ads)

@geindex Unicode categorization
@geindex Wide_Character

This package provides subprograms that allow categorization of
Wide_Character values according to Unicode categories.

@node Ada Wide_Text_IO C_Streams a-wtcstr ads,Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads,Ada Wide_Characters Unicode a-wichun ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-text-io-c-streams-a-wtcstr-ads}@anchor{2b0}@anchor{gnat_rm/the_gnat_library id30}@anchor{2b1}
@section @cite{Ada.Wide_Text_IO.C_Streams} (@code{a-wtcstr.ads})


@geindex Ada.Wide_Text_IO.C_Streams (a-wtcstr.ads)

@geindex C Streams
@geindex Interfacing with `Wide_Text_IO`

This package provides subprograms that allow interfacing between
C streams and @cite{Wide_Text_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads,Ada Wide_Wide_Characters Unicode a-zchuni ads,Ada Wide_Text_IO C_Streams a-wtcstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library ada-wide-text-io-reset-standard-files-a-wrstfi-ads}@anchor{2b2}@anchor{gnat_rm/the_gnat_library id31}@anchor{2b3}
@section @cite{Ada.Wide_Text_IO.Reset_Standard_Files} (@code{a-wrstfi.ads})


@geindex Ada.Wide_Text_IO.Reset_Standard_Files (a-wrstfi.ads)

@geindex Wide_Text_IO resetting standard files

This procedure is used to reset the status of the standard files used
by Ada.Wide_Text_IO.  This is useful in a situation (such as a restart in an
embedded application) where the status of the files may change during
execution (for example a standard input file may be redefined to be
interactive).

@node Ada Wide_Wide_Characters Unicode a-zchuni ads,Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads,Ada Wide_Text_IO Reset_Standard_Files a-wrstfi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id32}@anchor{2b4}@anchor{gnat_rm/the_gnat_library ada-wide-wide-characters-unicode-a-zchuni-ads}@anchor{2b5}
@section @cite{Ada.Wide_Wide_Characters.Unicode} (@code{a-zchuni.ads})


@geindex Ada.Wide_Wide_Characters.Unicode (a-zchuni.ads)

@geindex Unicode categorization
@geindex Wide_Wide_Character

This package provides subprograms that allow categorization of
Wide_Wide_Character values according to Unicode categories.

@node Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads,Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads,Ada Wide_Wide_Characters Unicode a-zchuni ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id33}@anchor{2b6}@anchor{gnat_rm/the_gnat_library ada-wide-wide-text-io-c-streams-a-ztcstr-ads}@anchor{2b7}
@section @cite{Ada.Wide_Wide_Text_IO.C_Streams} (@code{a-ztcstr.ads})


@geindex Ada.Wide_Wide_Text_IO.C_Streams (a-ztcstr.ads)

@geindex C Streams
@geindex Interfacing with `Wide_Wide_Text_IO`

This package provides subprograms that allow interfacing between
C streams and @cite{Wide_Wide_Text_IO}.  The stream identifier can be
extracted from a file opened on the Ada side, and an Ada file
can be constructed from a stream opened on the C side.

@node Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads,GNAT Altivec g-altive ads,Ada Wide_Wide_Text_IO C_Streams a-ztcstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id34}@anchor{2b8}@anchor{gnat_rm/the_gnat_library ada-wide-wide-text-io-reset-standard-files-a-zrstfi-ads}@anchor{2b9}
@section @cite{Ada.Wide_Wide_Text_IO.Reset_Standard_Files} (@code{a-zrstfi.ads})


@geindex Ada.Wide_Wide_Text_IO.Reset_Standard_Files (a-zrstfi.ads)

@geindex Wide_Wide_Text_IO resetting standard files

This procedure is used to reset the status of the standard files used
by Ada.Wide_Wide_Text_IO. This is useful in a situation (such as a
restart in an embedded application) where the status of the files may
change during execution (for example a standard input file may be
redefined to be interactive).

@node GNAT Altivec g-altive ads,GNAT Altivec Conversions g-altcon ads,Ada Wide_Wide_Text_IO Reset_Standard_Files a-zrstfi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-g-altive-ads}@anchor{2ba}@anchor{gnat_rm/the_gnat_library id35}@anchor{2bb}
@section @cite{GNAT.Altivec} (@code{g-altive.ads})


@geindex GNAT.Altivec (g-altive.ads)

@geindex AltiVec

This is the root package of the GNAT AltiVec binding. It provides
definitions of constants and types common to all the versions of the
binding.

@node GNAT Altivec Conversions g-altcon ads,GNAT Altivec Vector_Operations g-alveop ads,GNAT Altivec g-altive ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id36}@anchor{2bc}@anchor{gnat_rm/the_gnat_library gnat-altivec-conversions-g-altcon-ads}@anchor{2bd}
@section @cite{GNAT.Altivec.Conversions} (@code{g-altcon.ads})


@geindex GNAT.Altivec.Conversions (g-altcon.ads)

@geindex AltiVec

This package provides the Vector/View conversion routines.

@node GNAT Altivec Vector_Operations g-alveop ads,GNAT Altivec Vector_Types g-alvety ads,GNAT Altivec Conversions g-altcon ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id37}@anchor{2be}@anchor{gnat_rm/the_gnat_library gnat-altivec-vector-operations-g-alveop-ads}@anchor{2bf}
@section @cite{GNAT.Altivec.Vector_Operations} (@code{g-alveop.ads})


@geindex GNAT.Altivec.Vector_Operations (g-alveop.ads)

@geindex AltiVec

This package exposes the Ada interface to the AltiVec operations on
vector objects. A soft emulation is included by default in the GNAT
library. The hard binding is provided as a separate package. This unit
is common to both bindings.

@node GNAT Altivec Vector_Types g-alvety ads,GNAT Altivec Vector_Views g-alvevi ads,GNAT Altivec Vector_Operations g-alveop ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-vector-types-g-alvety-ads}@anchor{2c0}@anchor{gnat_rm/the_gnat_library id38}@anchor{2c1}
@section @cite{GNAT.Altivec.Vector_Types} (@code{g-alvety.ads})


@geindex GNAT.Altivec.Vector_Types (g-alvety.ads)

@geindex AltiVec

This package exposes the various vector types part of the Ada binding
to AltiVec facilities.

@node GNAT Altivec Vector_Views g-alvevi ads,GNAT Array_Split g-arrspl ads,GNAT Altivec Vector_Types g-alvety ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-altivec-vector-views-g-alvevi-ads}@anchor{2c2}@anchor{gnat_rm/the_gnat_library id39}@anchor{2c3}
@section @cite{GNAT.Altivec.Vector_Views} (@code{g-alvevi.ads})


@geindex GNAT.Altivec.Vector_Views (g-alvevi.ads)

@geindex AltiVec

This package provides public 'View' data types from/to which private
vector representations can be converted via
GNAT.Altivec.Conversions. This allows convenient access to individual
vector elements and provides a simple way to initialize vector
objects.

@node GNAT Array_Split g-arrspl ads,GNAT AWK g-awk ads,GNAT Altivec Vector_Views g-alvevi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-array-split-g-arrspl-ads}@anchor{2c4}@anchor{gnat_rm/the_gnat_library id40}@anchor{2c5}
@section @cite{GNAT.Array_Split} (@code{g-arrspl.ads})


@geindex GNAT.Array_Split (g-arrspl.ads)

@geindex Array splitter

Useful array-manipulation routines: given a set of separators, split
an array wherever the separators appear, and provide direct access
to the resulting slices.

@node GNAT AWK g-awk ads,GNAT Bounded_Buffers g-boubuf ads,GNAT Array_Split g-arrspl ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id41}@anchor{2c6}@anchor{gnat_rm/the_gnat_library gnat-awk-g-awk-ads}@anchor{2c7}
@section @cite{GNAT.AWK} (@code{g-awk.ads})


@geindex GNAT.AWK (g-awk.ads)

@geindex Parsing

@geindex AWK

Provides AWK-like parsing functions, with an easy interface for parsing one
or more files containing formatted data.  The file is viewed as a database
where each record is a line and a field is a data element in this line.

@node GNAT Bounded_Buffers g-boubuf ads,GNAT Bounded_Mailboxes g-boumai ads,GNAT AWK g-awk ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bounded-buffers-g-boubuf-ads}@anchor{2c8}@anchor{gnat_rm/the_gnat_library id42}@anchor{2c9}
@section @cite{GNAT.Bounded_Buffers} (@code{g-boubuf.ads})


@geindex GNAT.Bounded_Buffers (g-boubuf.ads)

@geindex Parsing

@geindex Bounded Buffers

Provides a concurrent generic bounded buffer abstraction.  Instances are
useful directly or as parts of the implementations of other abstractions,
such as mailboxes.

@node GNAT Bounded_Mailboxes g-boumai ads,GNAT Bubble_Sort g-bubsor ads,GNAT Bounded_Buffers g-boubuf ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bounded-mailboxes-g-boumai-ads}@anchor{2ca}@anchor{gnat_rm/the_gnat_library id43}@anchor{2cb}
@section @cite{GNAT.Bounded_Mailboxes} (@code{g-boumai.ads})


@geindex GNAT.Bounded_Mailboxes (g-boumai.ads)

@geindex Parsing

@geindex Mailboxes

Provides a thread-safe asynchronous intertask mailbox communication facility.

@node GNAT Bubble_Sort g-bubsor ads,GNAT Bubble_Sort_A g-busora ads,GNAT Bounded_Mailboxes g-boumai ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-bubble-sort-g-bubsor-ads}@anchor{2cc}@anchor{gnat_rm/the_gnat_library id44}@anchor{2cd}
@section @cite{GNAT.Bubble_Sort} (@code{g-bubsor.ads})


@geindex GNAT.Bubble_Sort (g-bubsor.ads)

@geindex Sorting

@geindex Bubble sort

Provides a general implementation of bubble sort usable for sorting arbitrary
data items.  Exchange and comparison procedures are provided by passing
access-to-procedure values.

@node GNAT Bubble_Sort_A g-busora ads,GNAT Bubble_Sort_G g-busorg ads,GNAT Bubble_Sort g-bubsor ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id45}@anchor{2ce}@anchor{gnat_rm/the_gnat_library gnat-bubble-sort-a-g-busora-ads}@anchor{2cf}
@section @cite{GNAT.Bubble_Sort_A} (@code{g-busora.ads})


@geindex GNAT.Bubble_Sort_A (g-busora.ads)

@geindex Sorting

@geindex Bubble sort

Provides a general implementation of bubble sort usable for sorting arbitrary
data items.  Move and comparison procedures are provided by passing
access-to-procedure values. This is an older version, retained for
compatibility. Usually @cite{GNAT.Bubble_Sort} will be preferable.

@node GNAT Bubble_Sort_G g-busorg ads,GNAT Byte_Order_Mark g-byorma ads,GNAT Bubble_Sort_A g-busora ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id46}@anchor{2d0}@anchor{gnat_rm/the_gnat_library gnat-bubble-sort-g-g-busorg-ads}@anchor{2d1}
@section @cite{GNAT.Bubble_Sort_G} (@code{g-busorg.ads})


@geindex GNAT.Bubble_Sort_G (g-busorg.ads)

@geindex Sorting

@geindex Bubble sort

Similar to @cite{Bubble_Sort_A} except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.

@node GNAT Byte_Order_Mark g-byorma ads,GNAT Byte_Swapping g-bytswa ads,GNAT Bubble_Sort_G g-busorg ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id47}@anchor{2d2}@anchor{gnat_rm/the_gnat_library gnat-byte-order-mark-g-byorma-ads}@anchor{2d3}
@section @cite{GNAT.Byte_Order_Mark} (@code{g-byorma.ads})


@geindex GNAT.Byte_Order_Mark (g-byorma.ads)

@geindex UTF-8 representation

@geindex Wide characte representations

Provides a routine which given a string, reads the start of the string to
see whether it is one of the standard byte order marks (BOM's) which signal
the encoding of the string. The routine includes detection of special XML
sequences for various UCS input formats.

@node GNAT Byte_Swapping g-bytswa ads,GNAT Calendar g-calend ads,GNAT Byte_Order_Mark g-byorma ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-byte-swapping-g-bytswa-ads}@anchor{2d4}@anchor{gnat_rm/the_gnat_library id48}@anchor{2d5}
@section @cite{GNAT.Byte_Swapping} (@code{g-bytswa.ads})


@geindex GNAT.Byte_Swapping (g-bytswa.ads)

@geindex Byte swapping

@geindex Endianness

General routines for swapping the bytes in 2-, 4-, and 8-byte quantities.
Machine-specific implementations are available in some cases.

@node GNAT Calendar g-calend ads,GNAT Calendar Time_IO g-catiio ads,GNAT Byte_Swapping g-bytswa ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-calendar-g-calend-ads}@anchor{2d6}@anchor{gnat_rm/the_gnat_library id49}@anchor{2d7}
@section @cite{GNAT.Calendar} (@code{g-calend.ads})


@geindex GNAT.Calendar (g-calend.ads)

@geindex Calendar

Extends the facilities provided by @cite{Ada.Calendar} to include handling
of days of the week, an extended @cite{Split} and @cite{Time_Of} capability.
Also provides conversion of @cite{Ada.Calendar.Time} values to and from the
C @cite{timeval} format.

@node GNAT Calendar Time_IO g-catiio ads,GNAT CRC32 g-crc32 ads,GNAT Calendar g-calend ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-calendar-time-io-g-catiio-ads}@anchor{2d8}@anchor{gnat_rm/the_gnat_library id50}@anchor{2d9}
@section @cite{GNAT.Calendar.Time_IO} (@code{g-catiio.ads})


@geindex Calendar

@geindex Time

@geindex GNAT.Calendar.Time_IO (g-catiio.ads)

@node GNAT CRC32 g-crc32 ads,GNAT Case_Util g-casuti ads,GNAT Calendar Time_IO g-catiio ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id51}@anchor{2da}@anchor{gnat_rm/the_gnat_library gnat-crc32-g-crc32-ads}@anchor{2db}
@section @cite{GNAT.CRC32} (@code{g-crc32.ads})


@geindex GNAT.CRC32 (g-crc32.ads)

@geindex CRC32

@geindex Cyclic Redundancy Check

This package implements the CRC-32 algorithm.  For a full description
of this algorithm see
@emph{Computation of Cyclic Redundancy Checks via Table Look-Up},
@cite{Communications of the ACM}, Vol. 31 No. 8, pp. 1008-1013,
Aug. 1988.  Sarwate, D.V.

@node GNAT Case_Util g-casuti ads,GNAT CGI g-cgi ads,GNAT CRC32 g-crc32 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-case-util-g-casuti-ads}@anchor{2dc}@anchor{gnat_rm/the_gnat_library id52}@anchor{2dd}
@section @cite{GNAT.Case_Util} (@code{g-casuti.ads})


@geindex GNAT.Case_Util (g-casuti.ads)

@geindex Casing utilities

@geindex Character handling (`GNAT.Case_Util`)

A set of simple routines for handling upper and lower casing of strings
without the overhead of the full casing tables
in @cite{Ada.Characters.Handling}.

@node GNAT CGI g-cgi ads,GNAT CGI Cookie g-cgicoo ads,GNAT Case_Util g-casuti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id53}@anchor{2de}@anchor{gnat_rm/the_gnat_library gnat-cgi-g-cgi-ads}@anchor{2df}
@section @cite{GNAT.CGI} (@code{g-cgi.ads})


@geindex GNAT.CGI (g-cgi.ads)

@geindex CGI (Common Gateway Interface)

This is a package for interfacing a GNAT program with a Web server via the
Common Gateway Interface (CGI).  Basically this package parses the CGI
parameters, which are a set of key/value pairs sent by the Web server.  It
builds a table whose index is the key and provides some services to deal
with this table.

@node GNAT CGI Cookie g-cgicoo ads,GNAT CGI Debug g-cgideb ads,GNAT CGI g-cgi ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id54}@anchor{2e0}@anchor{gnat_rm/the_gnat_library gnat-cgi-cookie-g-cgicoo-ads}@anchor{2e1}
@section @cite{GNAT.CGI.Cookie} (@code{g-cgicoo.ads})


@geindex GNAT.CGI.Cookie (g-cgicoo.ads)

@geindex CGI (Common Gateway Interface) cookie support

@geindex Cookie support in CGI

This is a package to interface a GNAT program with a Web server via the
Common Gateway Interface (CGI).  It exports services to deal with Web
cookies (piece of information kept in the Web client software).

@node GNAT CGI Debug g-cgideb ads,GNAT Command_Line g-comlin ads,GNAT CGI Cookie g-cgicoo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-cgi-debug-g-cgideb-ads}@anchor{2e2}@anchor{gnat_rm/the_gnat_library id55}@anchor{2e3}
@section @cite{GNAT.CGI.Debug} (@code{g-cgideb.ads})


@geindex GNAT.CGI.Debug (g-cgideb.ads)

@geindex CGI (Common Gateway Interface) debugging

This is a package to help debugging CGI (Common Gateway Interface)
programs written in Ada.

@node GNAT Command_Line g-comlin ads,GNAT Compiler_Version g-comver ads,GNAT CGI Debug g-cgideb ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id56}@anchor{2e4}@anchor{gnat_rm/the_gnat_library gnat-command-line-g-comlin-ads}@anchor{2e5}
@section @cite{GNAT.Command_Line} (@code{g-comlin.ads})


@geindex GNAT.Command_Line (g-comlin.ads)

@geindex Command line

Provides a high level interface to @cite{Ada.Command_Line} facilities,
including the ability to scan for named switches with optional parameters
and expand file names using wild card notations.

@node GNAT Compiler_Version g-comver ads,GNAT Ctrl_C g-ctrl_c ads,GNAT Command_Line g-comlin ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-compiler-version-g-comver-ads}@anchor{2e6}@anchor{gnat_rm/the_gnat_library id57}@anchor{2e7}
@section @cite{GNAT.Compiler_Version} (@code{g-comver.ads})


@geindex GNAT.Compiler_Version (g-comver.ads)

@geindex Compiler Version

@geindex Version
@geindex of compiler

Provides a routine for obtaining the version of the compiler used to
compile the program. More accurately this is the version of the binder
used to bind the program (this will normally be the same as the version
of the compiler if a consistent tool set is used to compile all units
of a partition).

@node GNAT Ctrl_C g-ctrl_c ads,GNAT Current_Exception g-curexc ads,GNAT Compiler_Version g-comver ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id58}@anchor{2e8}@anchor{gnat_rm/the_gnat_library gnat-ctrl-c-g-ctrl-c-ads}@anchor{2e9}
@section @cite{GNAT.Ctrl_C} (@code{g-ctrl_c.ads})


@geindex GNAT.Ctrl_C (g-ctrl_c.ads)

@geindex Interrupt

Provides a simple interface to handle Ctrl-C keyboard events.

@node GNAT Current_Exception g-curexc ads,GNAT Debug_Pools g-debpoo ads,GNAT Ctrl_C g-ctrl_c ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id59}@anchor{2ea}@anchor{gnat_rm/the_gnat_library gnat-current-exception-g-curexc-ads}@anchor{2eb}
@section @cite{GNAT.Current_Exception} (@code{g-curexc.ads})


@geindex GNAT.Current_Exception (g-curexc.ads)

@geindex Current exception

@geindex Exception retrieval

Provides access to information on the current exception that has been raised
without the need for using the Ada 95 / Ada 2005 exception choice parameter
specification syntax.
This is particularly useful in simulating typical facilities for
obtaining information about exceptions provided by Ada 83 compilers.

@node GNAT Debug_Pools g-debpoo ads,GNAT Debug_Utilities g-debuti ads,GNAT Current_Exception g-curexc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-debug-pools-g-debpoo-ads}@anchor{2ec}@anchor{gnat_rm/the_gnat_library id60}@anchor{2ed}
@section @cite{GNAT.Debug_Pools} (@code{g-debpoo.ads})


@geindex GNAT.Debug_Pools (g-debpoo.ads)

@geindex Debugging

@geindex Debug pools

@geindex Memory corruption debugging

Provide a debugging storage pools that helps tracking memory corruption
problems.
See @cite{The GNAT Debug_Pool Facility} section in the @cite{GNAT User's Guide}.

@node GNAT Debug_Utilities g-debuti ads,GNAT Decode_String g-decstr ads,GNAT Debug_Pools g-debpoo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-debug-utilities-g-debuti-ads}@anchor{2ee}@anchor{gnat_rm/the_gnat_library id61}@anchor{2ef}
@section @cite{GNAT.Debug_Utilities} (@code{g-debuti.ads})


@geindex GNAT.Debug_Utilities (g-debuti.ads)

@geindex Debugging

Provides a few useful utilities for debugging purposes, including conversion
to and from string images of address values. Supports both C and Ada formats
for hexadecimal literals.

@node GNAT Decode_String g-decstr ads,GNAT Decode_UTF8_String g-deutst ads,GNAT Debug_Utilities g-debuti ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-decode-string-g-decstr-ads}@anchor{2f0}@anchor{gnat_rm/the_gnat_library id62}@anchor{2f1}
@section @cite{GNAT.Decode_String} (@code{g-decstr.ads})


@geindex GNAT.Decode_String (g-decstr.ads)

@geindex Decoding strings

@geindex String decoding

@geindex Wide character encoding

@geindex UTF-8

@geindex Unicode

A generic package providing routines for decoding wide character and wide wide
character strings encoded as sequences of 8-bit characters using a specified
encoding method. Includes validation routines, and also routines for stepping
to next or previous encoded character in an encoded string.
Useful in conjunction with Unicode character coding. Note there is a
preinstantiation for UTF-8. See next entry.

@node GNAT Decode_UTF8_String g-deutst ads,GNAT Directory_Operations g-dirope ads,GNAT Decode_String g-decstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-decode-utf8-string-g-deutst-ads}@anchor{2f2}@anchor{gnat_rm/the_gnat_library id63}@anchor{2f3}
@section @cite{GNAT.Decode_UTF8_String} (@code{g-deutst.ads})


@geindex GNAT.Decode_UTF8_String (g-deutst.ads)

@geindex Decoding strings

@geindex Decoding UTF-8 strings

@geindex UTF-8 string decoding

@geindex Wide character decoding

@geindex UTF-8

@geindex Unicode

A preinstantiation of GNAT.Decode_Strings for UTF-8 encoding.

@node GNAT Directory_Operations g-dirope ads,GNAT Directory_Operations Iteration g-diopit ads,GNAT Decode_UTF8_String g-deutst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id64}@anchor{2f4}@anchor{gnat_rm/the_gnat_library gnat-directory-operations-g-dirope-ads}@anchor{2f5}
@section @cite{GNAT.Directory_Operations} (@code{g-dirope.ads})


@geindex GNAT.Directory_Operations (g-dirope.ads)

@geindex Directory operations

Provides a set of routines for manipulating directories, including changing
the current directory, making new directories, and scanning the files in a
directory.

@node GNAT Directory_Operations Iteration g-diopit ads,GNAT Dynamic_HTables g-dynhta ads,GNAT Directory_Operations g-dirope ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id65}@anchor{2f6}@anchor{gnat_rm/the_gnat_library gnat-directory-operations-iteration-g-diopit-ads}@anchor{2f7}
@section @cite{GNAT.Directory_Operations.Iteration} (@code{g-diopit.ads})


@geindex GNAT.Directory_Operations.Iteration (g-diopit.ads)

@geindex Directory operations iteration

A child unit of GNAT.Directory_Operations providing additional operations
for iterating through directories.

@node GNAT Dynamic_HTables g-dynhta ads,GNAT Dynamic_Tables g-dyntab ads,GNAT Directory_Operations Iteration g-diopit ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-dynamic-htables-g-dynhta-ads}@anchor{2f8}@anchor{gnat_rm/the_gnat_library id66}@anchor{2f9}
@section @cite{GNAT.Dynamic_HTables} (@code{g-dynhta.ads})


@geindex GNAT.Dynamic_HTables (g-dynhta.ads)

@geindex Hash tables

A generic implementation of hash tables that can be used to hash arbitrary
data.  Provided in two forms, a simple form with built in hash functions,
and a more complex form in which the hash function is supplied.

This package provides a facility similar to that of @cite{GNAT.HTable},
except that this package declares a type that can be used to define
dynamic instances of the hash table, while an instantiation of
@cite{GNAT.HTable} creates a single instance of the hash table.

@node GNAT Dynamic_Tables g-dyntab ads,GNAT Encode_String g-encstr ads,GNAT Dynamic_HTables g-dynhta ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-dynamic-tables-g-dyntab-ads}@anchor{2fa}@anchor{gnat_rm/the_gnat_library id67}@anchor{2fb}
@section @cite{GNAT.Dynamic_Tables} (@code{g-dyntab.ads})


@geindex GNAT.Dynamic_Tables (g-dyntab.ads)

@geindex Table implementation

@geindex Arrays
@geindex extendable

A generic package providing a single dimension array abstraction where the
length of the array can be dynamically modified.

This package provides a facility similar to that of @cite{GNAT.Table},
except that this package declares a type that can be used to define
dynamic instances of the table, while an instantiation of
@cite{GNAT.Table} creates a single instance of the table type.

@node GNAT Encode_String g-encstr ads,GNAT Encode_UTF8_String g-enutst ads,GNAT Dynamic_Tables g-dyntab ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id68}@anchor{2fc}@anchor{gnat_rm/the_gnat_library gnat-encode-string-g-encstr-ads}@anchor{2fd}
@section @cite{GNAT.Encode_String} (@code{g-encstr.ads})


@geindex GNAT.Encode_String (g-encstr.ads)

@geindex Encoding strings

@geindex String encoding

@geindex Wide character encoding

@geindex UTF-8

@geindex Unicode

A generic package providing routines for encoding wide character and wide
wide character strings as sequences of 8-bit characters using a specified
encoding method. Useful in conjunction with Unicode character coding.
Note there is a preinstantiation for UTF-8. See next entry.

@node GNAT Encode_UTF8_String g-enutst ads,GNAT Exception_Actions g-excact ads,GNAT Encode_String g-encstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-encode-utf8-string-g-enutst-ads}@anchor{2fe}@anchor{gnat_rm/the_gnat_library id69}@anchor{2ff}
@section @cite{GNAT.Encode_UTF8_String} (@code{g-enutst.ads})


@geindex GNAT.Encode_UTF8_String (g-enutst.ads)

@geindex Encoding strings

@geindex Encoding UTF-8 strings

@geindex UTF-8 string encoding

@geindex Wide character encoding

@geindex UTF-8

@geindex Unicode

A preinstantiation of GNAT.Encode_Strings for UTF-8 encoding.

@node GNAT Exception_Actions g-excact ads,GNAT Exception_Traces g-exctra ads,GNAT Encode_UTF8_String g-enutst ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id70}@anchor{300}@anchor{gnat_rm/the_gnat_library gnat-exception-actions-g-excact-ads}@anchor{301}
@section @cite{GNAT.Exception_Actions} (@code{g-excact.ads})


@geindex GNAT.Exception_Actions (g-excact.ads)

@geindex Exception actions

Provides callbacks when an exception is raised. Callbacks can be registered
for specific exceptions, or when any exception is raised. This
can be used for instance to force a core dump to ease debugging.

@node GNAT Exception_Traces g-exctra ads,GNAT Exceptions g-expect ads,GNAT Exception_Actions g-excact ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-exception-traces-g-exctra-ads}@anchor{302}@anchor{gnat_rm/the_gnat_library id71}@anchor{303}
@section @cite{GNAT.Exception_Traces} (@code{g-exctra.ads})


@geindex GNAT.Exception_Traces (g-exctra.ads)

@geindex Exception traces

@geindex Debugging

Provides an interface allowing to control automatic output upon exception
occurrences.

@node GNAT Exceptions g-expect ads,GNAT Expect g-expect ads,GNAT Exception_Traces g-exctra ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id72}@anchor{304}@anchor{gnat_rm/the_gnat_library gnat-exceptions-g-expect-ads}@anchor{305}
@section @cite{GNAT.Exceptions} (@code{g-expect.ads})


@geindex GNAT.Exceptions (g-expect.ads)

@geindex Exceptions
@geindex Pure

@geindex Pure packages
@geindex exceptions

Normally it is not possible to raise an exception with
a message from a subprogram in a pure package, since the
necessary types and subprograms are in @cite{Ada.Exceptions}
which is not a pure unit. @cite{GNAT.Exceptions} provides a
facility for getting around this limitation for a few
predefined exceptions, and for example allow raising
@cite{Constraint_Error} with a message from a pure subprogram.

@node GNAT Expect g-expect ads,GNAT Expect TTY g-exptty ads,GNAT Exceptions g-expect ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id73}@anchor{306}@anchor{gnat_rm/the_gnat_library gnat-expect-g-expect-ads}@anchor{307}
@section @cite{GNAT.Expect} (@code{g-expect.ads})


@geindex GNAT.Expect (g-expect.ads)

Provides a set of subprograms similar to what is available
with the standard Tcl Expect tool.
It allows you to easily spawn and communicate with an external process.
You can send commands or inputs to the process, and compare the output
with some expected regular expression. Currently @cite{GNAT.Expect}
is implemented on all native GNAT ports.
It is not implemented for cross ports, and in particular is not
implemented for VxWorks or LynxOS.

@node GNAT Expect TTY g-exptty ads,GNAT Float_Control g-flocon ads,GNAT Expect g-expect ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id74}@anchor{308}@anchor{gnat_rm/the_gnat_library gnat-expect-tty-g-exptty-ads}@anchor{309}
@section @cite{GNAT.Expect.TTY} (@code{g-exptty.ads})


@geindex GNAT.Expect.TTY (g-exptty.ads)

As GNAT.Expect but using pseudo-terminal.
Currently @cite{GNAT.Expect.TTY} is implemented on all native GNAT
ports. It is not implemented for cross ports, and
in particular is not implemented for VxWorks or LynxOS.

@node GNAT Float_Control g-flocon ads,GNAT Formatted_String g-forstr ads,GNAT Expect TTY g-exptty ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id75}@anchor{30a}@anchor{gnat_rm/the_gnat_library gnat-float-control-g-flocon-ads}@anchor{30b}
@section @cite{GNAT.Float_Control} (@code{g-flocon.ads})


@geindex GNAT.Float_Control (g-flocon.ads)

@geindex Floating-Point Processor

Provides an interface for resetting the floating-point processor into the
mode required for correct semantic operation in Ada.  Some third party
library calls may cause this mode to be modified, and the Reset procedure
in this package can be used to reestablish the required mode.

@node GNAT Formatted_String g-forstr ads,GNAT Heap_Sort g-heasor ads,GNAT Float_Control g-flocon ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id76}@anchor{30c}@anchor{gnat_rm/the_gnat_library gnat-formatted-string-g-forstr-ads}@anchor{30d}
@section @cite{GNAT.Formatted_String} (@code{g-forstr.ads})


@geindex GNAT.Formatted_String (g-forstr.ads)

@geindex Formatted String

Provides support for C/C++ printf() formatted strings. The format is
copied from the printf() routine and should therefore gives identical
output. Some generic routines are provided to be able to use types
derived from Integer, Float or enumerations as values for the
formatted string.

@node GNAT Heap_Sort g-heasor ads,GNAT Heap_Sort_A g-hesora ads,GNAT Formatted_String g-forstr ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-heap-sort-g-heasor-ads}@anchor{30e}@anchor{gnat_rm/the_gnat_library id77}@anchor{30f}
@section @cite{GNAT.Heap_Sort} (@code{g-heasor.ads})


@geindex GNAT.Heap_Sort (g-heasor.ads)

@geindex Sorting

Provides a general implementation of heap sort usable for sorting arbitrary
data items. Exchange and comparison procedures are provided by passing
access-to-procedure values.  The algorithm used is a modified heap sort
that performs approximately N*log(N) comparisons in the worst case.

@node GNAT Heap_Sort_A g-hesora ads,GNAT Heap_Sort_G g-hesorg ads,GNAT Heap_Sort g-heasor ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id78}@anchor{310}@anchor{gnat_rm/the_gnat_library gnat-heap-sort-a-g-hesora-ads}@anchor{311}
@section @cite{GNAT.Heap_Sort_A} (@code{g-hesora.ads})


@geindex GNAT.Heap_Sort_A (g-hesora.ads)

@geindex Sorting

Provides a general implementation of heap sort usable for sorting arbitrary
data items. Move and comparison procedures are provided by passing
access-to-procedure values.  The algorithm used is a modified heap sort
that performs approximately N*log(N) comparisons in the worst case.
This differs from @cite{GNAT.Heap_Sort} in having a less convenient
interface, but may be slightly more efficient.

@node GNAT Heap_Sort_G g-hesorg ads,GNAT HTable g-htable ads,GNAT Heap_Sort_A g-hesora ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id79}@anchor{312}@anchor{gnat_rm/the_gnat_library gnat-heap-sort-g-g-hesorg-ads}@anchor{313}
@section @cite{GNAT.Heap_Sort_G} (@code{g-hesorg.ads})


@geindex GNAT.Heap_Sort_G (g-hesorg.ads)

@geindex Sorting

Similar to @cite{Heap_Sort_A} except that the move and sorting procedures
are provided as generic parameters, this improves efficiency, especially
if the procedures can be inlined, at the expense of duplicating code for
multiple instantiations.

@node GNAT HTable g-htable ads,GNAT IO g-io ads,GNAT Heap_Sort_G g-hesorg ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id80}@anchor{314}@anchor{gnat_rm/the_gnat_library gnat-htable-g-htable-ads}@anchor{315}
@section @cite{GNAT.HTable} (@code{g-htable.ads})


@geindex GNAT.HTable (g-htable.ads)

@geindex Hash tables

A generic implementation of hash tables that can be used to hash arbitrary
data.  Provides two approaches, one a simple static approach, and the other
allowing arbitrary dynamic hash tables.

@node GNAT IO g-io ads,GNAT IO_Aux g-io_aux ads,GNAT HTable g-htable ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id81}@anchor{316}@anchor{gnat_rm/the_gnat_library gnat-io-g-io-ads}@anchor{317}
@section @cite{GNAT.IO} (@code{g-io.ads})


@geindex GNAT.IO (g-io.ads)

@geindex Simple I/O

@geindex Input/Output facilities

A simple preelaborable input-output package that provides a subset of
simple Text_IO functions for reading characters and strings from
Standard_Input, and writing characters, strings and integers to either
Standard_Output or Standard_Error.

@node GNAT IO_Aux g-io_aux ads,GNAT Lock_Files g-locfil ads,GNAT IO g-io ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id82}@anchor{318}@anchor{gnat_rm/the_gnat_library gnat-io-aux-g-io-aux-ads}@anchor{319}
@section @cite{GNAT.IO_Aux} (@code{g-io_aux.ads})


@geindex GNAT.IO_Aux (g-io_aux.ads)

@geindex Text_IO

@geindex Input/Output facilities

Provides some auxiliary functions for use with Text_IO, including a test
for whether a file exists, and functions for reading a line of text.

@node GNAT Lock_Files g-locfil ads,GNAT MBBS_Discrete_Random g-mbdira ads,GNAT IO_Aux g-io_aux ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id83}@anchor{31a}@anchor{gnat_rm/the_gnat_library gnat-lock-files-g-locfil-ads}@anchor{31b}
@section @cite{GNAT.Lock_Files} (@code{g-locfil.ads})


@geindex GNAT.Lock_Files (g-locfil.ads)

@geindex File locking

@geindex Locking using files

Provides a general interface for using files as locks.  Can be used for
providing program level synchronization.

@node GNAT MBBS_Discrete_Random g-mbdira ads,GNAT MBBS_Float_Random g-mbflra ads,GNAT Lock_Files g-locfil ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-mbbs-discrete-random-g-mbdira-ads}@anchor{31c}@anchor{gnat_rm/the_gnat_library id84}@anchor{31d}
@section @cite{GNAT.MBBS_Discrete_Random} (@code{g-mbdira.ads})


@geindex GNAT.MBBS_Discrete_Random (g-mbdira.ads)

@geindex Random number generation

The original implementation of @cite{Ada.Numerics.Discrete_Random}.  Uses
a modified version of the Blum-Blum-Shub generator.

@node GNAT MBBS_Float_Random g-mbflra ads,GNAT MD5 g-md5 ads,GNAT MBBS_Discrete_Random g-mbdira ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id85}@anchor{31e}@anchor{gnat_rm/the_gnat_library gnat-mbbs-float-random-g-mbflra-ads}@anchor{31f}
@section @cite{GNAT.MBBS_Float_Random} (@code{g-mbflra.ads})


@geindex GNAT.MBBS_Float_Random (g-mbflra.ads)

@geindex Random number generation

The original implementation of @cite{Ada.Numerics.Float_Random}.  Uses
a modified version of the Blum-Blum-Shub generator.

@node GNAT MD5 g-md5 ads,GNAT Memory_Dump g-memdum ads,GNAT MBBS_Float_Random g-mbflra ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id86}@anchor{320}@anchor{gnat_rm/the_gnat_library gnat-md5-g-md5-ads}@anchor{321}
@section @cite{GNAT.MD5} (@code{g-md5.ads})


@geindex GNAT.MD5 (g-md5.ads)

@geindex Message Digest MD5

Implements the MD5 Message-Digest Algorithm as described in RFC 1321, and
the HMAC-MD5 message authentication function as described in RFC 2104 and
FIPS PUB 198.

@node GNAT Memory_Dump g-memdum ads,GNAT Most_Recent_Exception g-moreex ads,GNAT MD5 g-md5 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id87}@anchor{322}@anchor{gnat_rm/the_gnat_library gnat-memory-dump-g-memdum-ads}@anchor{323}
@section @cite{GNAT.Memory_Dump} (@code{g-memdum.ads})


@geindex GNAT.Memory_Dump (g-memdum.ads)

@geindex Dump Memory

Provides a convenient routine for dumping raw memory to either the
standard output or standard error files. Uses GNAT.IO for actual
output.

@node GNAT Most_Recent_Exception g-moreex ads,GNAT OS_Lib g-os_lib ads,GNAT Memory_Dump g-memdum ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id88}@anchor{324}@anchor{gnat_rm/the_gnat_library gnat-most-recent-exception-g-moreex-ads}@anchor{325}
@section @cite{GNAT.Most_Recent_Exception} (@code{g-moreex.ads})


@geindex GNAT.Most_Recent_Exception (g-moreex.ads)

@geindex Exception
@geindex obtaining most recent

Provides access to the most recently raised exception.  Can be used for
various logging purposes, including duplicating functionality of some
Ada 83 implementation dependent extensions.

@node GNAT OS_Lib g-os_lib ads,GNAT Perfect_Hash_Generators g-pehage ads,GNAT Most_Recent_Exception g-moreex ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id89}@anchor{326}@anchor{gnat_rm/the_gnat_library gnat-os-lib-g-os-lib-ads}@anchor{327}
@section @cite{GNAT.OS_Lib} (@code{g-os_lib.ads})


@geindex GNAT.OS_Lib (g-os_lib.ads)

@geindex Operating System interface

@geindex Spawn capability

Provides a range of target independent operating system interface functions,
including time/date management, file operations, subprocess management,
including a portable spawn procedure, and access to environment variables
and error return codes.

@node GNAT Perfect_Hash_Generators g-pehage ads,GNAT Random_Numbers g-rannum ads,GNAT OS_Lib g-os_lib ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id90}@anchor{328}@anchor{gnat_rm/the_gnat_library gnat-perfect-hash-generators-g-pehage-ads}@anchor{329}
@section @cite{GNAT.Perfect_Hash_Generators} (@code{g-pehage.ads})


@geindex GNAT.Perfect_Hash_Generators (g-pehage.ads)

@geindex Hash functions

Provides a generator of static minimal perfect hash functions. No
collisions occur and each item can be retrieved from the table in one
probe (perfect property). The hash table size corresponds to the exact
size of the key set and no larger (minimal property). The key set has to
be know in advance (static property). The hash functions are also order
preserving. If w2 is inserted after w1 in the generator, their
hashcode are in the same order. These hashing functions are very
convenient for use with realtime applications.

@node GNAT Random_Numbers g-rannum ads,GNAT Regexp g-regexp ads,GNAT Perfect_Hash_Generators g-pehage ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id91}@anchor{32a}@anchor{gnat_rm/the_gnat_library gnat-random-numbers-g-rannum-ads}@anchor{32b}
@section @cite{GNAT.Random_Numbers} (@code{g-rannum.ads})


@geindex GNAT.Random_Numbers (g-rannum.ads)

@geindex Random number generation

Provides random number capabilities which extend those available in the
standard Ada library and are more convenient to use.

@node GNAT Regexp g-regexp ads,GNAT Registry g-regist ads,GNAT Random_Numbers g-rannum ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-regexp-g-regexp-ads}@anchor{1ff}@anchor{gnat_rm/the_gnat_library id92}@anchor{32c}
@section @cite{GNAT.Regexp} (@code{g-regexp.ads})


@geindex GNAT.Regexp (g-regexp.ads)

@geindex Regular expressions

@geindex Pattern matching

A simple implementation of regular expressions, using a subset of regular
expression syntax copied from familiar Unix style utilities.  This is the
simplest of the three pattern matching packages provided, and is particularly
suitable for 'file globbing' applications.

@node GNAT Registry g-regist ads,GNAT Regpat g-regpat ads,GNAT Regexp g-regexp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id93}@anchor{32d}@anchor{gnat_rm/the_gnat_library gnat-registry-g-regist-ads}@anchor{32e}
@section @cite{GNAT.Registry} (@code{g-regist.ads})


@geindex GNAT.Registry (g-regist.ads)

@geindex Windows Registry

This is a high level binding to the Windows registry.  It is possible to
do simple things like reading a key value, creating a new key.  For full
registry API, but at a lower level of abstraction, refer to the Win32.Winreg
package provided with the Win32Ada binding

@node GNAT Regpat g-regpat ads,GNAT Rewrite_Data g-rewdat ads,GNAT Registry g-regist ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id94}@anchor{32f}@anchor{gnat_rm/the_gnat_library gnat-regpat-g-regpat-ads}@anchor{330}
@section @cite{GNAT.Regpat} (@code{g-regpat.ads})


@geindex GNAT.Regpat (g-regpat.ads)

@geindex Regular expressions

@geindex Pattern matching

A complete implementation of Unix-style regular expression matching, copied
from the original V7 style regular expression library written in C by
Henry Spencer (and binary compatible with this C library).

@node GNAT Rewrite_Data g-rewdat ads,GNAT Secondary_Stack_Info g-sestin ads,GNAT Regpat g-regpat ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id95}@anchor{331}@anchor{gnat_rm/the_gnat_library gnat-rewrite-data-g-rewdat-ads}@anchor{332}
@section @cite{GNAT.Rewrite_Data} (@code{g-rewdat.ads})


@geindex GNAT.Rewrite_Data (g-rewdat.ads)

@geindex Rewrite data

A unit to rewrite on-the-fly string occurrences in a stream of
data. The implementation has a very minimal memory footprint as the
full content to be processed is not loaded into memory all at once. This makes
this interface usable for large files or socket streams.

@node GNAT Secondary_Stack_Info g-sestin ads,GNAT Semaphores g-semaph ads,GNAT Rewrite_Data g-rewdat ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-secondary-stack-info-g-sestin-ads}@anchor{333}@anchor{gnat_rm/the_gnat_library id96}@anchor{334}
@section @cite{GNAT.Secondary_Stack_Info} (@code{g-sestin.ads})


@geindex GNAT.Secondary_Stack_Info (g-sestin.ads)

@geindex Secondary Stack Info

Provide the capability to query the high water mark of the current task's
secondary stack.

@node GNAT Semaphores g-semaph ads,GNAT Serial_Communications g-sercom ads,GNAT Secondary_Stack_Info g-sestin ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-semaphores-g-semaph-ads}@anchor{335}@anchor{gnat_rm/the_gnat_library id97}@anchor{336}
@section @cite{GNAT.Semaphores} (@code{g-semaph.ads})


@geindex GNAT.Semaphores (g-semaph.ads)

@geindex Semaphores

Provides classic counting and binary semaphores using protected types.

@node GNAT Serial_Communications g-sercom ads,GNAT SHA1 g-sha1 ads,GNAT Semaphores g-semaph ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id98}@anchor{337}@anchor{gnat_rm/the_gnat_library gnat-serial-communications-g-sercom-ads}@anchor{338}
@section @cite{GNAT.Serial_Communications} (@code{g-sercom.ads})


@geindex GNAT.Serial_Communications (g-sercom.ads)

@geindex Serial_Communications

Provides a simple interface to send and receive data over a serial
port. This is only supported on GNU/Linux and Windows.

@node GNAT SHA1 g-sha1 ads,GNAT SHA224 g-sha224 ads,GNAT Serial_Communications g-sercom ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sha1-g-sha1-ads}@anchor{339}@anchor{gnat_rm/the_gnat_library id99}@anchor{33a}
@section @cite{GNAT.SHA1} (@code{g-sha1.ads})


@geindex GNAT.SHA1 (g-sha1.ads)

@geindex Secure Hash Algorithm SHA-1

Implements the SHA-1 Secure Hash Algorithm as described in FIPS PUB 180-3
and RFC 3174, and the HMAC-SHA1 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA224 g-sha224 ads,GNAT SHA256 g-sha256 ads,GNAT SHA1 g-sha1 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id100}@anchor{33b}@anchor{gnat_rm/the_gnat_library gnat-sha224-g-sha224-ads}@anchor{33c}
@section @cite{GNAT.SHA224} (@code{g-sha224.ads})


@geindex GNAT.SHA224 (g-sha224.ads)

@geindex Secure Hash Algorithm SHA-224

Implements the SHA-224 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA224 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA256 g-sha256 ads,GNAT SHA384 g-sha384 ads,GNAT SHA224 g-sha224 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id101}@anchor{33d}@anchor{gnat_rm/the_gnat_library gnat-sha256-g-sha256-ads}@anchor{33e}
@section @cite{GNAT.SHA256} (@code{g-sha256.ads})


@geindex GNAT.SHA256 (g-sha256.ads)

@geindex Secure Hash Algorithm SHA-256

Implements the SHA-256 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA256 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA384 g-sha384 ads,GNAT SHA512 g-sha512 ads,GNAT SHA256 g-sha256 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id102}@anchor{33f}@anchor{gnat_rm/the_gnat_library gnat-sha384-g-sha384-ads}@anchor{340}
@section @cite{GNAT.SHA384} (@code{g-sha384.ads})


@geindex GNAT.SHA384 (g-sha384.ads)

@geindex Secure Hash Algorithm SHA-384

Implements the SHA-384 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA384 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT SHA512 g-sha512 ads,GNAT Signals g-signal ads,GNAT SHA384 g-sha384 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id103}@anchor{341}@anchor{gnat_rm/the_gnat_library gnat-sha512-g-sha512-ads}@anchor{342}
@section @cite{GNAT.SHA512} (@code{g-sha512.ads})


@geindex GNAT.SHA512 (g-sha512.ads)

@geindex Secure Hash Algorithm SHA-512

Implements the SHA-512 Secure Hash Algorithm as described in FIPS PUB 180-3,
and the HMAC-SHA512 message authentication function as described
in RFC 2104 and FIPS PUB 198.

@node GNAT Signals g-signal ads,GNAT Sockets g-socket ads,GNAT SHA512 g-sha512 ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-signals-g-signal-ads}@anchor{343}@anchor{gnat_rm/the_gnat_library id104}@anchor{344}
@section @cite{GNAT.Signals} (@code{g-signal.ads})


@geindex GNAT.Signals (g-signal.ads)

@geindex Signals

Provides the ability to manipulate the blocked status of signals on supported
targets.

@node GNAT Sockets g-socket ads,GNAT Source_Info g-souinf ads,GNAT Signals g-signal ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sockets-g-socket-ads}@anchor{345}@anchor{gnat_rm/the_gnat_library id105}@anchor{346}
@section @cite{GNAT.Sockets} (@code{g-socket.ads})


@geindex GNAT.Sockets (g-socket.ads)

@geindex Sockets

A high level and portable interface to develop sockets based applications.
This package is based on the sockets thin binding found in
@cite{GNAT.Sockets.Thin}. Currently @cite{GNAT.Sockets} is implemented
on all native GNAT ports and on VxWorks cross prots.  It is not implemented for
the LynxOS cross port.

@node GNAT Source_Info g-souinf ads,GNAT Spelling_Checker g-speche ads,GNAT Sockets g-socket ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-source-info-g-souinf-ads}@anchor{347}@anchor{gnat_rm/the_gnat_library id106}@anchor{348}
@section @cite{GNAT.Source_Info} (@code{g-souinf.ads})


@geindex GNAT.Source_Info (g-souinf.ads)

@geindex Source Information

Provides subprograms that give access to source code information known at
compile time, such as the current file name and line number. Also provides
subprograms yielding the date and time of the current compilation (like the
C macros @cite{__DATE__} and @cite{__TIME__})

@node GNAT Spelling_Checker g-speche ads,GNAT Spelling_Checker_Generic g-spchge ads,GNAT Source_Info g-souinf ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spelling-checker-g-speche-ads}@anchor{349}@anchor{gnat_rm/the_gnat_library id107}@anchor{34a}
@section @cite{GNAT.Spelling_Checker} (@code{g-speche.ads})


@geindex GNAT.Spelling_Checker (g-speche.ads)

@geindex Spell checking

Provides a function for determining whether one string is a plausible
near misspelling of another string.

@node GNAT Spelling_Checker_Generic g-spchge ads,GNAT Spitbol Patterns g-spipat ads,GNAT Spelling_Checker g-speche ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-spelling-checker-generic-g-spchge-ads}@anchor{34b}@anchor{gnat_rm/the_gnat_library id108}@anchor{34c}
@section @cite{GNAT.Spelling_Checker_Generic} (@code{g-spchge.ads})


@geindex GNAT.Spelling_Checker_Generic (g-spchge.ads)

@geindex Spell checking

Provides a generic function that can be instantiated with a string type for
determining whether one string is a plausible near misspelling of another
string.

@node GNAT Spitbol Patterns g-spipat ads,GNAT Spitbol g-spitbo ads,GNAT Spelling_Checker_Generic g-spchge ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id109}@anchor{34d}@anchor{gnat_rm/the_gnat_library gnat-spitbol-patterns-g-spipat-ads}@anchor{34e}
@section @cite{GNAT.Spitbol.Patterns} (@code{g-spipat.ads})


@geindex GNAT.Spitbol.Patterns (g-spipat.ads)

@geindex SPITBOL pattern matching

@geindex Pattern matching

A complete implementation of SNOBOL4 style pattern matching.  This is the
most elaborate of the pattern matching packages provided.  It fully duplicates
the SNOBOL4 dynamic pattern construction and matching capabilities, using the
efficient algorithm developed by Robert Dewar for the SPITBOL system.

@node GNAT Spitbol g-spitbo ads,GNAT Spitbol Table_Boolean g-sptabo ads,GNAT Spitbol Patterns g-spipat ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id110}@anchor{34f}@anchor{gnat_rm/the_gnat_library gnat-spitbol-g-spitbo-ads}@anchor{350}
@section @cite{GNAT.Spitbol} (@code{g-spitbo.ads})


@geindex GNAT.Spitbol (g-spitbo.ads)

@geindex SPITBOL interface

The top level package of the collection of SPITBOL-style functionality, this
package provides basic SNOBOL4 string manipulation functions, such as
Pad, Reverse, Trim, Substr capability, as well as a generic table function
useful for constructing arbitrary mappings from strings in the style of
the SNOBOL4 TABLE function.

@node GNAT Spitbol Table_Boolean g-sptabo ads,GNAT Spitbol Table_Integer g-sptain ads,GNAT Spitbol g-spitbo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id111}@anchor{351}@anchor{gnat_rm/the_gnat_library gnat-spitbol-table-boolean-g-sptabo-ads}@anchor{352}
@section @cite{GNAT.Spitbol.Table_Boolean} (@code{g-sptabo.ads})


@geindex GNAT.Spitbol.Table_Boolean (g-sptabo.ads)

@geindex Sets of strings

@geindex SPITBOL Tables

A library level of instantiation of @cite{GNAT.Spitbol.Patterns.Table}
for type @cite{Standard.Boolean}, giving an implementation of sets of
string values.

@node GNAT Spitbol Table_Integer g-sptain ads,GNAT Spitbol Table_VString g-sptavs ads,GNAT Spitbol Table_Boolean g-sptabo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id112}@anchor{353}@anchor{gnat_rm/the_gnat_library gnat-spitbol-table-integer-g-sptain-ads}@anchor{354}
@section @cite{GNAT.Spitbol.Table_Integer} (@code{g-sptain.ads})


@geindex GNAT.Spitbol.Table_Integer (g-sptain.ads)

@geindex Integer maps

@geindex Maps

@geindex SPITBOL Tables

A library level of instantiation of @cite{GNAT.Spitbol.Patterns.Table}
for type @cite{Standard.Integer}, giving an implementation of maps
from string to integer values.

@node GNAT Spitbol Table_VString g-sptavs ads,GNAT SSE g-sse ads,GNAT Spitbol Table_Integer g-sptain ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id113}@anchor{355}@anchor{gnat_rm/the_gnat_library gnat-spitbol-table-vstring-g-sptavs-ads}@anchor{356}
@section @cite{GNAT.Spitbol.Table_VString} (@code{g-sptavs.ads})


@geindex GNAT.Spitbol.Table_VString (g-sptavs.ads)

@geindex String maps

@geindex Maps

@geindex SPITBOL Tables

A library level of instantiation of @cite{GNAT.Spitbol.Patterns.Table} for
a variable length string type, giving an implementation of general
maps from strings to strings.

@node GNAT SSE g-sse ads,GNAT SSE Vector_Types g-ssvety ads,GNAT Spitbol Table_VString g-sptavs ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id114}@anchor{357}@anchor{gnat_rm/the_gnat_library gnat-sse-g-sse-ads}@anchor{358}
@section @cite{GNAT.SSE} (@code{g-sse.ads})


@geindex GNAT.SSE (g-sse.ads)

Root of a set of units aimed at offering Ada bindings to a subset of
the Intel(r) Streaming SIMD Extensions with GNAT on the x86 family of
targets.  It exposes vector component types together with a general
introduction to the binding contents and use.

@node GNAT SSE Vector_Types g-ssvety ads,GNAT Strings g-string ads,GNAT SSE g-sse ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-sse-vector-types-g-ssvety-ads}@anchor{359}@anchor{gnat_rm/the_gnat_library id115}@anchor{35a}
@section @cite{GNAT.SSE.Vector_Types} (@code{g-ssvety.ads})


@geindex GNAT.SSE.Vector_Types (g-ssvety.ads)

SSE vector types for use with SSE related intrinsics.

@node GNAT Strings g-string ads,GNAT String_Split g-strspl ads,GNAT SSE Vector_Types g-ssvety ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id116}@anchor{35b}@anchor{gnat_rm/the_gnat_library gnat-strings-g-string-ads}@anchor{35c}
@section @cite{GNAT.Strings} (@code{g-string.ads})


@geindex GNAT.Strings (g-string.ads)

Common String access types and related subprograms. Basically it
defines a string access and an array of string access types.

@node GNAT String_Split g-strspl ads,GNAT Table g-table ads,GNAT Strings g-string ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-string-split-g-strspl-ads}@anchor{35d}@anchor{gnat_rm/the_gnat_library id117}@anchor{35e}
@section @cite{GNAT.String_Split} (@code{g-strspl.ads})


@geindex GNAT.String_Split (g-strspl.ads)

@geindex String splitter

Useful string manipulation routines: given a set of separators, split
a string wherever the separators appear, and provide direct access
to the resulting slices. This package is instantiated from
@cite{GNAT.Array_Split}.

@node GNAT Table g-table ads,GNAT Task_Lock g-tasloc ads,GNAT String_Split g-strspl ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id118}@anchor{35f}@anchor{gnat_rm/the_gnat_library gnat-table-g-table-ads}@anchor{360}
@section @cite{GNAT.Table} (@code{g-table.ads})


@geindex GNAT.Table (g-table.ads)

@geindex Table implementation

@geindex Arrays
@geindex extendable

A generic package providing a single dimension array abstraction where the
length of the array can be dynamically modified.

This package provides a facility similar to that of @cite{GNAT.Dynamic_Tables},
except that this package declares a single instance of the table type,
while an instantiation of @cite{GNAT.Dynamic_Tables} creates a type that can be
used to define dynamic instances of the table.

@node GNAT Task_Lock g-tasloc ads,GNAT Time_Stamp g-timsta ads,GNAT Table g-table ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-task-lock-g-tasloc-ads}@anchor{361}@anchor{gnat_rm/the_gnat_library id119}@anchor{362}
@section @cite{GNAT.Task_Lock} (@code{g-tasloc.ads})


@geindex GNAT.Task_Lock (g-tasloc.ads)

@geindex Task synchronization

@geindex Task locking

@geindex Locking

A very simple facility for locking and unlocking sections of code using a
single global task lock.  Appropriate for use in situations where contention
between tasks is very rarely expected.

@node GNAT Time_Stamp g-timsta ads,GNAT Threads g-thread ads,GNAT Task_Lock g-tasloc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-time-stamp-g-timsta-ads}@anchor{363}@anchor{gnat_rm/the_gnat_library id120}@anchor{364}
@section @cite{GNAT.Time_Stamp} (@code{g-timsta.ads})


@geindex GNAT.Time_Stamp (g-timsta.ads)

@geindex Time stamp

@geindex Current time

Provides a simple function that returns a string YYYY-MM-DD HH:MM:SS.SS that
represents the current date and time in ISO 8601 format. This is a very simple
routine with minimal code and there are no dependencies on any other unit.

@node GNAT Threads g-thread ads,GNAT Traceback g-traceb ads,GNAT Time_Stamp g-timsta ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-threads-g-thread-ads}@anchor{365}@anchor{gnat_rm/the_gnat_library id121}@anchor{366}
@section @cite{GNAT.Threads} (@code{g-thread.ads})


@geindex GNAT.Threads (g-thread.ads)

@geindex Foreign threads

@geindex Threads
@geindex foreign

Provides facilities for dealing with foreign threads which need to be known
by the GNAT run-time system. Consult the documentation of this package for
further details if your program has threads that are created by a non-Ada
environment which then accesses Ada code.

@node GNAT Traceback g-traceb ads,GNAT Traceback Symbolic g-trasym ads,GNAT Threads g-thread ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id122}@anchor{367}@anchor{gnat_rm/the_gnat_library gnat-traceback-g-traceb-ads}@anchor{368}
@section @cite{GNAT.Traceback} (@code{g-traceb.ads})


@geindex GNAT.Traceback (g-traceb.ads)

@geindex Trace back facilities

Provides a facility for obtaining non-symbolic traceback information, useful
in various debugging situations.

@node GNAT Traceback Symbolic g-trasym ads,GNAT UTF_32 g-table ads,GNAT Traceback g-traceb ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-traceback-symbolic-g-trasym-ads}@anchor{369}@anchor{gnat_rm/the_gnat_library id123}@anchor{36a}
@section @cite{GNAT.Traceback.Symbolic} (@code{g-trasym.ads})


@geindex GNAT.Traceback.Symbolic (g-trasym.ads)

@geindex Trace back facilities

@node GNAT UTF_32 g-table ads,GNAT Wide_Spelling_Checker g-u3spch ads,GNAT Traceback Symbolic g-trasym ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id124}@anchor{36b}@anchor{gnat_rm/the_gnat_library gnat-utf-32-g-table-ads}@anchor{36c}
@section @cite{GNAT.UTF_32} (@code{g-table.ads})


@geindex GNAT.UTF_32 (g-table.ads)

@geindex Wide character codes

This is a package intended to be used in conjunction with the
@cite{Wide_Character} type in Ada 95 and the
@cite{Wide_Wide_Character} type in Ada 2005 (available
in @cite{GNAT} in Ada 2005 mode). This package contains
Unicode categorization routines, as well as lexical
categorization routines corresponding to the Ada 2005
lexical rules for identifiers and strings, and also a
lower case to upper case fold routine corresponding to
the Ada 2005 rules for identifier equivalence.

@node GNAT Wide_Spelling_Checker g-u3spch ads,GNAT Wide_Spelling_Checker g-wispch ads,GNAT UTF_32 g-table ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-spelling-checker-g-u3spch-ads}@anchor{36d}@anchor{gnat_rm/the_gnat_library id125}@anchor{36e}
@section @cite{GNAT.Wide_Spelling_Checker} (@code{g-u3spch.ads})


@geindex GNAT.Wide_Spelling_Checker (g-u3spch.ads)

@geindex Spell checking

Provides a function for determining whether one wide wide string is a plausible
near misspelling of another wide wide string, where the strings are represented
using the UTF_32_String type defined in System.Wch_Cnv.

@node GNAT Wide_Spelling_Checker g-wispch ads,GNAT Wide_String_Split g-wistsp ads,GNAT Wide_Spelling_Checker g-u3spch ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-spelling-checker-g-wispch-ads}@anchor{36f}@anchor{gnat_rm/the_gnat_library id126}@anchor{370}
@section @cite{GNAT.Wide_Spelling_Checker} (@code{g-wispch.ads})


@geindex GNAT.Wide_Spelling_Checker (g-wispch.ads)

@geindex Spell checking

Provides a function for determining whether one wide string is a plausible
near misspelling of another wide string.

@node GNAT Wide_String_Split g-wistsp ads,GNAT Wide_Wide_Spelling_Checker g-zspche ads,GNAT Wide_Spelling_Checker g-wispch ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id127}@anchor{371}@anchor{gnat_rm/the_gnat_library gnat-wide-string-split-g-wistsp-ads}@anchor{372}
@section @cite{GNAT.Wide_String_Split} (@code{g-wistsp.ads})


@geindex GNAT.Wide_String_Split (g-wistsp.ads)

@geindex Wide_String splitter

Useful wide string manipulation routines: given a set of separators, split
a wide string wherever the separators appear, and provide direct access
to the resulting slices. This package is instantiated from
@cite{GNAT.Array_Split}.

@node GNAT Wide_Wide_Spelling_Checker g-zspche ads,GNAT Wide_Wide_String_Split g-zistsp ads,GNAT Wide_String_Split g-wistsp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-wide-spelling-checker-g-zspche-ads}@anchor{373}@anchor{gnat_rm/the_gnat_library id128}@anchor{374}
@section @cite{GNAT.Wide_Wide_Spelling_Checker} (@code{g-zspche.ads})


@geindex GNAT.Wide_Wide_Spelling_Checker (g-zspche.ads)

@geindex Spell checking

Provides a function for determining whether one wide wide string is a plausible
near misspelling of another wide wide string.

@node GNAT Wide_Wide_String_Split g-zistsp ads,Interfaces C Extensions i-cexten ads,GNAT Wide_Wide_Spelling_Checker g-zspche ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library gnat-wide-wide-string-split-g-zistsp-ads}@anchor{375}@anchor{gnat_rm/the_gnat_library id129}@anchor{376}
@section @cite{GNAT.Wide_Wide_String_Split} (@code{g-zistsp.ads})


@geindex GNAT.Wide_Wide_String_Split (g-zistsp.ads)

@geindex Wide_Wide_String splitter

Useful wide wide string manipulation routines: given a set of separators, split
a wide wide string wherever the separators appear, and provide direct access
to the resulting slices. This package is instantiated from
@cite{GNAT.Array_Split}.

@node Interfaces C Extensions i-cexten ads,Interfaces C Streams i-cstrea ads,GNAT Wide_Wide_String_Split g-zistsp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id130}@anchor{377}@anchor{gnat_rm/the_gnat_library interfaces-c-extensions-i-cexten-ads}@anchor{378}
@section @cite{Interfaces.C.Extensions} (@code{i-cexten.ads})


@geindex Interfaces.C.Extensions (i-cexten.ads)

This package contains additional C-related definitions, intended
for use with either manually or automatically generated bindings
to C libraries.

@node Interfaces C Streams i-cstrea ads,Interfaces Packed_Decimal i-pacdec ads,Interfaces C Extensions i-cexten ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id131}@anchor{379}@anchor{gnat_rm/the_gnat_library interfaces-c-streams-i-cstrea-ads}@anchor{37a}
@section @cite{Interfaces.C.Streams} (@code{i-cstrea.ads})


@geindex Interfaces.C.Streams (i-cstrea.ads)

@geindex C streams
@geindex interfacing

This package is a binding for the most commonly used operations
on C streams.

@node Interfaces Packed_Decimal i-pacdec ads,Interfaces VxWorks i-vxwork ads,Interfaces C Streams i-cstrea ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library interfaces-packed-decimal-i-pacdec-ads}@anchor{37b}@anchor{gnat_rm/the_gnat_library id132}@anchor{37c}
@section @cite{Interfaces.Packed_Decimal} (@code{i-pacdec.ads})


@geindex Interfaces.Packed_Decimal (i-pacdec.ads)

@geindex IBM Packed Format

@geindex Packed Decimal

This package provides a set of routines for conversions to and
from a packed decimal format compatible with that used on IBM
mainframes.

@node Interfaces VxWorks i-vxwork ads,Interfaces VxWorks IO i-vxwoio ads,Interfaces Packed_Decimal i-pacdec ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id133}@anchor{37d}@anchor{gnat_rm/the_gnat_library interfaces-vxworks-i-vxwork-ads}@anchor{37e}
@section @cite{Interfaces.VxWorks} (@code{i-vxwork.ads})


@geindex Interfaces.VxWorks (i-vxwork.ads)

@geindex Interfacing to VxWorks

@geindex VxWorks
@geindex interfacing

This package provides a limited binding to the VxWorks API.
In particular, it interfaces with the
VxWorks hardware interrupt facilities.

@node Interfaces VxWorks IO i-vxwoio ads,System Address_Image s-addima ads,Interfaces VxWorks i-vxwork ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library interfaces-vxworks-io-i-vxwoio-ads}@anchor{37f}@anchor{gnat_rm/the_gnat_library id134}@anchor{380}
@section @cite{Interfaces.VxWorks.IO} (@code{i-vxwoio.ads})


@geindex Interfaces.VxWorks.IO (i-vxwoio.ads)

@geindex Interfacing to VxWorks' I/O

@geindex VxWorks
@geindex I/O interfacing

@geindex VxWorks
@geindex Get_Immediate

@geindex Get_Immediate
@geindex VxWorks

This package provides a binding to the ioctl (IO/Control)
function of VxWorks, defining a set of option values and
function codes. A particular use of this package is
to enable the use of Get_Immediate under VxWorks.

@node System Address_Image s-addima ads,System Assertions s-assert ads,Interfaces VxWorks IO i-vxwoio ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-address-image-s-addima-ads}@anchor{381}@anchor{gnat_rm/the_gnat_library id135}@anchor{382}
@section @cite{System.Address_Image} (@code{s-addima.ads})


@geindex System.Address_Image (s-addima.ads)

@geindex Address image

@geindex Image
@geindex of an address

This function provides a useful debugging
function that gives an (implementation dependent)
string which identifies an address.

@node System Assertions s-assert ads,System Atomic_Counters s-atocou ads,System Address_Image s-addima ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-assertions-s-assert-ads}@anchor{383}@anchor{gnat_rm/the_gnat_library id136}@anchor{384}
@section @cite{System.Assertions} (@code{s-assert.ads})


@geindex System.Assertions (s-assert.ads)

@geindex Assertions

@geindex Assert_Failure
@geindex exception

This package provides the declaration of the exception raised
by an run-time assertion failure, as well as the routine that
is used internally to raise this assertion.

@node System Atomic_Counters s-atocou ads,System Memory s-memory ads,System Assertions s-assert ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id137}@anchor{385}@anchor{gnat_rm/the_gnat_library system-atomic-counters-s-atocou-ads}@anchor{386}
@section @cite{System.Atomic_Counters} (@code{s-atocou.ads})


@geindex System.Atomic_Counters (s-atocou.ads)

This package provides the declaration of an atomic counter type,
together with efficient routines (using hardware
synchronization primitives) for incrementing, decrementing,
and testing of these counters. This package is implemented
on most targets, including all Alpha, ia64, PowerPC, SPARC V9,
x86, and x86_64 platforms.

@node System Memory s-memory ads,System Multiprocessors s-multip ads,System Atomic_Counters s-atocou ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-memory-s-memory-ads}@anchor{387}@anchor{gnat_rm/the_gnat_library id138}@anchor{388}
@section @cite{System.Memory} (@code{s-memory.ads})


@geindex System.Memory (s-memory.ads)

@geindex Memory allocation

This package provides the interface to the low level routines used
by the generated code for allocation and freeing storage for the
default storage pool (analogous to the C routines malloc and free.
It also provides a reallocation interface analogous to the C routine
realloc. The body of this unit may be modified to provide alternative
allocation mechanisms for the default pool, and in addition, direct
calls to this unit may be made for low level allocation uses (for
example see the body of @cite{GNAT.Tables}).

@node System Multiprocessors s-multip ads,System Multiprocessors Dispatching_Domains s-mudido ads,System Memory s-memory ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id139}@anchor{389}@anchor{gnat_rm/the_gnat_library system-multiprocessors-s-multip-ads}@anchor{38a}
@section @cite{System.Multiprocessors} (@code{s-multip.ads})


@geindex System.Multiprocessors (s-multip.ads)

@geindex Multiprocessor interface

This is an Ada 2012 unit defined in the Ada 2012 Reference Manual, but
in GNAT we also make it available in Ada 95 and Ada 2005 (where it is
technically an implementation-defined addition).

@node System Multiprocessors Dispatching_Domains s-mudido ads,System Partition_Interface s-parint ads,System Multiprocessors s-multip ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-multiprocessors-dispatching-domains-s-mudido-ads}@anchor{38b}@anchor{gnat_rm/the_gnat_library id140}@anchor{38c}
@section @cite{System.Multiprocessors.Dispatching_Domains} (@code{s-mudido.ads})


@geindex System.Multiprocessors.Dispatching_Domains (s-mudido.ads)

@geindex Multiprocessor interface

This is an Ada 2012 unit defined in the Ada 2012 Reference Manual, but
in GNAT we also make it available in Ada 95 and Ada 2005 (where it is
technically an implementation-defined addition).

@node System Partition_Interface s-parint ads,System Pool_Global s-pooglo ads,System Multiprocessors Dispatching_Domains s-mudido ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id141}@anchor{38d}@anchor{gnat_rm/the_gnat_library system-partition-interface-s-parint-ads}@anchor{38e}
@section @cite{System.Partition_Interface} (@code{s-parint.ads})


@geindex System.Partition_Interface (s-parint.ads)

@geindex Partition interfacing functions

This package provides facilities for partition interfacing.  It
is used primarily in a distribution context when using Annex E
with @cite{GLADE}.

@node System Pool_Global s-pooglo ads,System Pool_Local s-pooloc ads,System Partition_Interface s-parint ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id142}@anchor{38f}@anchor{gnat_rm/the_gnat_library system-pool-global-s-pooglo-ads}@anchor{390}
@section @cite{System.Pool_Global} (@code{s-pooglo.ads})


@geindex System.Pool_Global (s-pooglo.ads)

@geindex Storage pool
@geindex global

@geindex Global storage pool

This package provides a storage pool that is equivalent to the default
storage pool used for access types for which no pool is specifically
declared. It uses malloc/free to allocate/free and does not attempt to
do any automatic reclamation.

@node System Pool_Local s-pooloc ads,System Restrictions s-restri ads,System Pool_Global s-pooglo ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-pool-local-s-pooloc-ads}@anchor{391}@anchor{gnat_rm/the_gnat_library id143}@anchor{392}
@section @cite{System.Pool_Local} (@code{s-pooloc.ads})


@geindex System.Pool_Local (s-pooloc.ads)

@geindex Storage pool
@geindex local

@geindex Local storage pool

This package provides a storage pool that is intended for use with locally
defined access types. It uses malloc/free for allocate/free, and maintains
a list of allocated blocks, so that all storage allocated for the pool can
be freed automatically when the pool is finalized.

@node System Restrictions s-restri ads,System Rident s-rident ads,System Pool_Local s-pooloc ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id144}@anchor{393}@anchor{gnat_rm/the_gnat_library system-restrictions-s-restri-ads}@anchor{394}
@section @cite{System.Restrictions} (@code{s-restri.ads})


@geindex System.Restrictions (s-restri.ads)

@geindex Run-time restrictions access

This package provides facilities for accessing at run time
the status of restrictions specified at compile time for
the partition. Information is available both with regard
to actual restrictions specified, and with regard to
compiler determined information on which restrictions
are violated by one or more packages in the partition.

@node System Rident s-rident ads,System Strings Stream_Ops s-ststop ads,System Restrictions s-restri ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id145}@anchor{395}@anchor{gnat_rm/the_gnat_library system-rident-s-rident-ads}@anchor{396}
@section @cite{System.Rident} (@code{s-rident.ads})


@geindex System.Rident (s-rident.ads)

@geindex Restrictions definitions

This package provides definitions of the restrictions
identifiers supported by GNAT, and also the format of
the restrictions provided in package System.Restrictions.
It is not normally necessary to @cite{with} this generic package
since the necessary instantiation is included in
package System.Restrictions.

@node System Strings Stream_Ops s-ststop ads,System Unsigned_Types s-unstyp ads,System Rident s-rident ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library id146}@anchor{397}@anchor{gnat_rm/the_gnat_library system-strings-stream-ops-s-ststop-ads}@anchor{398}
@section @cite{System.Strings.Stream_Ops} (@code{s-ststop.ads})


@geindex System.Strings.Stream_Ops (s-ststop.ads)

@geindex Stream operations

@geindex String stream operations

This package provides a set of stream subprograms for standard string types.
It is intended primarily to support implicit use of such subprograms when
stream attributes are applied to string types, but the subprograms in this
package can be used directly by application programs.

@node System Unsigned_Types s-unstyp ads,System Wch_Cnv s-wchcnv ads,System Strings Stream_Ops s-ststop ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-unsigned-types-s-unstyp-ads}@anchor{399}@anchor{gnat_rm/the_gnat_library id147}@anchor{39a}
@section @cite{System.Unsigned_Types} (@code{s-unstyp.ads})


@geindex System.Unsigned_Types (s-unstyp.ads)

This package contains definitions of standard unsigned types that
correspond in size to the standard signed types declared in Standard,
and (unlike the types in Interfaces) have corresponding names. It
also contains some related definitions for other specialized types
used by the compiler in connection with packed array types.

@node System Wch_Cnv s-wchcnv ads,System Wch_Con s-wchcon ads,System Unsigned_Types s-unstyp ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-wch-cnv-s-wchcnv-ads}@anchor{39b}@anchor{gnat_rm/the_gnat_library id148}@anchor{39c}
@section @cite{System.Wch_Cnv} (@code{s-wchcnv.ads})


@geindex System.Wch_Cnv (s-wchcnv.ads)

@geindex Wide Character
@geindex Representation

@geindex Wide String
@geindex Conversion

@geindex Representation of wide characters

This package provides routines for converting between
wide and wide wide characters and a representation as a value of type
@cite{Standard.String}, using a specified wide character
encoding method.  It uses definitions in
package @cite{System.Wch_Con}.

@node System Wch_Con s-wchcon ads,,System Wch_Cnv s-wchcnv ads,The GNAT Library
@anchor{gnat_rm/the_gnat_library system-wch-con-s-wchcon-ads}@anchor{39d}@anchor{gnat_rm/the_gnat_library id149}@anchor{39e}
@section @cite{System.Wch_Con} (@code{s-wchcon.ads})


@geindex System.Wch_Con (s-wchcon.ads)

This package provides definitions and descriptions of
the various methods used for encoding wide characters
in ordinary strings.  These definitions are used by
the package @cite{System.Wch_Cnv}.

@node Interfacing to Other Languages,Specialized Needs Annexes,The GNAT Library,Top
@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-other-languages}@anchor{11}@anchor{gnat_rm/interfacing_to_other_languages doc}@anchor{39f}@anchor{gnat_rm/interfacing_to_other_languages id1}@anchor{3a0}
@chapter Interfacing to Other Languages


The facilities in Annex B of the Ada Reference Manual are fully
implemented in GNAT, and in addition, a full interface to C++ is
provided.

@menu
* Interfacing to C:: 
* Interfacing to C++:: 
* Interfacing to COBOL:: 
* Interfacing to Fortran:: 
* Interfacing to non-GNAT Ada code:: 

@end menu

@node Interfacing to C,Interfacing to C++,,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-c}@anchor{3a1}@anchor{gnat_rm/interfacing_to_other_languages id2}@anchor{3a2}
@section Interfacing to C


Interfacing to C with GNAT can use one of two approaches:


@itemize *

@item 
The types in the package @cite{Interfaces.C} may be used.

@item 
Standard Ada types may be used directly.  This may be less portable to
other compilers, but will work on all GNAT compilers, which guarantee
correspondence between the C and Ada types.
@end itemize

Pragma @cite{Convention C} may be applied to Ada types, but mostly has no
effect, since this is the default.  The following table shows the
correspondence between Ada scalar types and the corresponding C types.


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} 
@headitem

Ada Type

@tab

C Type

@item

@code{Integer}

@tab

@code{int}

@item

@code{Short_Integer}

@tab

@code{short}

@item

@code{Short_Short_Integer}

@tab

@code{signed char}

@item

@code{Long_Integer}

@tab

@code{long}

@item

@code{Long_Long_Integer}

@tab

@code{long long}

@item

@code{Short_Float}

@tab

@code{float}

@item

@code{Float}

@tab

@code{float}

@item

@code{Long_Float}

@tab

@code{double}

@item

@code{Long_Long_Float}

@tab

This is the longest floating-point type supported by the hardware.

@end multitable


Additionally, there are the following general correspondences between Ada
and C types:


@itemize *

@item 
Ada enumeration types map to C enumeration types directly if pragma
@cite{Convention C} is specified, which causes them to have int
length.  Without pragma @cite{Convention C}, Ada enumeration types map to
8, 16, or 32 bits (i.e., C types @cite{signed char}, @cite{short},
@cite{int}, respectively) depending on the number of values passed.
This is the only case in which pragma @cite{Convention C} affects the
representation of an Ada type.

@item 
Ada access types map to C pointers, except for the case of pointers to
unconstrained types in Ada, which have no direct C equivalent.

@item 
Ada arrays map directly to C arrays.

@item 
Ada records map directly to C structures.

@item 
Packed Ada records map to C structures where all members are bit fields
of the length corresponding to the @code{type'Size} value in Ada.
@end itemize

@node Interfacing to C++,Interfacing to COBOL,Interfacing to C,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id4}@anchor{3a3}@anchor{gnat_rm/interfacing_to_other_languages id3}@anchor{3f}
@section Interfacing to C++


The interface to C++ makes use of the following pragmas, which are
primarily intended to be constructed automatically using a binding generator
tool, although it is possible to construct them by hand.

Using these pragmas it is possible to achieve complete
inter-operability between Ada tagged types and C++ class definitions.
See @ref{7,,Implementation Defined Pragmas}, for more details.


@table @asis

@item @emph{pragma CPP_Class ([Entity =>] `LOCAL_NAME`)}

The argument denotes an entity in the current declarative region that is
declared as a tagged or untagged record type. It indicates that the type
corresponds to an externally declared C++ class type, and is to be laid
out the same way that C++ would lay out the type.

Note: Pragma @cite{CPP_Class} is currently obsolete. It is supported
for backward compatibility but its functionality is available
using pragma @cite{Import} with @cite{Convention} = @cite{CPP}.

@item @emph{pragma CPP_Constructor ([Entity =>] `LOCAL_NAME`)}

This pragma identifies an imported function (imported in the usual way
with pragma @cite{Import}) as corresponding to a C++ constructor.
@end table

A few restrictions are placed on the use of the @cite{Access} attribute
in conjunction with subprograms subject to convention @cite{CPP}: the
attribute may be used neither on primitive operations of a tagged
record type with convention @cite{CPP}, imported or not, nor on
subprograms imported with pragma @cite{CPP_Constructor}.

In addition, C++ exceptions are propagated and can be handled in an
@cite{others} choice of an exception handler. The corresponding Ada
occurrence has no message, and the simple name of the exception identity
contains @code{Foreign_Exception}. Finalization and awaiting dependent
tasks works properly when such foreign exceptions are propagated.

It is also possible to import a C++ exception using the following syntax:

@example
LOCAL_NAME : exception;
pragma Import (Cpp,
  [Entity =>] LOCAL_NAME,
  [External_Name =>] static_string_EXPRESSION);
@end example

The @cite{External_Name} is the name of the C++ RTTI symbol. You can then
cover a specific C++ exception in an exception handler.

@node Interfacing to COBOL,Interfacing to Fortran,Interfacing to C++,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id5}@anchor{3a4}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-cobol}@anchor{3a5}
@section Interfacing to COBOL


Interfacing to COBOL is achieved as described in section B.4 of
the Ada Reference Manual.

@node Interfacing to Fortran,Interfacing to non-GNAT Ada code,Interfacing to COBOL,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages id6}@anchor{3a6}@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-fortran}@anchor{3a7}
@section Interfacing to Fortran


Interfacing to Fortran is achieved as described in section B.5 of the
Ada Reference Manual.  The pragma @cite{Convention Fortran}, applied to a
multi-dimensional array causes the array to be stored in column-major
order as required for convenient interface to Fortran.

@node Interfacing to non-GNAT Ada code,,Interfacing to Fortran,Interfacing to Other Languages
@anchor{gnat_rm/interfacing_to_other_languages interfacing-to-non-gnat-ada-code}@anchor{3a8}@anchor{gnat_rm/interfacing_to_other_languages id7}@anchor{3a9}
@section Interfacing to non-GNAT Ada code


It is possible to specify the convention @cite{Ada} in a pragma
@cite{Import} or pragma @cite{Export}.  However this refers to
the calling conventions used by GNAT, which may or may not be
similar enough to those used by some other Ada 83 / Ada 95 / Ada 2005
compiler to allow interoperation.

If arguments types are kept simple, and if the foreign compiler generally
follows system calling conventions, then it may be possible to integrate
files compiled by other Ada compilers, provided that the elaboration
issues are adequately addressed (for example by eliminating the
need for any load time elaboration).

In particular, GNAT running on VMS is designed to
be highly compatible with the DEC Ada 83 compiler, so this is one
case in which it is possible to import foreign units of this type,
provided that the data items passed are restricted to simple scalar
values or simple record types without variants, or simple array
types with fixed bounds.

@node Specialized Needs Annexes,Implementation of Specific Ada Features,Interfacing to Other Languages,Top
@anchor{gnat_rm/specialized_needs_annexes specialized-needs-annexes}@anchor{12}@anchor{gnat_rm/specialized_needs_annexes doc}@anchor{3aa}@anchor{gnat_rm/specialized_needs_annexes id1}@anchor{3ab}
@chapter Specialized Needs Annexes


Ada 95, Ada 2005, and Ada 2012 define a number of Specialized Needs Annexes, which are not
required in all implementations.  However, as described in this chapter,
GNAT implements all of these annexes:


@table @asis

@item @emph{Systems Programming (Annex C)}

The Systems Programming Annex is fully implemented.

@item @emph{Real-Time Systems (Annex D)}

The Real-Time Systems Annex is fully implemented.

@item @emph{Distributed Systems (Annex E)}

Stub generation is fully implemented in the GNAT compiler.  In addition,
a complete compatible PCS is available as part of the GLADE system,
a separate product.  When the two
products are used in conjunction, this annex is fully implemented.

@item @emph{Information Systems (Annex F)}

The Information Systems annex is fully implemented.

@item @emph{Numerics (Annex G)}

The Numerics Annex is fully implemented.

@item @emph{Safety and Security / High-Integrity Systems (Annex H)}

The Safety and Security Annex (termed the High-Integrity Systems Annex
in Ada 2005) is fully implemented.
@end table

@node Implementation of Specific Ada Features,Implementation of Ada 2012 Features,Specialized Needs Annexes,Top
@anchor{gnat_rm/implementation_of_specific_ada_features implementation-of-specific-ada-features}@anchor{13}@anchor{gnat_rm/implementation_of_specific_ada_features doc}@anchor{3ac}@anchor{gnat_rm/implementation_of_specific_ada_features id1}@anchor{3ad}
@chapter Implementation of Specific Ada Features


This chapter describes the GNAT implementation of several Ada language
facilities.

@menu
* Machine Code Insertions:: 
* GNAT Implementation of Tasking:: 
* GNAT Implementation of Shared Passive Packages:: 
* Code Generation for Array Aggregates:: 
* The Size of Discriminated Records with Default Discriminants:: 
* Strict Conformance to the Ada Reference Manual:: 

@end menu

@node Machine Code Insertions,GNAT Implementation of Tasking,,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features machine-code-insertions}@anchor{11a}@anchor{gnat_rm/implementation_of_specific_ada_features id2}@anchor{3ae}
@section Machine Code Insertions


@geindex Machine Code insertions

Package @cite{Machine_Code} provides machine code support as described
in the Ada Reference Manual in two separate forms:


@itemize *

@item 
Machine code statements, consisting of qualified expressions that
fit the requirements of RM section 13.8.

@item 
An intrinsic callable procedure, providing an alternative mechanism of
including machine instructions in a subprogram.
@end itemize

The two features are similar, and both are closely related to the mechanism
provided by the asm instruction in the GNU C compiler.  Full understanding
and use of the facilities in this package requires understanding the asm
instruction, see the section on Extended Asm in
@cite{Using_the_GNU_Compiler_Collection_(GCC)}.

Calls to the function @cite{Asm} and the procedure @cite{Asm} have identical
semantic restrictions and effects as described below.  Both are provided so
that the procedure call can be used as a statement, and the function call
can be used to form a code_statement.

Consider this C @cite{asm} instruction:

@example
asm ("fsinx %1 %0" : "=f" (result) : "f" (angle));
@end example

The equivalent can be written for GNAT as:

@example
Asm ("fsinx %1 %0",
     My_Float'Asm_Output ("=f", result),
     My_Float'Asm_Input  ("f",  angle));
@end example

The first argument to @cite{Asm} is the assembler template, and is
identical to what is used in GNU C.  This string must be a static
expression.  The second argument is the output operand list.  It is
either a single @cite{Asm_Output} attribute reference, or a list of such
references enclosed in parentheses (technically an array aggregate of
such references).

The @cite{Asm_Output} attribute denotes a function that takes two
parameters.  The first is a string, the second is the name of a variable
of the type designated by the attribute prefix.  The first (string)
argument is required to be a static expression and designates the
constraint (see the section on Constraints in
@cite{Using_the_GNU_Compiler_Collection_(GCC)})
for the parameter; e.g., what kind of register is required.  The second
argument is the variable to be written or updated with the
result.  The possible values for constraint are the same as those used in
the RTL, and are dependent on the configuration file used to build the
GCC back end.  If there are no output operands, then this argument may
either be omitted, or explicitly given as @cite{No_Output_Operands}.
No support is provided for GNU C's symbolic names for output parameters.

The second argument of @code{my_float'Asm_Output} functions as
though it were an @cite{out} parameter, which is a little curious, but
all names have the form of expressions, so there is no syntactic
irregularity, even though normally functions would not be permitted
@cite{out} parameters.  The third argument is the list of input
operands.  It is either a single @cite{Asm_Input} attribute reference, or
a list of such references enclosed in parentheses (technically an array
aggregate of such references).

The @cite{Asm_Input} attribute denotes a function that takes two
parameters.  The first is a string, the second is an expression of the
type designated by the prefix.  The first (string) argument is required
to be a static expression, and is the constraint for the parameter,
(e.g., what kind of register is required).  The second argument is the
value to be used as the input argument.  The possible values for the
constraint are the same as those used in the RTL, and are dependent on
the configuration file used to built the GCC back end.
No support is provided for GNU C's symbolic names for input parameters.

If there are no input operands, this argument may either be omitted, or
explicitly given as @cite{No_Input_Operands}.  The fourth argument, not
present in the above example, is a list of register names, called the
@emph{clobber} argument.  This argument, if given, must be a static string
expression, and is a space or comma separated list of names of registers
that must be considered destroyed as a result of the @cite{Asm} call.  If
this argument is the null string (the default value), then the code
generator assumes that no additional registers are destroyed.
In addition to registers, the special clobbers @cite{memory} and
@cite{cc} as described in the GNU C docs are both supported.

The fifth argument, not present in the above example, called the
@emph{volatile} argument, is by default @cite{False}.  It can be set to
the literal value @cite{True} to indicate to the code generator that all
optimizations with respect to the instruction specified should be
suppressed, and in particular an instruction that has outputs
will still be generated, even if none of the outputs are
used.  See @cite{Using_the_GNU_Compiler_Collection_(GCC)}
for the full description.
Generally it is strongly advisable to use Volatile for any ASM statement
that is missing either input or output operands or to avoid unwanted
optimizations. A warning is generated if this advice is not followed.

No support is provided for GNU C's @cite{asm goto} feature.

The @cite{Asm} subprograms may be used in two ways.  First the procedure
forms can be used anywhere a procedure call would be valid, and
correspond to what the RM calls 'intrinsic' routines.  Such calls can
be used to intersperse machine instructions with other Ada statements.
Second, the function forms, which return a dummy value of the limited
private type @cite{Asm_Insn}, can be used in code statements, and indeed
this is the only context where such calls are allowed.  Code statements
appear as aggregates of the form:

@example
Asm_Insn'(Asm (...));
Asm_Insn'(Asm_Volatile (...));
@end example

In accordance with RM rules, such code statements are allowed only
within subprograms whose entire body consists of such statements.  It is
not permissible to intermix such statements with other Ada statements.

Typically the form using intrinsic procedure calls is more convenient
and more flexible.  The code statement form is provided to meet the RM
suggestion that such a facility should be made available.  The following
is the exact syntax of the call to @cite{Asm}. As usual, if named notation
is used, the arguments may be given in arbitrary order, following the
normal rules for use of positional and named arguments:

@example
ASM_CALL ::= Asm (
                 [Template =>] static_string_EXPRESSION
               [,[Outputs  =>] OUTPUT_OPERAND_LIST      ]
               [,[Inputs   =>] INPUT_OPERAND_LIST       ]
               [,[Clobber  =>] static_string_EXPRESSION ]
               [,[Volatile =>] static_boolean_EXPRESSION] )

OUTPUT_OPERAND_LIST ::=
  [PREFIX.]No_Output_Operands
| OUTPUT_OPERAND_ATTRIBUTE
| (OUTPUT_OPERAND_ATTRIBUTE @{,OUTPUT_OPERAND_ATTRIBUTE@})

OUTPUT_OPERAND_ATTRIBUTE ::=
  SUBTYPE_MARK'Asm_Output (static_string_EXPRESSION, NAME)

INPUT_OPERAND_LIST ::=
  [PREFIX.]No_Input_Operands
| INPUT_OPERAND_ATTRIBUTE
| (INPUT_OPERAND_ATTRIBUTE @{,INPUT_OPERAND_ATTRIBUTE@})

INPUT_OPERAND_ATTRIBUTE ::=
  SUBTYPE_MARK'Asm_Input (static_string_EXPRESSION, EXPRESSION)
@end example

The identifiers @cite{No_Input_Operands} and @cite{No_Output_Operands}
are declared in the package @cite{Machine_Code} and must be referenced
according to normal visibility rules. In particular if there is no
@cite{use} clause for this package, then appropriate package name
qualification is required.

@node GNAT Implementation of Tasking,GNAT Implementation of Shared Passive Packages,Machine Code Insertions,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id3}@anchor{3af}@anchor{gnat_rm/implementation_of_specific_ada_features gnat-implementation-of-tasking}@anchor{3b0}
@section GNAT Implementation of Tasking


This chapter outlines the basic GNAT approach to tasking (in particular,
a multi-layered library for portability) and discusses issues related
to compliance with the Real-Time Systems Annex.

@menu
* Mapping Ada Tasks onto the Underlying Kernel Threads:: 
* Ensuring Compliance with the Real-Time Annex:: 

@end menu

@node Mapping Ada Tasks onto the Underlying Kernel Threads,Ensuring Compliance with the Real-Time Annex,,GNAT Implementation of Tasking
@anchor{gnat_rm/implementation_of_specific_ada_features mapping-ada-tasks-onto-the-underlying-kernel-threads}@anchor{3b1}@anchor{gnat_rm/implementation_of_specific_ada_features id4}@anchor{3b2}
@subsection Mapping Ada Tasks onto the Underlying Kernel Threads


GNAT's run-time support comprises two layers:


@itemize *

@item 
GNARL (GNAT Run-time Layer)

@item 
GNULL (GNAT Low-level Library)
@end itemize

In GNAT, Ada's tasking services rely on a platform and OS independent
layer known as GNARL.  This code is responsible for implementing the
correct semantics of Ada's task creation, rendezvous, protected
operations etc.

GNARL decomposes Ada's tasking semantics into simpler lower level
operations such as create a thread, set the priority of a thread,
yield, create a lock, lock/unlock, etc.  The spec for these low-level
operations constitutes GNULLI, the GNULL Interface.  This interface is
directly inspired from the POSIX real-time API.

If the underlying executive or OS implements the POSIX standard
faithfully, the GNULL Interface maps as is to the services offered by
the underlying kernel.  Otherwise, some target dependent glue code maps
the services offered by the underlying kernel to the semantics expected
by GNARL.

Whatever the underlying OS (VxWorks, UNIX, Windows, etc.) the
key point is that each Ada task is mapped on a thread in the underlying
kernel.  For example, in the case of VxWorks, one Ada task = one VxWorks task.

In addition Ada task priorities map onto the underlying thread priorities.
Mapping Ada tasks onto the underlying kernel threads has several advantages:


@itemize *

@item 
The underlying scheduler is used to schedule the Ada tasks.  This
makes Ada tasks as efficient as kernel threads from a scheduling
standpoint.

@item 
Interaction with code written in C containing threads is eased
since at the lowest level Ada tasks and C threads map onto the same
underlying kernel concept.

@item 
When an Ada task is blocked during I/O the remaining Ada tasks are
able to proceed.

@item 
On multiprocessor systems Ada tasks can execute in parallel.
@end itemize

Some threads libraries offer a mechanism to fork a new process, with the
child process duplicating the threads from the parent.
GNAT does not
support this functionality when the parent contains more than one task.
.. index:: Forking a new process

@node Ensuring Compliance with the Real-Time Annex,,Mapping Ada Tasks onto the Underlying Kernel Threads,GNAT Implementation of Tasking
@anchor{gnat_rm/implementation_of_specific_ada_features id5}@anchor{3b3}@anchor{gnat_rm/implementation_of_specific_ada_features ensuring-compliance-with-the-real-time-annex}@anchor{3b4}
@subsection Ensuring Compliance with the Real-Time Annex


@geindex Real-Time Systems Annex compliance

Although mapping Ada tasks onto
the underlying threads has significant advantages, it does create some
complications when it comes to respecting the scheduling semantics
specified in the real-time annex (Annex D).

For instance the Annex D requirement for the @cite{FIFO_Within_Priorities}
scheduling policy states:

@quotation

@emph{When the active priority of a ready task that is not running
changes, or the setting of its base priority takes effect, the
task is removed from the ready queue for its old active priority
and is added at the tail of the ready queue for its new active
priority, except in the case where the active priority is lowered
due to the loss of inherited priority, in which case the task is
added at the head of the ready queue for its new active priority.}
@end quotation

While most kernels do put tasks at the end of the priority queue when
a task changes its priority, (which respects the main
FIFO_Within_Priorities requirement), almost none keep a thread at the
beginning of its priority queue when its priority drops from the loss
of inherited priority.

As a result most vendors have provided incomplete Annex D implementations.

The GNAT run-time, has a nice cooperative solution to this problem
which ensures that accurate FIFO_Within_Priorities semantics are
respected.

The principle is as follows.  When an Ada task T is about to start
running, it checks whether some other Ada task R with the same
priority as T has been suspended due to the loss of priority
inheritance.  If this is the case, T yields and is placed at the end of
its priority queue.  When R arrives at the front of the queue it
executes.

Note that this simple scheme preserves the relative order of the tasks
that were ready to execute in the priority queue where R has been
placed at the end.

@node GNAT Implementation of Shared Passive Packages,Code Generation for Array Aggregates,GNAT Implementation of Tasking,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id6}@anchor{3b5}@anchor{gnat_rm/implementation_of_specific_ada_features gnat-implementation-of-shared-passive-packages}@anchor{3b6}
@section GNAT Implementation of Shared Passive Packages


@geindex Shared passive packages

GNAT fully implements the pragma @cite{Shared_Passive} for
.. index:: pragma @cite{Shared_Passive}

the purpose of designating shared passive packages.
This allows the use of passive partitions in the
context described in the Ada Reference Manual; i.e., for communication
between separate partitions of a distributed application using the
features in Annex E.
.. index:: Annex E

@geindex Distribution Systems Annex

However, the implementation approach used by GNAT provides for more
extensive usage as follows:


@table @asis

@item @emph{Communication between separate programs}

This allows separate programs to access the data in passive
partitions, using protected objects for synchronization where
needed. The only requirement is that the two programs have a
common shared file system. It is even possible for programs
running on different machines with different architectures
(e.g., different endianness) to communicate via the data in
a passive partition.

@item @emph{Persistence between program runs}

The data in a passive package can persist from one run of a
program to another, so that a later program sees the final
values stored by a previous run of the same program.
@end table

The implementation approach used is to store the data in files. A
separate stream file is created for each object in the package, and
an access to an object causes the corresponding file to be read or
written.

@geindex SHARED_MEMORY_DIRECTORY environment variable

The environment variable @cite{SHARED_MEMORY_DIRECTORY} should be
set to the directory to be used for these files.
The files in this directory
have names that correspond to their fully qualified names. For
example, if we have the package

@example
package X is
  pragma Shared_Passive (X);
  Y : Integer;
  Z : Float;
end X;
@end example

and the environment variable is set to @cite{/stemp/}, then the files created
will have the names:

@example
/stemp/x.y
/stemp/x.z
@end example

These files are created when a value is initially written to the object, and
the files are retained until manually deleted. This provides the persistence
semantics. If no file exists, it means that no partition has assigned a value
to the variable; in this case the initial value declared in the package
will be used. This model ensures that there are no issues in synchronizing
the elaboration process, since elaboration of passive packages elaborates the
initial values, but does not create the files.

The files are written using normal @cite{Stream_IO} access.
If you want to be able
to communicate between programs or partitions running on different
architectures, then you should use the XDR versions of the stream attribute
routines, since these are architecture independent.

If active synchronization is required for access to the variables in the
shared passive package, then as described in the Ada Reference Manual, the
package may contain protected objects used for this purpose. In this case
a lock file (whose name is @code{___lock} (three underscores)
is created in the shared memory directory.

@geindex ___lock file (for shared passive packages)

This is used to provide the required locking
semantics for proper protected object synchronization.

GNAT supports shared passive packages on all platforms
except for OpenVMS.

@node Code Generation for Array Aggregates,The Size of Discriminated Records with Default Discriminants,GNAT Implementation of Shared Passive Packages,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features code-generation-for-array-aggregates}@anchor{3b7}@anchor{gnat_rm/implementation_of_specific_ada_features id7}@anchor{3b8}
@section Code Generation for Array Aggregates


Aggregates have a rich syntax and allow the user to specify the values of
complex data structures by means of a single construct.  As a result, the
code generated for aggregates can be quite complex and involve loops, case
statements and multiple assignments.  In the simplest cases, however, the
compiler will recognize aggregates whose components and constraints are
fully static, and in those cases the compiler will generate little or no
executable code.  The following is an outline of the code that GNAT generates
for various aggregate constructs.  For further details, you will find it
useful to examine the output produced by the -gnatG flag to see the expanded
source that is input to the code generator.  You may also want to examine
the assembly code generated at various levels of optimization.

The code generated for aggregates depends on the context, the component values,
and the type.  In the context of an object declaration the code generated is
generally simpler than in the case of an assignment.  As a general rule, static
component values and static subtypes also lead to simpler code.

@menu
* Static constant aggregates with static bounds:: 
* Constant aggregates with unconstrained nominal types:: 
* Aggregates with static bounds:: 
* Aggregates with non-static bounds:: 
* Aggregates in assignment statements:: 

@end menu

@node Static constant aggregates with static bounds,Constant aggregates with unconstrained nominal types,,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features static-constant-aggregates-with-static-bounds}@anchor{3b9}@anchor{gnat_rm/implementation_of_specific_ada_features id8}@anchor{3ba}
@subsection Static constant aggregates with static bounds


For the declarations:

@example
type One_Dim is array (1..10) of integer;
ar0 : constant One_Dim := (1, 2, 3, 4, 5, 6, 7, 8, 9, 0);
@end example

GNAT generates no executable code: the constant ar0 is placed in static memory.
The same is true for constant aggregates with named associations:

@example
Cr1 : constant One_Dim := (4 => 16, 2 => 4, 3 => 9, 1 => 1, 5 .. 10 => 0);
Cr3 : constant One_Dim := (others => 7777);
@end example

The same is true for multidimensional constant arrays such as:

@example
type two_dim is array (1..3, 1..3) of integer;
Unit : constant two_dim := ( (1,0,0), (0,1,0), (0,0,1));
@end example

The same is true for arrays of one-dimensional arrays: the following are
static:

@example
type ar1b  is array (1..3) of boolean;
type ar_ar is array (1..3) of ar1b;
None  : constant ar1b := (others => false);     --  fully static
None2 : constant ar_ar := (1..3 => None);       --  fully static
@end example

However, for multidimensional aggregates with named associations, GNAT will
generate assignments and loops, even if all associations are static.  The
following two declarations generate a loop for the first dimension, and
individual component assignments for the second dimension:

@example
Zero1: constant two_dim := (1..3 => (1..3 => 0));
Zero2: constant two_dim := (others => (others => 0));
@end example

@node Constant aggregates with unconstrained nominal types,Aggregates with static bounds,Static constant aggregates with static bounds,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features constant-aggregates-with-unconstrained-nominal-types}@anchor{3bb}@anchor{gnat_rm/implementation_of_specific_ada_features id9}@anchor{3bc}
@subsection Constant aggregates with unconstrained nominal types


In such cases the aggregate itself establishes the subtype, so that
associations with @cite{others} cannot be used.  GNAT determines the
bounds for the actual subtype of the aggregate, and allocates the
aggregate statically as well.  No code is generated for the following:

@example
type One_Unc is array (natural range <>) of integer;
Cr_Unc : constant One_Unc := (12,24,36);
@end example

@node Aggregates with static bounds,Aggregates with non-static bounds,Constant aggregates with unconstrained nominal types,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features id10}@anchor{3bd}@anchor{gnat_rm/implementation_of_specific_ada_features aggregates-with-static-bounds}@anchor{3be}
@subsection Aggregates with static bounds


In all previous examples the aggregate was the initial (and immutable) value
of a constant.  If the aggregate initializes a variable, then code is generated
for it as a combination of individual assignments and loops over the target
object.  The declarations

@example
Cr_Var1 : One_Dim := (2, 5, 7, 11, 0, 0, 0, 0, 0, 0);
Cr_Var2 : One_Dim := (others > -1);
@end example

generate the equivalent of

@example
Cr_Var1 (1) := 2;
Cr_Var1 (2) := 3;
Cr_Var1 (3) := 5;
Cr_Var1 (4) := 11;

for I in Cr_Var2'range loop
   Cr_Var2 (I) := -1;
end loop;
@end example

@node Aggregates with non-static bounds,Aggregates in assignment statements,Aggregates with static bounds,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features aggregates-with-non-static-bounds}@anchor{3bf}@anchor{gnat_rm/implementation_of_specific_ada_features id11}@anchor{3c0}
@subsection Aggregates with non-static bounds


If the bounds of the aggregate are not statically compatible with the bounds
of the nominal subtype  of the target, then constraint checks have to be
generated on the bounds.  For a multidimensional array, constraint checks may
have to be applied to sub-arrays individually, if they do not have statically
compatible subtypes.

@node Aggregates in assignment statements,,Aggregates with non-static bounds,Code Generation for Array Aggregates
@anchor{gnat_rm/implementation_of_specific_ada_features id12}@anchor{3c1}@anchor{gnat_rm/implementation_of_specific_ada_features aggregates-in-assignment-statements}@anchor{3c2}
@subsection Aggregates in assignment statements


In general, aggregate assignment requires the construction of a temporary,
and a copy from the temporary to the target of the assignment.  This is because
it is not always possible to convert the assignment into a series of individual
component assignments.  For example, consider the simple case:

@example
A := (A(2), A(1));
@end example

This cannot be converted into:

@example
A(1) := A(2);
A(2) := A(1);
@end example

So the aggregate has to be built first in a separate location, and then
copied into the target.  GNAT recognizes simple cases where this intermediate
step is not required, and the assignments can be performed in place, directly
into the target.  The following sufficient criteria are applied:


@itemize *

@item 
The bounds of the aggregate are static, and the associations are static.

@item 
The components of the aggregate are static constants, names of
simple variables that are not renamings, or expressions not involving
indexed components whose operands obey these rules.
@end itemize

If any of these conditions are violated, the aggregate will be built in
a temporary (created either by the front-end or the code generator) and then
that temporary will be copied onto the target.

@node The Size of Discriminated Records with Default Discriminants,Strict Conformance to the Ada Reference Manual,Code Generation for Array Aggregates,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features id13}@anchor{3c3}@anchor{gnat_rm/implementation_of_specific_ada_features the-size-of-discriminated-records-with-default-discriminants}@anchor{3c4}
@section The Size of Discriminated Records with Default Discriminants


If a discriminated type @cite{T} has discriminants with default values, it is
possible to declare an object of this type without providing an explicit
constraint:

@example
type Size is range 1..100;

type Rec (D : Size := 15) is record
   Name : String (1..D);
end T;

Word : Rec;
@end example

Such an object is said to be @emph{unconstrained}.
The discriminant of the object
can be modified by a full assignment to the object, as long as it preserves the
relation between the value of the discriminant, and the value of the components
that depend on it:

@example
Word := (3, "yes");

Word := (5, "maybe");

Word := (5, "no"); -- raises Constraint_Error
@end example

In order to support this behavior efficiently, an unconstrained object is
given the maximum size that any value of the type requires. In the case
above, @cite{Word} has storage for the discriminant and for
a @cite{String} of length 100.
It is important to note that unconstrained objects do not require dynamic
allocation. It would be an improper implementation to place on the heap those
components whose size depends on discriminants. (This improper implementation
was used by some Ada83 compilers, where the @cite{Name} component above
would have
been stored as a pointer to a dynamic string). Following the principle that
dynamic storage management should never be introduced implicitly,
an Ada compiler should reserve the full size for an unconstrained declared
object, and place it on the stack.

This maximum size approach
has been a source of surprise to some users, who expect the default
values of the discriminants to determine the size reserved for an
unconstrained object: "If the default is 15, why should the object occupy
a larger size?"
The answer, of course, is that the discriminant may be later modified,
and its full range of values must be taken into account. This is why the
declaration:

@example
type Rec (D : Positive := 15) is record
   Name : String (1..D);
end record;

Too_Large : Rec;
@end example

is flagged by the compiler with a warning:
an attempt to create @cite{Too_Large} will raise @cite{Storage_Error},
because the required size includes @cite{Positive'Last}
bytes. As the first example indicates, the proper approach is to declare an
index type of 'reasonable' range so that unconstrained objects are not too
large.

One final wrinkle: if the object is declared to be @cite{aliased}, or if it is
created in the heap by means of an allocator, then it is @emph{not}
unconstrained:
it is constrained by the default values of the discriminants, and those values
cannot be modified by full assignment. This is because in the presence of
aliasing all views of the object (which may be manipulated by different tasks,
say) must be consistent, so it is imperative that the object, once created,
remain invariant.

@node Strict Conformance to the Ada Reference Manual,,The Size of Discriminated Records with Default Discriminants,Implementation of Specific Ada Features
@anchor{gnat_rm/implementation_of_specific_ada_features strict-conformance-to-the-ada-reference-manual}@anchor{3c5}@anchor{gnat_rm/implementation_of_specific_ada_features id14}@anchor{3c6}
@section Strict Conformance to the Ada Reference Manual


The dynamic semantics defined by the Ada Reference Manual impose a set of
run-time checks to be generated. By default, the GNAT compiler will insert many
run-time checks into the compiled code, including most of those required by the
Ada Reference Manual. However, there are three checks that are not enabled
in the default mode for efficiency reasons: arithmetic overflow checking for
integer operations (including division by zero), checks for access before
elaboration on subprogram calls, and stack overflow checking (most operating
systems do not perform this check by default).

Strict conformance to the Ada Reference Manual can be achieved by adding
three compiler options for overflow checking for integer operations
(@emph{-gnato}), dynamic checks for access-before-elaboration on subprogram
calls and generic instantiations (@emph{-gnatE}), and stack overflow
checking (@emph{-fstack-check}).

Note that the result of a floating point arithmetic operation in overflow and
invalid situations, when the @cite{Machine_Overflows} attribute of the result
type is @cite{False}, is to generate IEEE NaN and infinite values. This is the
case for machines compliant with the IEEE floating-point standard, but on
machines that are not fully compliant with this standard, such as Alpha, the
@emph{-mieee} compiler flag must be used for achieving IEEE confirming
behavior (although at the cost of a significant performance penalty), so
infinite and NaN values are properly generated.

@node Implementation of Ada 2012 Features,Obsolescent Features,Implementation of Specific Ada Features,Top
@anchor{gnat_rm/implementation_of_ada_2012_features doc}@anchor{3c7}@anchor{gnat_rm/implementation_of_ada_2012_features implementation-of-ada-2012-features}@anchor{14}@anchor{gnat_rm/implementation_of_ada_2012_features id1}@anchor{3c8}
@chapter Implementation of Ada 2012 Features


@geindex Ada 2012 implementation status

@geindex -gnat12 option (gcc)

@geindex pragma Ada_2012

@geindex configuration pragma Ada_2012

@geindex Ada_2012 configuration pragma

This chapter contains a complete list of Ada 2012 features that have been
implemented.
Generally, these features are only
available if the @emph{-gnat12} (Ada 2012 features enabled) option is set,
which is the default behavior,
or if the configuration pragma @cite{Ada_2012} is used.

However, new pragmas, attributes, and restrictions are
unconditionally available, since the Ada 95 standard allows the addition of
new pragmas, attributes, and restrictions (there are exceptions, which are
documented in the individual descriptions), and also certain packages
were made available in earlier versions of Ada.

An ISO date (YYYY-MM-DD) appears in parentheses on the description line.
This date shows the implementation date of the feature. Any wavefront
subsequent to this date will contain the indicated feature, as will any
subsequent releases. A date of 0000-00-00 means that GNAT has always
implemented the feature, or implemented it as soon as it appeared as a
binding interpretation.

Each feature corresponds to an Ada Issue ('AI') approved by the Ada
standardization group (ISO/IEC JTC1/SC22/WG9) for inclusion in Ada 2012.
The features are ordered based on the relevant sections of the Ada
Reference Manual ("RM").  When a given AI relates to multiple points
in the RM, the earliest is used.

A complete description of the AIs may be found in
@indicateurl{http://www.ada-auth.org/ai05-summary.html}.

@geindex AI-0176 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0176 Quantified expressions (2010-09-29)}

Both universally and existentially quantified expressions are implemented.
They use the new syntax for iterators proposed in AI05-139-2, as well as
the standard Ada loop syntax.

RM References:  1.01.04 (12)   2.09 (2/2)   4.04 (7)   4.05.09 (0)
@end itemize

@geindex AI-0079 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0079 Allow other_format characters in source (2010-07-10)}

Wide characters in the unicode category @emph{other_format} are now allowed in
source programs between tokens, but not within a token such as an identifier.

RM References:  2.01 (4/2)   2.02 (7)
@end itemize

@geindex AI-0091 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0091 Do not allow other_format in identifiers (0000-00-00)}

Wide characters in the unicode category @emph{other_format} are not permitted
within  an identifier, since this can be a security problem. The error
message for this case has been improved to be more specific, but GNAT has
never allowed such characters to appear in identifiers.

RM References:  2.03 (3.1/2)   2.03 (4/2)   2.03 (5/2)   2.03 (5.1/2)   2.03 (5.2/2)   2.03 (5.3/2)   2.09 (2/2)
@end itemize

@geindex AI-0100 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0100 Placement of pragmas  (2010-07-01)}

This AI is an earlier version of AI-163. It simplifies the rules
for legal placement of pragmas. In the case of lists that allow pragmas, if
the list may have no elements, then the list may consist solely of pragmas.

RM References:  2.08 (7)
@end itemize

@geindex AI-0163 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0163 Pragmas in place of null (2010-07-01)}

A statement sequence may be composed entirely of pragmas. It is no longer
necessary to add a dummy @cite{null} statement to make the sequence legal.

RM References:  2.08 (7)   2.08 (16)
@end itemize

@geindex AI-0080 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0080 'View of' not needed if clear from context (0000-00-00)}

This is an editorial change only, described as non-testable in the AI.

RM References:  3.01 (7)
@end itemize

@geindex AI-0183 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0183 Aspect specifications (2010-08-16)}

Aspect specifications have been fully implemented except for pre and post-
conditions, and type invariants, which have their own separate AI's. All
forms of declarations listed in the AI are supported. The following is a
list of the aspects supported (with GNAT implementation aspects marked)
@end itemize


@multitable {xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx} {xxxxxxxxxxxxx} 
@headitem

Supported Aspect

@tab

Source

@item

@cite{Ada_2005}

@tab

-- GNAT

@item

@cite{Ada_2012}

@tab

-- GNAT

@item

@cite{Address}

@tab

@item

@cite{Alignment}

@tab

@item

@cite{Atomic}

@tab

@item

@cite{Atomic_Components}

@tab

@item

@cite{Bit_Order}

@tab

@item

@cite{Component_Size}

@tab

@item

@cite{Contract_Cases}

@tab

-- GNAT

@item

@cite{Discard_Names}

@tab

@item

@cite{External_Tag}

@tab

@item

@cite{Favor_Top_Level}

@tab

-- GNAT

@item

@cite{Inline}

@tab

@item

@cite{Inline_Always}

@tab

-- GNAT

@item

@cite{Invariant}

@tab

-- GNAT

@item

@cite{Machine_Radix}

@tab

@item

@cite{No_Return}

@tab

@item

@cite{Object_Size}

@tab

-- GNAT

@item

@cite{Pack}

@tab

@item

@cite{Persistent_BSS}

@tab

-- GNAT

@item

@cite{Post}

@tab

@item

@cite{Pre}

@tab

@item

@cite{Predicate}

@tab

@item

@cite{Preelaborable_Initialization}

@tab

@item

@cite{Pure_Function}

@tab

-- GNAT

@item

@cite{Remote_Access_Type}

@tab

-- GNAT

@item

@cite{Shared}

@tab

-- GNAT

@item

@cite{Size}

@tab

@item

@cite{Storage_Pool}

@tab

@item

@cite{Storage_Size}

@tab

@item

@cite{Stream_Size}

@tab

@item

@cite{Suppress}

@tab

@item

@cite{Suppress_Debug_Info}

@tab

-- GNAT

@item

@cite{Test_Case}

@tab

-- GNAT

@item

@cite{Thread_Local_Storage}

@tab

-- GNAT

@item

@cite{Type_Invariant}

@tab

@item

@cite{Unchecked_Union}

@tab

@item

@cite{Universal_Aliasing}

@tab

-- GNAT

@item

@cite{Unmodified}

@tab

-- GNAT

@item

@cite{Unreferenced}

@tab

-- GNAT

@item

@cite{Unreferenced_Objects}

@tab

-- GNAT

@item

@cite{Unsuppress}

@tab

@item

@cite{Value_Size}

@tab

-- GNAT

@item

@cite{Volatile}

@tab

@item

@cite{Volatile_Components}

@tab

@item

@cite{Warnings}

@tab

-- GNAT

@end multitable


@quotation

Note that for aspects with an expression, e.g. @cite{Size}, the expression is
treated like a default expression (visibility is analyzed at the point of
occurrence of the aspect, but evaluation of the expression occurs at the
freeze point of the entity involved).

RM References:  3.02.01 (3)   3.02.02 (2)   3.03.01 (2/2)   3.08 (6)
3.09.03 (1.1/2)   6.01 (2/2)   6.07 (2/2)   9.05.02 (2/2)   7.01 (3)   7.03
(2)   7.03 (3)   9.01 (2/2)   9.01 (3/2)   9.04 (2/2)   9.04 (3/2)
9.05.02 (2/2)   11.01 (2)   12.01 (3)   12.03 (2/2)   12.04 (2/2)   12.05 (2)
12.06 (2.1/2)   12.06 (2.2/2)   12.07 (2)   13.01 (0.1/2)   13.03 (5/1)
13.03.01 (0)
@end quotation

@geindex AI-0128 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0128 Inequality is a primitive operation (0000-00-00)}

If an equality operator ("=") is declared for a type, then the implicitly
declared inequality operator ("/=") is a primitive operation of the type.
This is the only reasonable interpretation, and is the one always implemented
by GNAT, but the RM was not entirely clear in making this point.

RM References:  3.02.03 (6)   6.06 (6)
@end itemize

@geindex AI-0003 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0003 Qualified expressions as names (2010-07-11)}

In Ada 2012, a qualified expression is considered to be syntactically a name,
meaning that constructs such as @cite{A'(F(X)).B} are now legal. This is
useful in disambiguating some cases of overloading.

RM References:  3.03 (11)   3.03 (21)   4.01 (2)   4.04 (7)   4.07 (3)
5.04 (7)
@end itemize

@geindex AI-0120 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0120 Constant instance of protected object (0000-00-00)}

This is an RM editorial change only. The section that lists objects that are
constant failed to include the current instance of a protected object
within a protected function. This has always been treated as a constant
in GNAT.

RM References:  3.03 (21)
@end itemize

@geindex AI-0008 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0008 General access to constrained objects (0000-00-00)}

The wording in the RM implied that if you have a general access to a
constrained object, it could be used to modify the discriminants. This was
obviously not intended. @cite{Constraint_Error} should be raised, and GNAT
has always done so in this situation.

RM References:  3.03 (23)   3.10.02 (26/2)   4.01 (9)   6.04.01 (17)   8.05.01 (5/2)
@end itemize

@geindex AI-0093 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0093 Additional rules use immutably limited (0000-00-00)}

This is an editorial change only, to make more widespread use of the Ada 2012
'immutably limited'.

RM References:  3.03 (23.4/3)
@end itemize

@geindex AI-0096 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0096 Deriving from formal private types (2010-07-20)}

In general it is illegal for a type derived from a formal limited type to be
nonlimited.  This AI makes an exception to this rule: derivation is legal
if it appears in the private part of the generic, and the formal type is not
tagged. If the type is tagged, the legality check must be applied to the
private part of the package.

RM References:  3.04 (5.1/2)   6.02 (7)
@end itemize

@geindex AI-0181 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0181 Soft hyphen is a non-graphic character (2010-07-23)}

From Ada 2005 on, soft hyphen is considered a non-graphic character, which
means that it has a special name (@cite{SOFT_HYPHEN}) in conjunction with the
@cite{Image} and @cite{Value} attributes for the character types. Strictly
speaking this is an inconsistency with Ada 95, but in practice the use of
these attributes is so obscure that it will not cause problems.

RM References:  3.05.02 (2/2)   A.01 (35/2)   A.03.03 (21)
@end itemize

@geindex AI-0182 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0182 Additional forms for `Character'Value} (0000-00-00)`

This AI allows @cite{Character'Value} to accept the string @cite{'?'} where
@cite{?} is any character including non-graphic control characters. GNAT has
always accepted such strings. It also allows strings such as
@cite{HEX_00000041} to be accepted, but GNAT does not take advantage of this
permission and raises @cite{Constraint_Error}, as is certainly still
permitted.

RM References:  3.05 (56/2)
@end itemize

@geindex AI-0214 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0214 Defaulted discriminants for limited tagged (2010-10-01)}

Ada 2012 relaxes the restriction that forbids discriminants of tagged types
to have default expressions by allowing them when the type is limited. It
is often useful to define a default value for a discriminant even though
it can't be changed by assignment.

RM References:  3.07 (9.1/2)   3.07.02 (3)
@end itemize

@geindex AI-0102 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0102 Some implicit conversions are illegal (0000-00-00)}

It is illegal to assign an anonymous access constant to an anonymous access
variable. The RM did not have a clear rule to prevent this, but GNAT has
always generated an error for this usage.

RM References:  3.07 (16)   3.07.01 (9)   6.04.01 (6)   8.06 (27/2)
@end itemize

@geindex AI-0158 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0158 Generalizing membership tests (2010-09-16)}

This AI extends the syntax of membership tests to simplify complex conditions
that can be expressed as membership in a subset of values of any type. It
introduces syntax for a list of expressions that may be used in loop contexts
as well.

RM References:  3.08.01 (5)   4.04 (3)   4.05.02 (3)   4.05.02 (5)   4.05.02 (27)
@end itemize

@geindex AI-0173 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0173 Testing if tags represent abstract types (2010-07-03)}

The function @cite{Ada.Tags.Type_Is_Abstract} returns @cite{True} if invoked
with the tag of an abstract type, and @cite{False} otherwise.

RM References:  3.09 (7.4/2)   3.09 (12.4/2)
@end itemize

@geindex AI-0076 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0076 function with controlling result (0000-00-00)}

This is an editorial change only. The RM defines calls with controlling
results, but uses the term 'function with controlling result' without an
explicit definition.

RM References:  3.09.02 (2/2)
@end itemize

@geindex AI-0126 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0126 Dispatching with no declared operation (0000-00-00)}

This AI clarifies dispatching rules, and simply confirms that dispatching
executes the operation of the parent type when there is no explicitly or
implicitly declared operation for the descendant type. This has always been
the case in all versions of GNAT.

RM References:  3.09.02 (20/2)   3.09.02 (20.1/2)   3.09.02 (20.2/2)
@end itemize

@geindex AI-0097 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0097 Treatment of abstract null extension (2010-07-19)}

The RM as written implied that in some cases it was possible to create an
object of an abstract type, by having an abstract extension inherit a non-
abstract constructor from its parent type. This mistake has been corrected
in GNAT and in the RM, and this construct is now illegal.

RM References:  3.09.03 (4/2)
@end itemize

@geindex AI-0203 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0203 Extended return cannot be abstract (0000-00-00)}

A return_subtype_indication cannot denote an abstract subtype. GNAT has never
permitted such usage.

RM References:  3.09.03 (8/3)
@end itemize

@geindex AI-0198 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0198 Inheriting abstract operators  (0000-00-00)}

This AI resolves a conflict between two rules involving inherited abstract
operations and predefined operators. If a derived numeric type inherits
an abstract operator, it overrides the predefined one. This interpretation
was always the one implemented in GNAT.

RM References:  3.09.03 (4/3)
@end itemize

@geindex AI-0073 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0073 Functions returning abstract types (2010-07-10)}

This AI covers a number of issues regarding returning abstract types. In
particular generic functions cannot have abstract result types or access
result types designated an abstract type. There are some other cases which
are detailed in the AI. Note that this binding interpretation has not been
retrofitted to operate before Ada 2012 mode, since it caused a significant
number of regressions.

RM References:  3.09.03 (8)   3.09.03 (10)   6.05 (8/2)
@end itemize

@geindex AI-0070 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0070 Elaboration of interface types (0000-00-00)}

This is an editorial change only, there are no testable consequences short of
checking for the absence of generated code for an interface declaration.

RM References:  3.09.04 (18/2)
@end itemize

@geindex AI-0208 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0208 Characteristics of incomplete views (0000-00-00)}

The wording in the Ada 2005 RM concerning characteristics of incomplete views
was incorrect and implied that some programs intended to be legal were now
illegal. GNAT had never considered such programs illegal, so it has always
implemented the intent of this AI.

RM References:  3.10.01 (2.4/2)   3.10.01 (2.6/2)
@end itemize

@geindex AI-0162 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0162 Incomplete type completed by partial view (2010-09-15)}

Incomplete types are made more useful by allowing them to be completed by
private types and private extensions.

RM References:  3.10.01 (2.5/2)   3.10.01 (2.6/2)   3.10.01 (3)   3.10.01 (4/2)
@end itemize

@geindex AI-0098 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0098 Anonymous subprogram access restrictions (0000-00-00)}

An unintentional omission in the RM implied some inconsistent restrictions on
the use of anonymous access to subprogram values. These restrictions were not
intentional, and have never been enforced by GNAT.

RM References:  3.10.01 (6)   3.10.01 (9.2/2)
@end itemize

@geindex AI-0199 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0199 Aggregate with anonymous access components (2010-07-14)}

A choice list in a record aggregate can include several components of
(distinct) anonymous access types as long as they have matching designated
subtypes.

RM References:  4.03.01 (16)
@end itemize

@geindex AI-0220 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0220 Needed components for aggregates (0000-00-00)}

This AI addresses a wording problem in the RM that appears to permit some
complex cases of aggregates with non-static discriminants. GNAT has always
implemented the intended semantics.

RM References:  4.03.01 (17)
@end itemize

@geindex AI-0147 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0147 Conditional expressions (2009-03-29)}

Conditional expressions are permitted. The form of such an expression is:

@example
(if expr then expr @{elsif expr then expr@} [else expr])
@end example

The parentheses can be omitted in contexts where parentheses are present
anyway, such as subprogram arguments and pragma arguments. If the @strong{else}
clause is omitted, @strong{else} @emph{True} is assumed;
thus @code{(if A then B)} is a way to conveniently represent
@emph{(A implies B)} in standard logic.

RM References:  4.03.03 (15)   4.04 (1)   4.04 (7)   4.05.07 (0)   4.07 (2)
4.07 (3)   4.09 (12)   4.09 (33)   5.03 (3)   5.03 (4)   7.05 (2.1/2)
@end itemize

@geindex AI-0037 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0037 Out-of-range box associations in aggregate (0000-00-00)}

This AI confirms that an association of the form @cite{Indx => <>} in an
array aggregate must raise @cite{Constraint_Error} if @cite{Indx}
is out of range. The RM specified a range check on other associations, but
not when the value of the association was defaulted. GNAT has always inserted
a constraint check on the index value.

RM References:  4.03.03 (29)
@end itemize

@geindex AI-0123 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0123 Composability of equality (2010-04-13)}

Equality of untagged record composes, so that the predefined equality for a
composite type that includes a component of some untagged record type
@cite{R} uses the equality operation of @cite{R} (which may be user-defined
or predefined). This makes the behavior of untagged records identical to that
of tagged types in this respect.

This change is an incompatibility with previous versions of Ada, but it
corrects a non-uniformity that was often a source of confusion. Analysis of
a large number of industrial programs indicates that in those rare cases
where a composite type had an untagged record component with a user-defined
equality, either there was no use of the composite equality, or else the code
expected the same composability as for tagged types, and thus had a bug that
would be fixed by this change.

RM References:  4.05.02 (9.7/2)   4.05.02 (14)   4.05.02 (15)   4.05.02 (24)
8.05.04 (8)
@end itemize

@geindex AI-0088 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0088 The value of exponentiation (0000-00-00)}

This AI clarifies the equivalence rule given for the dynamic semantics of
exponentiation: the value of the operation can be obtained by repeated
multiplication, but the operation can be implemented otherwise (for example
using the familiar divide-by-two-and-square algorithm, even if this is less
accurate), and does not imply repeated reads of a volatile base.

RM References:  4.05.06 (11)
@end itemize

@geindex AI-0188 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0188 Case expressions (2010-01-09)}

Case expressions are permitted. This allows use of constructs such as:

@example
X := (case Y is when 1 => 2, when 2 => 3, when others => 31)
@end example

RM References:  4.05.07 (0)   4.05.08 (0)   4.09 (12)   4.09 (33)
@end itemize

@geindex AI-0104 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0104 Null exclusion and uninitialized allocator (2010-07-15)}

The assignment @code{Ptr := new not null Some_Ptr;} will raise
@code{Constraint_Error} because the default value of the allocated object is
@strong{null}. This useless construct is illegal in Ada 2012.

RM References:  4.08 (2)
@end itemize

@geindex AI-0157 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0157 Allocation/Deallocation from empty pool (2010-07-11)}

Allocation and Deallocation from an empty storage pool (i.e. allocation or
deallocation of a pointer for which a static storage size clause of zero
has been given) is now illegal and is detected as such. GNAT
previously gave a warning but not an error.

RM References:  4.08 (5.3/2)   13.11.02 (4)   13.11.02 (17)
@end itemize

@geindex AI-0179 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0179 Statement not required after label (2010-04-10)}

It is not necessary to have a statement following a label, so a label
can appear at the end of a statement sequence without the need for putting a
null statement afterwards, but it is not allowable to have only labels and
no real statements in a statement sequence.

RM References:  5.01 (2)
@end itemize

@geindex AI-0139-2 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0139-2 Syntactic sugar for iterators (2010-09-29)}

The new syntax for iterating over arrays and containers is now implemented.
Iteration over containers is for now limited to read-only iterators. Only
default iterators are supported, with the syntax:  @cite{for Elem of C}.

RM References:  5.05
@end itemize

@geindex AI-0134 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0134 Profiles must match for full conformance (0000-00-00)}

For full conformance, the profiles of anonymous-access-to-subprogram
parameters must match. GNAT has always enforced this rule.

RM References:  6.03.01 (18)
@end itemize

@geindex AI-0207 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0207 Mode conformance and access constant (0000-00-00)}

This AI confirms that access_to_constant indication must match for mode
conformance. This was implemented in GNAT when the qualifier was originally
introduced in Ada 2005.

RM References:  6.03.01 (16/2)
@end itemize

@geindex AI-0046 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0046 Null exclusion match for full conformance (2010-07-17)}

For full conformance, in the case of access parameters, the null exclusion
must match (either both or neither must have @code{not null}).

RM References:  6.03.02 (18)
@end itemize

@geindex AI-0118 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0118 The association of parameter associations (0000-00-00)}

This AI clarifies the rules for named associations in subprogram calls and
generic instantiations. The rules have been in place since Ada 83.

RM References:  6.04.01 (2)   12.03 (9)
@end itemize

@geindex AI-0196 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0196 Null exclusion tests for out parameters (0000-00-00)}

Null exclusion checks are not made for @cite{**out**} parameters when
evaluating the actual parameters. GNAT has never generated these checks.

RM References:  6.04.01 (13)
@end itemize

@geindex AI-0015 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0015 Constant return objects (0000-00-00)}

The return object declared in an @emph{extended_return_statement} may be
declared constant. This was always intended, and GNAT has always allowed it.

RM References:  6.05 (2.1/2)   3.03 (10/2)   3.03 (21)   6.05 (5/2)
6.05 (5.7/2)
@end itemize

@geindex AI-0032 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0032 Extended return for class-wide functions (0000-00-00)}

If a function returns a class-wide type, the object of an extended return
statement can be declared with a specific type that is covered by the class-
wide type. This has been implemented in GNAT since the introduction of
extended returns. Note AI-0103 complements this AI by imposing matching
rules for constrained return types.

RM References:  6.05 (5.2/2)   6.05 (5.3/2)   6.05 (5.6/2)   6.05 (5.8/2)
6.05 (8/2)
@end itemize

@geindex AI-0103 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0103 Static matching for extended return (2010-07-23)}

If the return subtype of a function is an elementary type or a constrained
type, the subtype indication in an extended return statement must match
statically this return subtype.

RM References:  6.05 (5.2/2)
@end itemize

@geindex AI-0058 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0058 Abnormal completion of an extended return (0000-00-00)}

The RM had some incorrect wording implying wrong treatment of abnormal
completion in an extended return. GNAT has always implemented the intended
correct semantics as described by this AI.

RM References:  6.05 (22/2)
@end itemize

@geindex AI-0050 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0050 Raising Constraint_Error early for function call (0000-00-00)}

The implementation permissions for raising @cite{Constraint_Error} early on a function call
when it was clear an exception would be raised were over-permissive and allowed
mishandling of discriminants in some cases. GNAT did
not take advantage of these incorrect permissions in any case.

RM References:  6.05 (24/2)
@end itemize

@geindex AI-0125 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0125 Nonoverridable operations of an ancestor (2010-09-28)}

In Ada 2012, the declaration of a primitive operation of a type extension
or private extension can also override an inherited primitive that is not
visible at the point of this declaration.

RM References:  7.03.01 (6)   8.03 (23)   8.03.01 (5/2)   8.03.01 (6/2)
@end itemize

@geindex AI-0062 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0062 Null exclusions and deferred constants (0000-00-00)}

A full constant may have a null exclusion even if its associated deferred
constant does not. GNAT has always allowed this.

RM References:  7.04 (6/2)   7.04 (7.1/2)
@end itemize

@geindex AI-0178 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0178 Incomplete views are limited (0000-00-00)}

This AI clarifies the role of incomplete views and plugs an omission in the
RM. GNAT always correctly restricted the use of incomplete views and types.

RM References:  7.05 (3/2)   7.05 (6/2)
@end itemize

@geindex AI-0087 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0087 Actual for formal nonlimited derived type (2010-07-15)}

The actual for a formal nonlimited derived type cannot be limited. In
particular, a formal derived type that extends a limited interface but which
is not explicitly limited cannot be instantiated with a limited type.

RM References:  7.05 (5/2)   12.05.01 (5.1/2)
@end itemize

@geindex AI-0099 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0099 Tag determines whether finalization needed (0000-00-00)}

This AI clarifies that 'needs finalization' is part of dynamic semantics,
and therefore depends on the run-time characteristics of an object (i.e. its
tag) and not on its nominal type. As the AI indicates: "we do not expect
this to affect any implementation'@w{'}.

RM References:  7.06.01 (6)   7.06.01 (7)   7.06.01 (8)   7.06.01 (9/2)
@end itemize

@geindex AI-0064 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0064 Redundant finalization rule (0000-00-00)}

This is an editorial change only. The intended behavior is already checked
by an existing ACATS test, which GNAT has always executed correctly.

RM References:  7.06.01 (17.1/1)
@end itemize

@geindex AI-0026 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0026 Missing rules for Unchecked_Union (2010-07-07)}

Record representation clauses concerning Unchecked_Union types cannot mention
the discriminant of the type. The type of a component declared in the variant
part of an Unchecked_Union cannot be controlled, have controlled components,
nor have protected or task parts. If an Unchecked_Union type is declared
within the body of a generic unit or its descendants, then the type of a
component declared in the variant part cannot be a formal private type or a
formal private extension declared within the same generic unit.

RM References:  7.06 (9.4/2)   B.03.03 (9/2)   B.03.03 (10/2)
@end itemize

@geindex AI-0205 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0205 Extended return declares visible name (0000-00-00)}

This AI corrects a simple omission in the RM. Return objects have always
been visible within an extended return statement.

RM References:  8.03 (17)
@end itemize

@geindex AI-0042 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0042 Overriding versus implemented-by (0000-00-00)}

This AI fixes a wording gap in the RM. An operation of a synchronized
interface can be implemented by a protected or task entry, but the abstract
operation is not being overridden in the usual sense, and it must be stated
separately that this implementation is legal. This has always been the case
in GNAT.

RM References:  9.01 (9.2/2)   9.04 (11.1/2)
@end itemize

@geindex AI-0030 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0030 Requeue on synchronized interfaces (2010-07-19)}

Requeue is permitted to a protected, synchronized or task interface primitive
providing it is known that the overriding operation is an entry. Otherwise
the requeue statement has the same effect as a procedure call. Use of pragma
@cite{Implemented} provides a way to impose a static requirement on the
overriding operation by adhering to one of the implementation kinds: entry,
protected procedure or any of the above.

RM References:  9.05 (9)   9.05.04 (2)   9.05.04 (3)   9.05.04 (5)
9.05.04 (6)   9.05.04 (7)   9.05.04 (12)
@end itemize

@geindex AI-0201 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0201 Independence of atomic object components (2010-07-22)}

If an Atomic object has a pragma @cite{Pack} or a @cite{Component_Size}
attribute, then individual components may not be addressable by independent
tasks. However, if the representation clause has no effect (is confirming),
then independence is not compromised. Furthermore, in GNAT, specification of
other appropriately addressable component sizes (e.g. 16 for 8-bit
characters) also preserves independence. GNAT now gives very clear warnings
both for the declaration of such a type, and for any assignment to its components.

RM References:  9.10 (1/3)   C.06 (22/2)   C.06 (23/2)
@end itemize

@geindex AI-0009 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0009 Pragma Independent[_Components] (2010-07-23)}

This AI introduces the new pragmas @cite{Independent} and
@cite{Independent_Components},
which control guaranteeing independence of access to objects and components.
The AI also requires independence not unaffected by confirming rep clauses.

RM References:  9.10 (1)   13.01 (15/1)   13.02 (9)   13.03 (13)   C.06 (2)
C.06 (4)   C.06 (6)   C.06 (9)   C.06 (13)   C.06 (14)
@end itemize

@geindex AI-0072 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0072 Task signalling using 'Terminated (0000-00-00)}

This AI clarifies that task signalling for reading @cite{'Terminated} only
occurs if the result is True. GNAT semantics has always been consistent with
this notion of task signalling.

RM References:  9.10 (6.1/1)
@end itemize

@geindex AI-0108 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0108 Limited incomplete view and discriminants (0000-00-00)}

This AI confirms that an incomplete type from a limited view does not have
discriminants. This has always been the case in GNAT.

RM References:  10.01.01 (12.3/2)
@end itemize

@geindex AI-0129 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0129 Limited views and incomplete types (0000-00-00)}

This AI clarifies the description of limited views: a limited view of a
package includes only one view of a type that has an incomplete declaration
and a full declaration (there is no possible ambiguity in a client package).
This AI also fixes an omission: a nested package in the private part has no
limited view. GNAT always implemented this correctly.

RM References:  10.01.01 (12.2/2)   10.01.01 (12.3/2)
@end itemize

@geindex AI-0077 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0077 Limited withs and scope of declarations (0000-00-00)}

This AI clarifies that a declaration does not include a context clause,
and confirms that it is illegal to have a context in which both a limited
and a nonlimited view of a package are accessible. Such double visibility
was always rejected by GNAT.

RM References:  10.01.02 (12/2)   10.01.02 (21/2)   10.01.02 (22/2)
@end itemize

@geindex AI-0122 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0122 Private with and children of generics (0000-00-00)}

This AI clarifies the visibility of private children of generic units within
instantiations of a parent. GNAT has always handled this correctly.

RM References:  10.01.02 (12/2)
@end itemize

@geindex AI-0040 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0040 Limited with clauses on descendant (0000-00-00)}

This AI confirms that a limited with clause in a child unit cannot name
an ancestor of the unit. This has always been checked in GNAT.

RM References:  10.01.02 (20/2)
@end itemize

@geindex AI-0132 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0132 Placement of library unit pragmas (0000-00-00)}

This AI fills a gap in the description of library unit pragmas. The pragma
clearly must apply to a library unit, even if it does not carry the name
of the enclosing unit. GNAT has always enforced the required check.

RM References:  10.01.05 (7)
@end itemize

@geindex AI-0034 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0034 Categorization of limited views (0000-00-00)}

The RM makes certain limited with clauses illegal because of categorization
considerations, when the corresponding normal with would be legal. This is
not intended, and GNAT has always implemented the recommended behavior.

RM References:  10.02.01 (11/1)   10.02.01 (17/2)
@end itemize

@geindex AI-0035 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0035 Inconsistencies with Pure units (0000-00-00)}

This AI remedies some inconsistencies in the legality rules for Pure units.
Derived access types are legal in a pure unit (on the assumption that the
rule for a zero storage pool size has been enforced on the ancestor type).
The rules are enforced in generic instances and in subunits. GNAT has always
implemented the recommended behavior.

RM References:  10.02.01 (15.1/2)   10.02.01 (15.4/2)   10.02.01 (15.5/2)   10.02.01 (17/2)
@end itemize

@geindex AI-0219 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0219 Pure permissions and limited parameters (2010-05-25)}

This AI refines the rules for the cases with limited parameters which do not
allow the implementations to omit 'redundant'. GNAT now properly conforms
to the requirements of this binding interpretation.

RM References:  10.02.01 (18/2)
@end itemize

@geindex AI-0043 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0043 Rules about raising exceptions (0000-00-00)}

This AI covers various omissions in the RM regarding the raising of
exceptions. GNAT has always implemented the intended semantics.

RM References:  11.04.01 (10.1/2)   11 (2)
@end itemize

@geindex AI-0200 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0200 Mismatches in formal package declarations (0000-00-00)}

This AI plugs a gap in the RM which appeared to allow some obviously intended
illegal instantiations. GNAT has never allowed these instantiations.

RM References:  12.07 (16)
@end itemize

@geindex AI-0112 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0112 Detection of duplicate pragmas (2010-07-24)}

This AI concerns giving names to various representation aspects, but the
practical effect is simply to make the use of duplicate
@cite{Atomic[_Components]},
@cite{Volatile[_Components]}, and
@cite{Independent[_Components]} pragmas illegal, and GNAT
now performs this required check.

RM References:  13.01 (8)
@end itemize

@geindex AI-0106 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0106 No representation pragmas on generic formals (0000-00-00)}

The RM appeared to allow representation pragmas on generic formal parameters,
but this was not intended, and GNAT has never permitted this usage.

RM References:  13.01 (9.1/1)
@end itemize

@geindex AI-0012 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0012 Pack/Component_Size for aliased/atomic (2010-07-15)}

It is now illegal to give an inappropriate component size or a pragma
@cite{Pack} that attempts to change the component size in the case of atomic
or aliased components. Previously GNAT ignored such an attempt with a
warning.

RM References:  13.02 (6.1/2)   13.02 (7)   C.06 (10)   C.06 (11)   C.06 (21)
@end itemize

@geindex AI-0039 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0039 Stream attributes cannot be dynamic (0000-00-00)}

The RM permitted the use of dynamic expressions (such as @code{ptr.all})`
for stream attributes, but these were never useful and are now illegal. GNAT
has always regarded such expressions as illegal.

RM References:  13.03 (4)   13.03 (6)   13.13.02 (38/2)
@end itemize

@geindex AI-0095 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0095 Address of intrinsic subprograms (0000-00-00)}

The prefix of @cite{'Address} cannot statically denote a subprogram with
convention @cite{Intrinsic}. The use of the @cite{Address} attribute raises
@cite{Program_Error} if the prefix denotes a subprogram with convention
@cite{Intrinsic}.

RM References:  13.03 (11/1)
@end itemize

@geindex AI-0116 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0116 Alignment of class-wide objects (0000-00-00)}

This AI requires that the alignment of a class-wide object be no greater
than the alignment of any type in the class. GNAT has always followed this
recommendation.

RM References:  13.03 (29)   13.11 (16)
@end itemize

@geindex AI-0146 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0146 Type invariants (2009-09-21)}

Type invariants may be specified for private types using the aspect notation.
Aspect @cite{Type_Invariant} may be specified for any private type,
@cite{Type_Invariant'Class} can
only be specified for tagged types, and is inherited by any descendent of the
tagged types. The invariant is a boolean expression that is tested for being
true in the following situations: conversions to the private type, object
declarations for the private type that are default initialized, and
[@strong{in}] @strong{out}
parameters and returned result on return from any primitive operation for
the type that is visible to a client.
GNAT defines the synonyms @cite{Invariant} for @cite{Type_Invariant} and
@cite{Invariant'Class} for @cite{Type_Invariant'Class}.

RM References:  13.03.03 (00)
@end itemize

@geindex AI-0078 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0078 Relax Unchecked_Conversion alignment rules (0000-00-00)}

In Ada 2012, compilers are required to support unchecked conversion where the
target alignment is a multiple of the source alignment. GNAT always supported
this case (and indeed all cases of differing alignments, doing copies where
required if the alignment was reduced).

RM References:  13.09 (7)
@end itemize

@geindex AI-0195 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0195 Invalid value handling is implementation defined (2010-07-03)}

The handling of invalid values is now designated to be implementation
defined. This is a documentation change only, requiring Annex M in the GNAT
Reference Manual to document this handling.
In GNAT, checks for invalid values are made
only when necessary to avoid erroneous behavior. Operations like assignments
which cannot cause erroneous behavior ignore the possibility of invalid
values and do not do a check. The date given above applies only to the
documentation change, this behavior has always been implemented by GNAT.

RM References:  13.09.01 (10)
@end itemize

@geindex AI-0193 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0193 Alignment of allocators (2010-09-16)}

This AI introduces a new attribute @cite{Max_Alignment_For_Allocation},
analogous to @cite{Max_Size_In_Storage_Elements}, but for alignment instead
of size.

RM References:  13.11 (16)   13.11 (21)   13.11.01 (0)   13.11.01 (1)
13.11.01 (2)   13.11.01 (3)
@end itemize

@geindex AI-0177 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0177 Parameterized expressions (2010-07-10)}

The new Ada 2012 notion of parameterized expressions is implemented. The form
is:

@example
function-specification is (expression)
@end example

This is exactly equivalent to the
corresponding function body that returns the expression, but it can appear
in a package spec. Note that the expression must be parenthesized.

RM References:  13.11.01 (3/2)
@end itemize

@geindex AI-0033 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0033 Attach/Interrupt_Handler in generic (2010-07-24)}

Neither of these two pragmas may appear within a generic template, because
the generic might be instantiated at other than the library level.

RM References:  13.11.02 (16)   C.03.01 (7/2)   C.03.01 (8/2)
@end itemize

@geindex AI-0161 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0161 Restriction No_Default_Stream_Attributes (2010-09-11)}

A new restriction @cite{No_Default_Stream_Attributes} prevents the use of any
of the default stream attributes for elementary types. If this restriction is
in force, then it is necessary to provide explicit subprograms for any
stream attributes used.

RM References:  13.12.01 (4/2)   13.13.02 (40/2)   13.13.02 (52/2)
@end itemize

@geindex AI-0194 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0194 Value of Stream_Size attribute (0000-00-00)}

The @cite{Stream_Size} attribute returns the default number of bits in the
stream representation of the given type.
This value is not affected by the presence
of stream subprogram attributes for the type. GNAT has always implemented
this interpretation.

RM References:  13.13.02 (1.2/2)
@end itemize

@geindex AI-0109 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0109 Redundant check in S'Class'Input (0000-00-00)}

This AI is an editorial change only. It removes the need for a tag check
that can never fail.

RM References:  13.13.02 (34/2)
@end itemize

@geindex AI-0007 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0007 Stream read and private scalar types (0000-00-00)}

The RM as written appeared to limit the possibilities of declaring read
attribute procedures for private scalar types. This limitation was not
intended, and has never been enforced by GNAT.

RM References:  13.13.02 (50/2)   13.13.02 (51/2)
@end itemize

@geindex AI-0065 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0065 Remote access types and external streaming (0000-00-00)}

This AI clarifies the fact that all remote access types support external
streaming. This fixes an obvious oversight in the definition of the
language, and GNAT always implemented the intended correct rules.

RM References:  13.13.02 (52/2)
@end itemize

@geindex AI-0019 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0019 Freezing of primitives for tagged types (0000-00-00)}

The RM suggests that primitive subprograms of a specific tagged type are
frozen when the tagged type is frozen. This would be an incompatible change
and is not intended. GNAT has never attempted this kind of freezing and its
behavior is consistent with the recommendation of this AI.

RM References:  13.14 (2)   13.14 (3/1)   13.14 (8.1/1)   13.14 (10)   13.14 (14)   13.14 (15.1/2)
@end itemize

@geindex AI-0017 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0017 Freezing and incomplete types (0000-00-00)}

So-called 'Taft-amendment types' (i.e., types that are completed in package
bodies) are not frozen by the occurrence of bodies in the
enclosing declarative part. GNAT always implemented this properly.

RM References:  13.14 (3/1)
@end itemize

@geindex AI-0060 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0060 Extended definition of remote access types (0000-00-00)}

This AI extends the definition of remote access types to include access
to limited, synchronized, protected or task class-wide interface types.
GNAT already implemented this extension.

RM References:  A (4)   E.02.02 (9/1)   E.02.02 (9.2/1)   E.02.02 (14/2)   E.02.02 (18)
@end itemize

@geindex AI-0114 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0114 Classification of letters (0000-00-00)}

The code points 170 (@cite{FEMININE ORDINAL INDICATOR}),
181 (@cite{MICRO SIGN}), and
186 (@cite{MASCULINE ORDINAL INDICATOR}) are technically considered
lower case letters by Unicode.
However, they are not allowed in identifiers, and they
return @cite{False} to @cite{Ada.Characters.Handling.Is_Letter/Is_Lower}.
This behavior is consistent with that defined in Ada 95.

RM References:  A.03.02 (59)   A.04.06 (7)
@end itemize

@geindex AI-0185 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0185 Ada.Wide_[Wide_]Characters.Handling (2010-07-06)}

Two new packages @cite{Ada.Wide_[Wide_]Characters.Handling} provide
classification functions for @cite{Wide_Character} and
@cite{Wide_Wide_Character}, as well as providing
case folding routines for @cite{Wide_[Wide_]Character} and
@cite{Wide_[Wide_]String}.

RM References:  A.03.05 (0)   A.03.06 (0)
@end itemize

@geindex AI-0031 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0031 Add From parameter to Find_Token (2010-07-25)}

A new version of @cite{Find_Token} is added to all relevant string packages,
with an extra parameter @cite{From}. Instead of starting at the first
character of the string, the search for a matching Token starts at the
character indexed by the value of @cite{From}.
These procedures are available in all versions of Ada
but if used in versions earlier than Ada 2012 they will generate a warning
that an Ada 2012 subprogram is being used.

RM References:  A.04.03 (16)   A.04.03 (67)   A.04.03 (68/1)   A.04.04 (51)
A.04.05 (46)
@end itemize

@geindex AI-0056 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0056 Index on null string returns zero (0000-00-00)}

The wording in the Ada 2005 RM implied an incompatible handling of the
@cite{Index} functions, resulting in raising an exception instead of
returning zero in some situations.
This was not intended and has been corrected.
GNAT always returned zero, and is thus consistent with this AI.

RM References:  A.04.03 (56.2/2)   A.04.03 (58.5/2)
@end itemize

@geindex AI-0137 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0137 String encoding package (2010-03-25)}

The packages @cite{Ada.Strings.UTF_Encoding}, together with its child
packages, @cite{Conversions}, @cite{Strings}, @cite{Wide_Strings},
and @cite{Wide_Wide_Strings} have been
implemented. These packages (whose documentation can be found in the spec
files @code{a-stuten.ads}, @code{a-suenco.ads}, @code{a-suenst.ads},
@code{a-suewst.ads}, @code{a-suezst.ads}) allow encoding and decoding of
@cite{String}, @cite{Wide_String}, and @cite{Wide_Wide_String}
values using UTF coding schemes (including UTF-8, UTF-16LE, UTF-16BE, and
UTF-16), as well as conversions between the different UTF encodings. With
the exception of @cite{Wide_Wide_Strings}, these packages are available in
Ada 95 and Ada 2005 mode as well as Ada 2012 mode.
The @cite{Wide_Wide_Strings package}
is available in Ada 2005 mode as well as Ada 2012 mode (but not in Ada 95
mode since it uses @cite{Wide_Wide_Character}).

RM References:  A.04.11
@end itemize

@geindex AI-0038 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0038 Minor errors in Text_IO (0000-00-00)}

These are minor errors in the description on three points. The intent on
all these points has always been clear, and GNAT has always implemented the
correct intended semantics.

RM References:  A.10.05 (37)   A.10.07 (8/1)   A.10.07 (10)   A.10.07 (12)   A.10.08 (10)   A.10.08 (24)
@end itemize

@geindex AI-0044 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0044 Restrictions on container instantiations (0000-00-00)}

This AI places restrictions on allowed instantiations of generic containers.
These restrictions are not checked by the compiler, so there is nothing to
change in the implementation. This affects only the RM documentation.

RM References:  A.18 (4/2)   A.18.02 (231/2)   A.18.03 (145/2)   A.18.06 (56/2)   A.18.08 (66/2)   A.18.09 (79/2)   A.18.26 (5/2)   A.18.26 (9/2)
@end itemize

@geindex AI-0127 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0127 Adding Locale Capabilities (2010-09-29)}

This package provides an interface for identifying the current locale.

RM References:  A.19    A.19.01    A.19.02    A.19.03    A.19.05    A.19.06
A.19.07    A.19.08    A.19.09    A.19.10    A.19.11    A.19.12    A.19.13
@end itemize

@geindex AI-0002 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0002 Export C with unconstrained arrays (0000-00-00)}

The compiler is not required to support exporting an Ada subprogram with
convention C if there are parameters or a return type of an unconstrained
array type (such as @cite{String}). GNAT allows such declarations but
generates warnings. It is possible, but complicated, to write the
corresponding C code and certainly such code would be specific to GNAT and
non-portable.

RM References:  B.01 (17)   B.03 (62)   B.03 (71.1/2)
@end itemize

@geindex AI05-0216 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0216 No_Task_Hierarchy forbids local tasks (0000-00-00)}

It is clearly the intention that @cite{No_Task_Hierarchy} is intended to
forbid tasks declared locally within subprograms, or functions returning task
objects, and that is the implementation that GNAT has always provided.
However the language in the RM was not sufficiently clear on this point.
Thus this is a documentation change in the RM only.

RM References:  D.07 (3/3)
@end itemize

@geindex AI-0211 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0211 No_Relative_Delays forbids Set_Handler use (2010-07-09)}

The restriction @cite{No_Relative_Delays} forbids any calls to the subprogram
@cite{Ada.Real_Time.Timing_Events.Set_Handler}.

RM References:  D.07 (5)   D.07 (10/2)   D.07 (10.4/2)   D.07 (10.7/2)
@end itemize

@geindex AI-0190 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0190 pragma Default_Storage_Pool (2010-09-15)}

This AI introduces a new pragma @cite{Default_Storage_Pool}, which can be
used to control storage pools globally.
In particular, you can force every access
type that is used for allocation (@strong{new}) to have an explicit storage pool,
or you can declare a pool globally to be used for all access types that lack
an explicit one.

RM References:  D.07 (8)
@end itemize

@geindex AI-0189 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0189 No_Allocators_After_Elaboration (2010-01-23)}

This AI introduces a new restriction @cite{No_Allocators_After_Elaboration},
which says that no dynamic allocation will occur once elaboration is
completed.
In general this requires a run-time check, which is not required, and which
GNAT does not attempt. But the static cases of allocators in a task body or
in the body of the main program are detected and flagged at compile or bind
time.

RM References:  D.07 (19.1/2)   H.04 (23.3/2)
@end itemize

@geindex AI-0171 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0171 Pragma CPU and Ravenscar Profile (2010-09-24)}

A new package @cite{System.Multiprocessors} is added, together with the
definition of pragma @cite{CPU} for controlling task affinity. A new no
dependence restriction, on @cite{System.Multiprocessors.Dispatching_Domains},
is added to the Ravenscar profile.

RM References:  D.13.01 (4/2)   D.16
@end itemize

@geindex AI-0210 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0210 Correct Timing_Events metric (0000-00-00)}

This is a documentation only issue regarding wording of metric requirements,
that does not affect the implementation of the compiler.

RM References:  D.15 (24/2)
@end itemize

@geindex AI-0206 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0206 Remote types packages and preelaborate (2010-07-24)}

Remote types packages are now allowed to depend on preelaborated packages.
This was formerly considered illegal.

RM References:  E.02.02 (6)
@end itemize

@geindex AI-0152 (Ada 2012 feature)


@itemize *

@item 
@emph{AI-0152 Restriction No_Anonymous_Allocators (2010-09-08)}

Restriction @cite{No_Anonymous_Allocators} prevents the use of allocators
where the type of the returned value is an anonymous access type.

RM References:  H.04 (8/1)
@end itemize

@node Obsolescent Features,Compatibility and Porting Guide,Implementation of Ada 2012 Features,Top
@anchor{gnat_rm/obsolescent_features id1}@anchor{3c9}@anchor{gnat_rm/obsolescent_features doc}@anchor{3ca}@anchor{gnat_rm/obsolescent_features obsolescent-features}@anchor{15}
@chapter Obsolescent Features


This chapter describes features that are provided by GNAT, but are
considered obsolescent since there are preferred ways of achieving
the same effect. These features are provided solely for historical
compatibility purposes.

@menu
* pragma No_Run_Time:: 
* pragma Ravenscar:: 
* pragma Restricted_Run_Time:: 
* pragma Task_Info:: 
* package System.Task_Info (s-tasinf.ads): package System Task_Info s-tasinf ads. 

@end menu

@node pragma No_Run_Time,pragma Ravenscar,,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id2}@anchor{3cb}@anchor{gnat_rm/obsolescent_features pragma-no-run-time}@anchor{3cc}
@section pragma No_Run_Time


The pragma @cite{No_Run_Time} is used to achieve an affect similar
to the use of the "Zero Foot Print" configurable run time, but without
requiring a specially configured run time. The result of using this
pragma, which must be used for all units in a partition, is to restrict
the use of any language features requiring run-time support code. The
preferred usage is to use an appropriately configured run-time that
includes just those features that are to be made accessible.

@node pragma Ravenscar,pragma Restricted_Run_Time,pragma No_Run_Time,Obsolescent Features
@anchor{gnat_rm/obsolescent_features id3}@anchor{3cd}@anchor{gnat_rm/obsolescent_features pragma-ravenscar}@anchor{3ce}
@section pragma Ravenscar


The pragma @cite{Ravenscar} has exactly the same effect as pragma
@cite{Profile (Ravenscar)}. The latter usage is preferred since it
is part of the new Ada 2005 standard.

@node pragma Restricted_Run_Time,pragma Task_Info,pragma Ravenscar,Obsolescent Features
@anchor{gnat_rm/obsolescent_features pragma-restricted-run-time}@anchor{3cf}@anchor{gnat_rm/obsolescent_features id4}@anchor{3d0}
@section pragma Restricted_Run_Time


The pragma @cite{Restricted_Run_Time} has exactly the same effect as
pragma @cite{Profile (Restricted)}. The latter usage is
preferred since the Ada 2005 pragma @cite{Profile} is intended for
this kind of implementation dependent addition.

@node pragma Task_Info,package System Task_Info s-tasinf ads,pragma Restricted_Run_Time,Obsolescent Features
@anchor{gnat_rm/obsolescent_features pragma-task-info}@anchor{3d1}@anchor{gnat_rm/obsolescent_features id5}@anchor{3d2}
@section pragma Task_Info


The functionality provided by pragma @cite{Task_Info} is now part of the
Ada language. The @cite{CPU} aspect and the package
@cite{System.Multiprocessors} offer a less system-dependent way to specify
task affinity or to query the number of processsors.

Syntax

@example
pragma Task_Info (EXPRESSION);
@end example

This pragma appears within a task definition (like pragma
@cite{Priority}) and applies to the task in which it appears.  The
argument must be of type @cite{System.Task_Info.Task_Info_Type}.
The @cite{Task_Info} pragma provides system dependent control over
aspects of tasking implementation, for example, the ability to map
tasks to specific processors.  For details on the facilities available
for the version of GNAT that you are using, see the documentation
in the spec of package System.Task_Info in the runtime
library.

@node package System Task_Info s-tasinf ads,,pragma Task_Info,Obsolescent Features
@anchor{gnat_rm/obsolescent_features package-system-task-info}@anchor{3d3}@anchor{gnat_rm/obsolescent_features package-system-task-info-s-tasinf-ads}@anchor{3d4}
@section package System.Task_Info (@code{s-tasinf.ads})


This package provides target dependent functionality that is used
to support the @cite{Task_Info} pragma. The predefined Ada package
@cite{System.Multiprocessors} and the @cite{CPU} aspect now provide a
standard replacement for GNAT's @cite{Task_Info} functionality.

@node Compatibility and Porting Guide,GNU Free Documentation License,Obsolescent Features,Top
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-and-porting-guide}@anchor{16}@anchor{gnat_rm/compatibility_and_porting_guide doc}@anchor{3d5}@anchor{gnat_rm/compatibility_and_porting_guide id1}@anchor{3d6}
@chapter Compatibility and Porting Guide


This chapter presents some guidelines for developing portable Ada code,
describes the compatibility issues that may arise between
GNAT and other Ada compilation systems (including those for Ada 83),
and shows how GNAT can expedite porting
applications developed in other Ada environments.

@menu
* Writing Portable Fixed-Point Declarations:: 
* Compatibility with Ada 83:: 
* Compatibility between Ada 95 and Ada 2005:: 
* Implementation-dependent characteristics:: 
* Compatibility with Other Ada Systems:: 
* Representation Clauses:: 
* Compatibility with HP Ada 83:: 

@end menu

@node Writing Portable Fixed-Point Declarations,Compatibility with Ada 83,,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide id2}@anchor{3d7}@anchor{gnat_rm/compatibility_and_porting_guide writing-portable-fixed-point-declarations}@anchor{3d8}
@section Writing Portable Fixed-Point Declarations


The Ada Reference Manual gives an implementation freedom to choose bounds
that are narrower by @cite{Small} from the given bounds.
For example, if we write

@example
type F1 is delta 1.0 range -128.0 .. +128.0;
@end example

then the implementation is allowed to choose -128.0 .. +127.0 if it
likes, but is not required to do so.

This leads to possible portability problems, so let's have a closer
look at this, and figure out how to avoid these problems.

First, why does this freedom exist, and why would an implementation
take advantage of it? To answer this, take a closer look at the type
declaration for @cite{F1} above. If the compiler uses the given bounds,
it would need 9 bits to hold the largest positive value (and typically
that means 16 bits on all machines). But if the implementation chooses
the +127.0 bound then it can fit values of the type in 8 bits.

Why not make the user write +127.0 if that's what is wanted?
The rationale is that if you are thinking of fixed point
as a kind of 'poor man's floating-point', then you don't want
to be thinking about the scaled integers that are used in its
representation. Let's take another example:

@example
type F2 is delta 2.0**(-15) range -1.0 .. +1.0;
@end example

Looking at this declaration, it seems casually as though
it should fit in 16 bits, but again that extra positive value
+1.0 has the scaled integer equivalent of 2**15 which is one too
big for signed 16 bits. The implementation can treat this as:

@example
type F2 is delta 2.0**(-15) range -1.0 .. +1.0-(2.0**(-15));
@end example

and the Ada language design team felt that this was too annoying
to require. We don't need to debate this decision at this point,
since it is well established (the rule about narrowing the ranges
dates to Ada 83).

But the important point is that an implementation is not required
to do this narrowing, so we have a potential portability problem.
We could imagine three types of implementation:


@enumerate a

@item 
those that narrow the range automatically if they can figure
out that the narrower range will allow storage in a smaller machine unit,

@item 
those that will narrow only if forced to by a @cite{'Size} clause, and

@item 
those that will never narrow.
@end enumerate

Now if we are language theoreticians, we can imagine a fourth
approach: to narrow all the time, e.g. to treat

@example
type F3 is delta 1.0 range -10.0 .. +23.0;
@end example

as though it had been written:

@example
type F3 is delta 1.0 range -9.0 .. +22.0;
@end example

But although technically allowed, such a behavior would be hostile and silly,
and no real compiler would do this. All real compilers will fall into one of
the categories (a), (b) or (c) above.

So, how do you get the compiler to do what you want? The answer is give the
actual bounds you want, and then use a @cite{'Small} clause and a
@cite{'Size} clause to absolutely pin down what the compiler does.
E.g., for @cite{F2} above, we will write:

@example
My_Small : constant := 2.0**(-15);
My_First : constant := -1.0;
My_Last  : constant := +1.0 - My_Small;

type F2 is delta My_Small range My_First .. My_Last;
@end example

and then add

@example
for F2'Small use my_Small;
for F2'Size  use 16;
@end example

In practice all compilers will do the same thing here and will give you
what you want, so the above declarations are fully portable. If you really
want to play language lawyer and guard against ludicrous behavior by the
compiler you could add

@example
Test1 : constant := 1 / Boolean'Pos (F2'First = My_First);
Test2 : constant := 1 / Boolean'Pos (F2'Last  = My_Last);
@end example

One or other or both are allowed to be illegal if the compiler is
behaving in a silly manner, but at least the silly compiler will not
get away with silently messing with your (very clear) intentions.

If you follow this scheme you will be guaranteed that your fixed-point
types will be portable.

@node Compatibility with Ada 83,Compatibility between Ada 95 and Ada 2005,Writing Portable Fixed-Point Declarations,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-with-ada-83}@anchor{3d9}@anchor{gnat_rm/compatibility_and_porting_guide id3}@anchor{3da}
@section Compatibility with Ada 83


@geindex Compatibility (between Ada 83 and Ada 95 / Ada 2005 / Ada 2012)

Ada 95 and the subsequent revisions Ada 2005 and Ada 2012
are highly upwards compatible with Ada 83.  In
particular, the design intention was that the difficulties associated
with moving from Ada 83 to later versions of the standard should be no greater
than those that occur when moving from one Ada 83 system to another.

However, there are a number of points at which there are minor
incompatibilities.  The @cite{Ada 95 Annotated Reference Manual} contains
full details of these issues as they relate to Ada 95,
and should be consulted for a complete treatment.
In practice the
following subsections treat the most likely issues to be encountered.

@menu
* Legal Ada 83 programs that are illegal in Ada 95:: 
* More deterministic semantics:: 
* Changed semantics:: 
* Other language compatibility issues:: 

@end menu

@node Legal Ada 83 programs that are illegal in Ada 95,More deterministic semantics,,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide id4}@anchor{3db}@anchor{gnat_rm/compatibility_and_porting_guide legal-ada-83-programs-that-are-illegal-in-ada-95}@anchor{3dc}
@subsection Legal Ada 83 programs that are illegal in Ada 95


Some legal Ada 83 programs are illegal (i.e., they will fail to compile) in
Ada 95 and later versions of the standard:


@itemize *

@item 
@emph{Character literals}

Some uses of character literals are ambiguous.  Since Ada 95 has introduced
@cite{Wide_Character} as a new predefined character type, some uses of
character literals that were legal in Ada 83 are illegal in Ada 95.
For example:

@example
for Char in 'A' .. 'Z' loop ... end loop;
@end example

The problem is that 'A' and 'Z' could be from either
@cite{Character} or @cite{Wide_Character}.  The simplest correction
is to make the type explicit; e.g.:

@example
for Char in Character range 'A' .. 'Z' loop ... end loop;
@end example

@item 
@emph{New reserved words}

The identifiers @cite{abstract}, @cite{aliased}, @cite{protected},
@cite{requeue}, @cite{tagged}, and @cite{until} are reserved in Ada 95.
Existing Ada 83 code using any of these identifiers must be edited to
use some alternative name.

@item 
@emph{Freezing rules}

The rules in Ada 95 are slightly different with regard to the point at
which entities are frozen, and representation pragmas and clauses are
not permitted past the freeze point.  This shows up most typically in
the form of an error message complaining that a representation item
appears too late, and the appropriate corrective action is to move
the item nearer to the declaration of the entity to which it refers.

A particular case is that representation pragmas
cannot be applied to a subprogram body.  If necessary, a separate subprogram
declaration must be introduced to which the pragma can be applied.

@item 
@emph{Optional bodies for library packages}

In Ada 83, a package that did not require a package body was nevertheless
allowed to have one.  This lead to certain surprises in compiling large
systems (situations in which the body could be unexpectedly ignored by the
binder).  In Ada 95, if a package does not require a body then it is not
permitted to have a body.  To fix this problem, simply remove a redundant
body if it is empty, or, if it is non-empty, introduce a dummy declaration
into the spec that makes the body required.  One approach is to add a private
part to the package declaration (if necessary), and define a parameterless
procedure called @cite{Requires_Body}, which must then be given a dummy
procedure body in the package body, which then becomes required.
Another approach (assuming that this does not introduce elaboration
circularities) is to add an @cite{Elaborate_Body} pragma to the package spec,
since one effect of this pragma is to require the presence of a package body.

@item 
@emph{Numeric_Error is the same exception as Constraint_Error}

In Ada 95, the exception @cite{Numeric_Error} is a renaming of @cite{Constraint_Error}.
This means that it is illegal to have separate exception handlers for
the two exceptions.  The fix is simply to remove the handler for the
@cite{Numeric_Error} case (since even in Ada 83, a compiler was free to raise
@cite{Constraint_Error} in place of @cite{Numeric_Error} in all cases).

@item 
@emph{Indefinite subtypes in generics}

In Ada 83, it was permissible to pass an indefinite type (e.g, @cite{String})
as the actual for a generic formal private type, but then the instantiation
would be illegal if there were any instances of declarations of variables
of this type in the generic body.  In Ada 95, to avoid this clear violation
of the methodological principle known as the 'contract model',
the generic declaration explicitly indicates whether
or not such instantiations are permitted.  If a generic formal parameter
has explicit unknown discriminants, indicated by using @cite{(<>)} after the
subtype name, then it can be instantiated with indefinite types, but no
stand-alone variables can be declared of this type.  Any attempt to declare
such a variable will result in an illegality at the time the generic is
declared.  If the @cite{(<>)} notation is not used, then it is illegal
to instantiate the generic with an indefinite type.
This is the potential incompatibility issue when porting Ada 83 code to Ada 95.
It will show up as a compile time error, and
the fix is usually simply to add the @cite{(<>)} to the generic declaration.
@end itemize

@node More deterministic semantics,Changed semantics,Legal Ada 83 programs that are illegal in Ada 95,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide more-deterministic-semantics}@anchor{3dd}@anchor{gnat_rm/compatibility_and_porting_guide id5}@anchor{3de}
@subsection More deterministic semantics



@itemize *

@item 
@emph{Conversions}

Conversions from real types to integer types round away from 0.  In Ada 83
the conversion Integer(2.5) could deliver either 2 or 3 as its value.  This
implementation freedom was intended to support unbiased rounding in
statistical applications, but in practice it interfered with portability.
In Ada 95 the conversion semantics are unambiguous, and rounding away from 0
is required.  Numeric code may be affected by this change in semantics.
Note, though, that this issue is no worse than already existed in Ada 83
when porting code from one vendor to another.

@item 
@emph{Tasking}

The Real-Time Annex introduces a set of policies that define the behavior of
features that were implementation dependent in Ada 83, such as the order in
which open select branches are executed.
@end itemize

@node Changed semantics,Other language compatibility issues,More deterministic semantics,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide id6}@anchor{3df}@anchor{gnat_rm/compatibility_and_porting_guide changed-semantics}@anchor{3e0}
@subsection Changed semantics


The worst kind of incompatibility is one where a program that is legal in
Ada 83 is also legal in Ada 95 but can have an effect in Ada 95 that was not
possible in Ada 83.  Fortunately this is extremely rare, but the one
situation that you should be alert to is the change in the predefined type
@cite{Character} from 7-bit ASCII to 8-bit Latin-1.

@quotation

@geindex Latin-1
@end quotation


@itemize *

@item 
@emph{Range of type `Character`}

The range of @cite{Standard.Character} is now the full 256 characters
of Latin-1, whereas in most Ada 83 implementations it was restricted
to 128 characters. Although some of the effects of
this change will be manifest in compile-time rejection of legal
Ada 83 programs it is possible for a working Ada 83 program to have
a different effect in Ada 95, one that was not permitted in Ada 83.
As an example, the expression
@cite{Character'Pos(Character'Last)} returned @cite{127} in Ada 83 and now
delivers @cite{255} as its value.
In general, you should look at the logic of any
character-processing Ada 83 program and see whether it needs to be adapted
to work correctly with Latin-1.  Note that the predefined Ada 95 API has a
character handling package that may be relevant if code needs to be adapted
to account for the additional Latin-1 elements.
The desirable fix is to
modify the program to accommodate the full character set, but in some cases
it may be convenient to define a subtype or derived type of Character that
covers only the restricted range.
@end itemize

@node Other language compatibility issues,,Changed semantics,Compatibility with Ada 83
@anchor{gnat_rm/compatibility_and_porting_guide other-language-compatibility-issues}@anchor{3e1}@anchor{gnat_rm/compatibility_and_porting_guide id7}@anchor{3e2}
@subsection Other language compatibility issues



@itemize *

@item 
@emph{-gnat83} switch

All implementations of GNAT provide a switch that causes GNAT to operate
in Ada 83 mode.  In this mode, some but not all compatibility problems
of the type described above are handled automatically.  For example, the
new reserved words introduced in Ada 95 and Ada 2005 are treated simply
as identifiers as in Ada 83.  However,
in practice, it is usually advisable to make the necessary modifications
to the program to remove the need for using this switch.
See the @cite{Compiling Different Versions of Ada} section in
the @cite{GNAT User's Guide}.

@item 
Support for removed Ada 83 pragmas and attributes

A number of pragmas and attributes from Ada 83 were removed from Ada 95,
generally because they were replaced by other mechanisms.  Ada 95 and Ada 2005
compilers are allowed, but not required, to implement these missing
elements.  In contrast with some other compilers, GNAT implements all
such pragmas and attributes, eliminating this compatibility concern.  These
include @cite{pragma Interface} and the floating point type attributes
(@cite{Emax}, @cite{Mantissa}, etc.), among other items.
@end itemize

@node Compatibility between Ada 95 and Ada 2005,Implementation-dependent characteristics,Compatibility with Ada 83,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-between-ada-95-and-ada-2005}@anchor{3e3}@anchor{gnat_rm/compatibility_and_porting_guide id8}@anchor{3e4}
@section Compatibility between Ada 95 and Ada 2005


@geindex Compatibility between Ada 95 and Ada 2005

Although Ada 2005 was designed to be upwards compatible with Ada 95, there are
a number of incompatibilities. Several are enumerated below;
for a complete description please see the
@cite{Annotated Ada 2005 Reference Manual}, or section 9.1.1 in
@cite{Rationale for Ada 2005}.


@itemize *

@item 
@emph{New reserved words.}

The words @cite{interface}, @cite{overriding} and @cite{synchronized} are
reserved in Ada 2005.
A pre-Ada 2005 program that uses any of these as an identifier will be
illegal.

@item 
@emph{New declarations in predefined packages.}

A number of packages in the predefined environment contain new declarations:
@cite{Ada.Exceptions}, @cite{Ada.Real_Time}, @cite{Ada.Strings},
@cite{Ada.Strings.Fixed}, @cite{Ada.Strings.Bounded},
@cite{Ada.Strings.Unbounded}, @cite{Ada.Strings.Wide_Fixed},
@cite{Ada.Strings.Wide_Bounded}, @cite{Ada.Strings.Wide_Unbounded},
@cite{Ada.Tags}, @cite{Ada.Text_IO}, and @cite{Interfaces.C}.
If an Ada 95 program does a @cite{with} and @cite{use} of any of these
packages, the new declarations may cause name clashes.

@item 
@emph{Access parameters.}

A nondispatching subprogram with an access parameter cannot be renamed
as a dispatching operation.  This was permitted in Ada 95.

@item 
@emph{Access types, discriminants, and constraints.}

Rule changes in this area have led to some incompatibilities; for example,
constrained subtypes of some access types are not permitted in Ada 2005.

@item 
@emph{Aggregates for limited types.}

The allowance of aggregates for limited types in Ada 2005 raises the
possibility of ambiguities in legal Ada 95 programs, since additional types
now need to be considered in expression resolution.

@item 
@emph{Fixed-point multiplication and division.}

Certain expressions involving '*' or '/' for a fixed-point type, which
were legal in Ada 95 and invoked the predefined versions of these operations,
are now ambiguous.
The ambiguity may be resolved either by applying a type conversion to the
expression, or by explicitly invoking the operation from package
@cite{Standard}.

@item 
@emph{Return-by-reference types.}

The Ada 95 return-by-reference mechanism has been removed.  Instead, the user
can declare a function returning a value from an anonymous access type.
@end itemize

@node Implementation-dependent characteristics,Compatibility with Other Ada Systems,Compatibility between Ada 95 and Ada 2005,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide implementation-dependent-characteristics}@anchor{3e5}@anchor{gnat_rm/compatibility_and_porting_guide id9}@anchor{3e6}
@section Implementation-dependent characteristics


Although the Ada language defines the semantics of each construct as
precisely as practical, in some situations (for example for reasons of
efficiency, or where the effect is heavily dependent on the host or target
platform) the implementation is allowed some freedom.  In porting Ada 83
code to GNAT, you need to be aware of whether / how the existing code
exercised such implementation dependencies.  Such characteristics fall into
several categories, and GNAT offers specific support in assisting the
transition from certain Ada 83 compilers.

@menu
* Implementation-defined pragmas:: 
* Implementation-defined attributes:: 
* Libraries:: 
* Elaboration order:: 
* Target-specific aspects:: 

@end menu

@node Implementation-defined pragmas,Implementation-defined attributes,,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide implementation-defined-pragmas}@anchor{3e7}@anchor{gnat_rm/compatibility_and_porting_guide id10}@anchor{3e8}
@subsection Implementation-defined pragmas


Ada compilers are allowed to supplement the language-defined pragmas, and
these are a potential source of non-portability.  All GNAT-defined pragmas
are described in the @cite{Implementation Defined Pragmas} chapter of the
@cite{GNAT Reference Manual}, and these include several that are specifically
intended to correspond to other vendors' Ada 83 pragmas.
For migrating from VADS, the pragma @cite{Use_VADS_Size} may be useful.
For compatibility with HP Ada 83, GNAT supplies the pragmas
@cite{Extend_System}, @cite{Ident}, @cite{Inline_Generic},
@cite{Interface_Name}, @cite{Passive}, @cite{Suppress_All},
and @cite{Volatile}.
Other relevant pragmas include @cite{External} and @cite{Link_With}.
Some vendor-specific
Ada 83 pragmas (@cite{Share_Generic}, @cite{Subtitle}, and @cite{Title}) are
recognized, thus
avoiding compiler rejection of units that contain such pragmas; they are not
relevant in a GNAT context and hence are not otherwise implemented.

@node Implementation-defined attributes,Libraries,Implementation-defined pragmas,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide id11}@anchor{3e9}@anchor{gnat_rm/compatibility_and_porting_guide implementation-defined-attributes}@anchor{3ea}
@subsection Implementation-defined attributes


Analogous to pragmas, the set of attributes may be extended by an
implementation.  All GNAT-defined attributes are described in
@cite{Implementation Defined Attributes} section of the
@cite{GNAT Reference Manual}, and these include several that are specifically intended
to correspond to other vendors' Ada 83 attributes.  For migrating from VADS,
the attribute @cite{VADS_Size} may be useful.  For compatibility with HP
Ada 83, GNAT supplies the attributes @cite{Bit}, @cite{Machine_Size} and
@cite{Type_Class}.

@node Libraries,Elaboration order,Implementation-defined attributes,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide libraries}@anchor{3eb}@anchor{gnat_rm/compatibility_and_porting_guide id12}@anchor{3ec}
@subsection Libraries


Vendors may supply libraries to supplement the standard Ada API.  If Ada 83
code uses vendor-specific libraries then there are several ways to manage
this in Ada 95 and later versions of the standard:


@itemize *

@item 
If the source code for the libraries (specs and bodies) are
available, then the libraries can be migrated in the same way as the
application.

@item 
If the source code for the specs but not the bodies are
available, then you can reimplement the bodies.

@item 
Some features introduced by Ada 95 obviate the need for library support.  For
example most Ada 83 vendors supplied a package for unsigned integers.  The
Ada 95 modular type feature is the preferred way to handle this need, so
instead of migrating or reimplementing the unsigned integer package it may
be preferable to retrofit the application using modular types.
@end itemize

@node Elaboration order,Target-specific aspects,Libraries,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide elaboration-order}@anchor{3ed}@anchor{gnat_rm/compatibility_and_porting_guide id13}@anchor{3ee}
@subsection Elaboration order


The implementation can choose any elaboration order consistent with the unit
dependency relationship.  This freedom means that some orders can result in
Program_Error being raised due to an 'Access Before Elaboration': an attempt
to invoke a subprogram its body has been elaborated, or to instantiate a
generic before the generic body has been elaborated.  By default GNAT
attempts to choose a safe order (one that will not encounter access before
elaboration problems) by implicitly inserting @cite{Elaborate} or
@cite{Elaborate_All} pragmas where
needed.  However, this can lead to the creation of elaboration circularities
and a resulting rejection of the program by gnatbind.  This issue is
thoroughly described in the @cite{Elaboration Order Handling in GNAT} appendix
in the @cite{GNAT User's Guide}.
In brief, there are several
ways to deal with this situation:


@itemize *

@item 
Modify the program to eliminate the circularities, e.g., by moving
elaboration-time code into explicitly-invoked procedures

@item 
Constrain the elaboration order by including explicit @cite{Elaborate_Body} or
@cite{Elaborate} pragmas, and then inhibit the generation of implicit
@cite{Elaborate_All}
pragmas either globally (as an effect of the @emph{-gnatE} switch) or locally
(by selectively suppressing elaboration checks via pragma
@cite{Suppress(Elaboration_Check)} when it is safe to do so).
@end itemize

@node Target-specific aspects,,Elaboration order,Implementation-dependent characteristics
@anchor{gnat_rm/compatibility_and_porting_guide target-specific-aspects}@anchor{3ef}@anchor{gnat_rm/compatibility_and_porting_guide id14}@anchor{3f0}
@subsection Target-specific aspects


Low-level applications need to deal with machine addresses, data
representations, interfacing with assembler code, and similar issues.  If
such an Ada 83 application is being ported to different target hardware (for
example where the byte endianness has changed) then you will need to
carefully examine the program logic; the porting effort will heavily depend
on the robustness of the original design.  Moreover, Ada 95 (and thus
Ada 2005 and Ada 2012) are sometimes
incompatible with typical Ada 83 compiler practices regarding implicit
packing, the meaning of the Size attribute, and the size of access values.
GNAT's approach to these issues is described in @ref{3f1,,Representation Clauses}.

@node Compatibility with Other Ada Systems,Representation Clauses,Implementation-dependent characteristics,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide id15}@anchor{3f2}@anchor{gnat_rm/compatibility_and_porting_guide compatibility-with-other-ada-systems}@anchor{3f3}
@section Compatibility with Other Ada Systems


If programs avoid the use of implementation dependent and
implementation defined features, as documented in the
@cite{Ada Reference Manual}, there should be a high degree of portability between
GNAT and other Ada systems.  The following are specific items which
have proved troublesome in moving Ada 95 programs from GNAT to other Ada 95
compilers, but do not affect porting code to GNAT.
(As of January 2007, GNAT is the only compiler available for Ada 2005;
the following issues may or may not arise for Ada 2005 programs
when other compilers appear.)


@itemize *

@item 
@emph{Ada 83 Pragmas and Attributes}

Ada 95 compilers are allowed, but not required, to implement the missing
Ada 83 pragmas and attributes that are no longer defined in Ada 95.
GNAT implements all such pragmas and attributes, eliminating this as
a compatibility concern, but some other Ada 95 compilers reject these
pragmas and attributes.

@item 
@emph{Specialized Needs Annexes}

GNAT implements the full set of special needs annexes.  At the
current time, it is the only Ada 95 compiler to do so.  This means that
programs making use of these features may not be portable to other Ada
95 compilation systems.

@item 
@emph{Representation Clauses}

Some other Ada 95 compilers implement only the minimal set of
representation clauses required by the Ada 95 reference manual.  GNAT goes
far beyond this minimal set, as described in the next section.
@end itemize

@node Representation Clauses,Compatibility with HP Ada 83,Compatibility with Other Ada Systems,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide representation-clauses}@anchor{3f1}@anchor{gnat_rm/compatibility_and_porting_guide id16}@anchor{3f4}
@section Representation Clauses


The Ada 83 reference manual was quite vague in describing both the minimal
required implementation of representation clauses, and also their precise
effects.  Ada 95 (and thus also Ada 2005) are much more explicit, but the
minimal set of capabilities required is still quite limited.

GNAT implements the full required set of capabilities in
Ada 95 and Ada 2005, but also goes much further, and in particular
an effort has been made to be compatible with existing Ada 83 usage to the
greatest extent possible.

A few cases exist in which Ada 83 compiler behavior is incompatible with
the requirements in Ada 95 (and thus also Ada 2005).  These are instances of
intentional or accidental dependence on specific implementation dependent
characteristics of these Ada 83 compilers.  The following is a list of
the cases most likely to arise in existing Ada 83 code.


@itemize *

@item 
@emph{Implicit Packing}

Some Ada 83 compilers allowed a Size specification to cause implicit
packing of an array or record.  This could cause expensive implicit
conversions for change of representation in the presence of derived
types, and the Ada design intends to avoid this possibility.
Subsequent AI's were issued to make it clear that such implicit
change of representation in response to a Size clause is inadvisable,
and this recommendation is represented explicitly in the Ada 95 (and Ada 2005)
Reference Manuals as implementation advice that is followed by GNAT.
The problem will show up as an error
message rejecting the size clause.  The fix is simply to provide
the explicit pragma @cite{Pack}, or for more fine tuned control, provide
a Component_Size clause.

@item 
@emph{Meaning of Size Attribute}

The Size attribute in Ada 95 (and Ada 2005) for discrete types is defined as
the minimal number of bits required to hold values of the type.  For example,
on a 32-bit machine, the size of @cite{Natural} will typically be 31 and not
32 (since no sign bit is required).  Some Ada 83 compilers gave 31, and
some 32 in this situation.  This problem will usually show up as a compile
time error, but not always.  It is a good idea to check all uses of the
'Size attribute when porting Ada 83 code.  The GNAT specific attribute
Object_Size can provide a useful way of duplicating the behavior of
some Ada 83 compiler systems.

@item 
@emph{Size of Access Types}

A common assumption in Ada 83 code is that an access type is in fact a pointer,
and that therefore it will be the same size as a System.Address value.  This
assumption is true for GNAT in most cases with one exception.  For the case of
a pointer to an unconstrained array type (where the bounds may vary from one
value of the access type to another), the default is to use a 'fat pointer',
which is represented as two separate pointers, one to the bounds, and one to
the array.  This representation has a number of advantages, including improved
efficiency.  However, it may cause some difficulties in porting existing Ada 83
code which makes the assumption that, for example, pointers fit in 32 bits on
a machine with 32-bit addressing.

To get around this problem, GNAT also permits the use of 'thin pointers' for
access types in this case (where the designated type is an unconstrained array
type).  These thin pointers are indeed the same size as a System.Address value.
To specify a thin pointer, use a size clause for the type, for example:

@example
type X is access all String;
for X'Size use Standard'Address_Size;
@end example

which will cause the type X to be represented using a single pointer.
When using this representation, the bounds are right behind the array.
This representation is slightly less efficient, and does not allow quite
such flexibility in the use of foreign pointers or in using the
Unrestricted_Access attribute to create pointers to non-aliased objects.
But for any standard portable use of the access type it will work in
a functionally correct manner and allow porting of existing code.
Note that another way of forcing a thin pointer representation
is to use a component size clause for the element size in an array,
or a record representation clause for an access field in a record.

See the documentation of Unrestricted_Access in the GNAT RM for a
full discussion of possible problems using this attribute in conjunction
with thin pointers.
@end itemize

@node Compatibility with HP Ada 83,,Representation Clauses,Compatibility and Porting Guide
@anchor{gnat_rm/compatibility_and_porting_guide compatibility-with-hp-ada-83}@anchor{3f5}@anchor{gnat_rm/compatibility_and_porting_guide id17}@anchor{3f6}
@section Compatibility with HP Ada 83


All the HP Ada 83 pragmas and attributes are recognized, although only a subset
of them can sensibly be implemented.  The description of pragmas in
@ref{7,,Implementation Defined Pragmas} indicates whether or not they are
applicable to GNAT.


@itemize *

@item 
@emph{Default floating-point representation}

In GNAT, the default floating-point format is IEEE, whereas in HP Ada 83,
it is VMS format.

@item 
@emph{System}

the package System in GNAT exactly corresponds to the definition in the
Ada 95 reference manual, which means that it excludes many of the
HP Ada 83 extensions.  However, a separate package Aux_DEC is provided
that contains the additional definitions, and a special pragma,
Extend_System allows this package to be treated transparently as an
extension of package System.
@end itemize

@node GNU Free Documentation License,Index,Compatibility and Porting Guide,Top
@anchor{share/gnu_free_documentation_license gnu-fdl}@anchor{1}@anchor{share/gnu_free_documentation_license doc}@anchor{3f7}@anchor{share/gnu_free_documentation_license gnu-free-documentation-license}@anchor{3f8}
@chapter GNU Free Documentation License


Version 1.3, 3 November 2008

Copyright  2000, 2001, 2002, 2007, 2008  Free Software Foundation, Inc
@indicateurl{http://fsf.org/}

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@enumerate A

@item 
Use in the Title Page (and on the covers, if any) a title distinct
from that of the Document, and from those of previous versions
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of the Document).  You may use the same title as a previous version
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@item 
List on the Title Page, as authors, one or more persons or entities
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unless they release you from this requirement.

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Modified Version, as the publisher.

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@item 
Add an appropriate copyright notice for your modifications
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Include an unaltered copy of this License.

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Preserve the section Entitled "History", Preserve its Title, and add
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Preserve the network location, if any, given in the Document for
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For any section Entitled "Acknowledgements" or "Dedications",
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@end enumerate

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reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License.  If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does
not give you any rights to use it.

@strong{10. FUTURE REVISIONS OF THIS LICENSE}

The Free Software Foundation may publish new, revised versions
of the GNU Free Documentation License from time to time.  Such new
versions will be similar in spirit to the present version, but may
differ in detail to address new problems or concerns.  See
@indicateurl{http://www.gnu.org/copyleft/}.

Each version of the License is given a distinguishing version number.
If the Document specifies that a particular numbered version of this
License "or any later version" applies to it, you have the option of
following the terms and conditions either of that specified version or
of any later version that has been published (not as a draft) by the
Free Software Foundation.  If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.  If the Document
specifies that a proxy can decide which future versions of this
License can be used, that proxy's public statement of acceptance of a
version permanently authorizes you to choose that version for the
Document.

@strong{11. RELICENSING}

"Massive Multiauthor Collaboration Site" (or "MMC Site") means any
World Wide Web server that publishes copyrightable works and also
provides prominent facilities for anybody to edit those works.  A
public wiki that anybody can edit is an example of such a server.  A
"Massive Multiauthor Collaboration" (or "MMC") contained in the
site means any set of copyrightable works thus published on the MMC
site.

"CC-BY-SA" means the Creative Commons Attribution-Share Alike 3.0
license published by Creative Commons Corporation, a not-for-profit
corporation with a principal place of business in San Francisco,
California, as well as future copyleft versions of that license
published by that same organization.

"Incorporate" means to publish or republish a Document, in whole or
in part, as part of another Document.

An MMC is "eligible for relicensing" if it is licensed under this
License, and if all works that were first published under this License
somewhere other than this MMC, and subsequently incorporated in whole
or in part into the MMC, (1) had no cover texts or invariant sections,
and (2) were thus incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009,
provided the MMC is eligible for relicensing.

@strong{ADDENDUM: How to use this License for your documents}

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and
license notices just after the title page:

@quotation

Copyright © YEAR  YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".
@end quotation

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts,
replace the "with ... Texts." line with this:

@quotation

with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
@end quotation

If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License,
to permit their use in free software.

@node Index,,GNU Free Documentation License,Top
@unnumbered Index


@printindex ge


@c %**end of body
@bye
