CFFI-SYS Interface Specification

Table of Contents

cffi-sys

Copyright © 2005-2006, James Bielman <jamesjb at jamesjb.com>

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1 Introduction

CFFI, the Common Foreign Function Interface, purports to be a portable foreign function interface for Common Lisp.

This specification defines a set of low-level primitives that must be defined for each Lisp implementation supported by CFFI. These operators are defined in the CFFI-SYS package.

The CFFI package uses the CFFI-SYS interface to implement an extensible foreign type system with support for typedefs, structures, and unions, a declarative interface for defining foreign function calls, and automatic conversion of foreign function arguments to/from Lisp types.

Please note the following conventions that apply to everything in CFFI-SYS:

• Functions in CFFI-SYS that are low-level versions of functions exported from the CFFI package begin with a leading percent-sign (eg. %mem-ref).
• Where “foreign type” is mentioned as the kind of an argument, the meaning is restricted to that subset of all foreign types defined in Built-In Foreign Types. Support for higher-level types is always defined in terms of those lower-level types in CFFI proper.

2 Built-In Foreign Types

Foreign Type: :char

Foreign Type: :unsigned-char

Foreign Type: :short

Foreign Type: :unsigned-short

Foreign Type: :int

Foreign Type: :unsigned-int

Foreign Type: :long

Foreign Type: :unsigned-long

Foreign Type: :long-long

Foreign Type: :unsigned-long-long

These types correspond to the native C integer types according to the ABI of the system the Lisp implementation is compiled against.

Foreign Type: :int8

Foreign Type: :uint8

Foreign Type: :int16

Foreign Type: :uint16

Foreign Type: :int32

Foreign Type: :uint32

Foreign Type: :int64

Foreign Type: :uint64

Foreign integer types of specific sizes, corresponding to the C types defined in stdint.h.

Foreign Type: :size

Foreign Type: :ssize

Foreign Type: :ptrdiff

Foreign Type: :time

Foreign integer types corresponding to the standard C types (without the _t suffix).

Implementor’s note: I’m sure there are more of these that could be useful, let’s add any types that can’t be defined portably to this list as necessary.

Foreign Type: :float

Foreign Type: :double

The :float type represents a C float and a Lisp single-float. :double represents a C double and a Lisp double-float.

Foreign Type: :pointer

A foreign pointer to an object of any type, corresponding to void *.

Foreign Type: :void

No type at all. Only valid as the return type of a function.

3 Operations on Built-in Foreign Types

Function: %foreign-type-size type ⇒ size

Return the size, in bytes, of objects having foreign type type. An error is signalled if type is not a known built-in foreign type.

Function: %foreign-type-alignment type ⇒ alignment

Return the default alignment in bytes for structure members of foreign type type. An error is signalled if type is not a known built-in foreign type.

Implementor’s note: Maybe this should take an optional keyword argument specifying an alternate alignment system, eg. :mac68k for 68000-compatible alignment on Darwin.

4 Basic Pointer Operations

Function: pointerp ptr ⇒ boolean

Return true if ptr is a foreign pointer.

Function: null-pointer ⇒ pointer

Return a null foreign pointer.

Function: null-pointer-p ptr ⇒ boolean

Return true if ptr is a null foreign pointer.

Function: make-pointer address ⇒ pointer

Return a pointer corresponding to the numeric integer address.

Function: inc-pointer ptr offset ⇒ pointer

Return the result of numerically incrementing ptr by offset.

5 Foreign Memory Allocation

Function: foreign-alloc size ⇒ pointer

Allocate size bytes of foreign-addressable memory and return a pointer to the allocated block. An implementation-specific error is signalled if the memory cannot be allocated.

Function: foreign-free ptr ⇒ unspecified

Free a pointer ptr allocated by foreign-alloc. The results are undefined if ptr is used after being freed.

Macro: with-foreign-pointer (var size &optional size-var) &body body

Bind var to a pointer to size bytes of foreign-accessible memory during body. Both ptr and the memory block it points to have dynamic extent and may be stack allocated if supported by the implementation. If size-var is supplied, it will be bound to size during body.

6 Memory Access

Accessor: %mem-ref ptr type &optional offset

Dereference a pointer offset bytes from ptr to an object for reading (or writing when used with setf) of built-in type type.

Example

  ;; An impractical example, since time returns the time as well,
  ;; but it demonstrates %MEM-REF. Better (simple) examples wanted!
  (with-foreign-pointer (p (foreign-type-size :time))
    (foreign-funcall "time" :pointer p :time)
    (%mem-ref p :time))

7 Foreign Function Calling

Macro: %foreign-funcall name {arg-type arg}* &optional result-type ⇒ object

Macro: %foreign-funcall-pointer ptr {arg-type arg}* &optional result-type ⇒ object

Invoke a foreign function called name in the foreign source code.

Each arg-type is a foreign type specifier, followed by arg, Lisp data to be converted to foreign data of type arg-type. result-type is the foreign type of the function’s return value, and is assumed to be :void if not supplied.

%foreign-funcall-pointer takes a pointer ptr to the function, as returned by foreign-symbol-pointer, rather than a string name.

Examples

  ;; Calling a standard C library function:
  (%foreign-funcall "sqrtf" :float 16.0 :float) ⇒ 4.0

  ;; Dynamic allocation of a buffer and passing to a function:
  (with-foreign-ptr (buf 255 buf-size)
    (%foreign-funcall "gethostname" :pointer buf :size buf-size :int)
    ;; Convert buf to a Lisp string using MAKE-STRING and %MEM-REF or
    ;; a portable CFFI function such as CFFI:FOREIGN-STRING-TO-LISP.
    )

8 Loading Foreign Libraries

Function: %load-foreign-library name ⇒ unspecified

Load the foreign shared library name.

Implementor’s note: There is a lot of behavior to decide here. Currently I lean toward not requiring NAME to be a full path to the library so we can search the system library directories (maybe even get LD_LIBRARY_PATH from the environment) as necessary.

9 Foreign Globals

Function: foreign-symbol-pointer name ⇒ pointer

Return a pointer to a foreign symbol name.

Symbol Index