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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
<header>
    <copyright>
      <year>1997</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
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    <title>Debugger</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>debugger_chapter.xml</file>
  </header>

  <section>
    <title>Introduction</title>

    <p><em>Debugger</em> is a graphical user interface for the Erlang
      interpreter, which can be used for debugging and testing of
      Erlang programs. For example, breakpoints can be set, code can be
      single stepped and variable values can be displayed and changed.
    </p>

    <p>The Erlang interpreter can also be accessed via the interface
      module <c>int</c>, see <seealso marker="int">int(3)</seealso>.
    </p>

    <p><em>Warning:</em> Note that the Debugger at some point might
      start tracing on the processes which execute the interpreted
      code. This means that a conflict will occur if tracing by other
      means is started on any of these processes.</p>
  </section>  

  <section>
    <title>Getting Started with Debugger</title>

    <p>Start Debugger by calling <c>debugger:start()</c>. It will start
      the <seealso marker="#monitor">Monitor window</seealso> showing
      information about all debugged processes, interpreted modules and
      selected options.</p>

    <p>Initially there are normally no debugged processes. First, it
      must be specified which modules should be <em>debugged</em>, or
      <em>interpreted</em> as it is also called. This is done by
      choosing <em>Module->Interpret...</em> in the Monitor window and
      then selecting the appropriate modules from the
      <seealso marker="#interpret">Interpret Dialog window</seealso>.
    </p>

    <note>
      <p>Only modules compiled with the option <c>debug_info</c> set
	can be interpreted. Non-interpretable modules are shown within
	parenthesis in the Interpret Dialog window.</p>
    </note>

    <p>When a module is interpreted, it can be viewed in a
      <seealso marker="#view">View Module window</seealso>. This is done
      by selecting the module from the <em>Module->module->View</em>
      menu. The contents of the source file is shown and it is possible
      to set <seealso marker="#breakpoints">breakpoints</seealso>.</p>

    <p>Now the program that should be debugged can be started. This is
      done the normal way from the Erlang shell. All processes executing
      code in interpreted modules will be displayed in the Monitor
      window. It is possible to <em>attach</em> to one of these
      processes, by double-clicking it, or by selecting the process and
      then choosing <em>Process->Attach</em>.</p>

    <p>Attaching to a process will result in a
      <seealso marker="#attach">Attach Process window</seealso> being
      opened for this process. From the Attach Process window, it is
      possible to control the process execution, inspect variable
      values, set breakpoints etc.</p>
  </section>

  <section>
    <marker id="breakpoints"/>
    <title>Breakpoints and Break Dialogue Windows</title>
    
    <p>Once the appropriate modules are interpreted, breakpoints can
      be set at relevant locations in the source code. Breakpoints are
      specified on a line basis. When a process reaches a breakpoint,
      it stops and waits for commands (step, skip, continue,...) from
      the user.</p>

    <note>
      <p>When a process reaches a breakpoint, only that process is
	stopped. Other processes are not affected.</p>
    </note>

    <p>Breakpoints are created and deleted using the Break menu of
      the Monitor window, View Module window and Attach Process window.
    </p>

    <section>
      <title>Executable Lines</title>

      <p>To have effect, a breakpoint must be set at an
	<em>executable line</em>, which is a line of code containing an
	executable expression such as a matching or a function call.
	A blank line or a line containing a comment, function head or
	pattern in a <c>case</c>- or <c>receive</c> statement is not
	executable.</p>

      <p>In the example below, lines number 2, 4, 6, 8 and 11 are
	executable lines:</p>
      <pre>
1: is_loaded(Module,Compiled) ->
2:   case get_file(Module,Compiled) of
3:     {ok,File} ->
4:       case code:which(Module) of
5:         ?TAG ->
6:           {loaded,File};
7:         _ ->
8:           unloaded
9:       end;
10:    false ->
11:      false
12:  end.</pre>
    </section>
    
    <section>		  
      <title>Status and Trigger Action</title>

      <p>A breakpoint can be either <em>active</em> or
	<em>inactive</em>. Inactive breakpoints are ignored.</p>

      <p>Each breakpoint has a <em>trigger action</em> which specifies
	what should happen when a process has reached it (and stopped):
      </p>
      <list>
	<item><em>enable</em> Breakpoint should remain active (default).
	</item>
	<item><em>disable</em> Breakpoint should be made inactive.
	</item>
	<item><em>delete</em> Breakpoint should be deleted.</item>
      </list>
    </section>

    <section>
      <title>Line Breakpoints</title>

      <p>A line breakpoint is created at a certain line in a module.</p>

      <image file="images/line_break_dialog.jpg">
	<icaption>The Line Break Dialog Window.</icaption>
      </image>

      <p>Right-clicking the Module entry will open a popup menu from
	which the appropriate module can be selected.</p>

      <p>A line breakpoint can also be created (and deleted) by
	double-clicking the line when the module is displayed in
	the View Module or Attach Process window.</p>
    </section>

    <section>
      <title>Conditional Breakpoints</title>

      <p>A conditional breakpoint is created at a certain line in
	the module, but a process reaching the breakpoint will stop
	only if a given condition is true.</p>

      <p>The condition is specified by the user as a module name
	<c>CModule</c> and a function name <c>CFunction</c>. When a
	process reaches the breakpoint,
	<c>CModule:CFunction(Bindings)</c> will be evaluated. If and
	only if this function call returns <c>true</c>, the process
	will stop. If the function call returns <c>false</c>,
	the breakpoint will be silently ignored.</p>

      <p><c>Bindings</c> is a list of variable bindings. Use
	the function <c>int:get_binding(Variable,Bindings)</c> to
	retrieve the value of <c>Variable</c> (given as an atom).
	The function returns <c>unbound</c> or <c>{value,Value}</c>.</p>
	
      <image file="images/cond_break_dialog.jpg">
	<icaption>The Conditional Break Dialog Window.</icaption>
      </image>

      <p>Right-clicking the Module entry will open a popup menu from
	which the appropriate module can be selected.</p>

      <p>Example: A conditional breakpoint calling
	<c>c_test:c_break/1</c> is added at line 6 in the module
	<c>fact</c>. Each time the breakpoint is reached, the function is
	called, and when <c>N</c> is equal to 3 it returns <c>true</c>,
	and the process stops.</p>
	
      <p>Extract from <c>fact.erl</c>:</p>
      <pre>
5.   fac(0) -> 1;
6.   fac(N) when N > 0, is_integer(N) -> N * fac(N-1).</pre>

      <p>Definition of <c>c_test:c_break/1</c>:</p>
      <pre>
-module(c_test).
-export([c_break/1]).

c_break(Bindings) ->
    case int:get_binding('N', Bindings) of
        {value, 3} ->
            true;
        _ ->
            false
    end.</pre>
    </section>

    <section>
      <title>Function Breakpoints</title>

      <p>A function breakpoint is a set of line breakpoints, one at
	the first line of each clause in the given function.</p>

      <image file="images/function_break_dialog.jpg">
	<icaption>The Function Break Dialog Window.</icaption>
      </image>

      <p>Right-clicking the Module entry will open a popup menu from
	which the appropriate module can be selected.</p>

      <p>Clicking the Ok button (or 'Return' or 'Tab') when a module
	name has been given, will bring up all functions of the module
	in the listbox.</p>
    </section>
  </section>

  <section>
    <marker id="stack_trace"/>
    <title>Stack Trace</title>

    <p>The Erlang emulator keeps track of a <em>stack trace</em>,
      information about recent function calls. This information is
      used, for example, if an error occurs:</p>
    <pre>
1> <input>catch a+1.</input>
{'EXIT',{badarith,[{erlang,'+',[a,1],[]},
                   {erl_eval,do_apply,6,[{file,"erl_eval.erl"},{line,573}]},
                   {erl_eval,expr,5,[{file,"erl_eval.erl"},{line,357}]},
                   {shell,exprs,7,[{file,"shell.erl"},{line,674}]},
                   {shell,eval_exprs,7,[{file,"shell.erl"},{line,629}]},
                   {shell,eval_loop,3,[{file,"shell.erl"},{line,614}]}]}}</pre>

    <p>See the Erlang Reference Manual,
      <seealso marker="doc/reference_manual:errors">
      Errors and Error Handling</seealso>,
      for more information about the stack trace.</p>

    <p>The Debugger emulates the stack trace by keeping track of recently
      called interpreted functions. (The real stack trace cannot be
      used, as it shows which functions of the Debugger have been
      called, rather than which interpreted functions).</p>

    <p>This information can be used to traverse the chain of function
      calls, using the 'Up' and 'Down' buttons of
      <seealso marker="#attach">the Attach Process window</seealso>.
    </p>

    <p>By default, the Debugger only saves information about recursive
      function calls, that is, function calls that have not yet returned
      a value (option 'Stack On, No Tail').</p>

    <p>Sometimes, however, it can be useful to save all calls, even
      tail-recursive calls. That can be done with the 'Stack On, Tail'
      option. Note that this option will consume more memory and slow
      down execution of interpreted functions when there are many
      tail-recursive calls.
    </p>

    <p>It is also possible to turn off the Debugger stack trace
      facility ('Stack Off'). <em>Note:</em> If an error occurs, in this
      case the stack trace will be empty.</p>

    <p>See the section about <seealso marker="#monitor">the Monitor
      Window</seealso> for information about how to change the stack
      trace option.</p>
  </section>

  <section>
    <marker id="monitor"/>
    <title>The Monitor Window</title>

    <p>The Monitor window is the main window of Debugger and shows a
      listbox containing the names of all interpreted modules
      (double-clicking a module brings up the View Module window),
      which options are selected, and information about all debugged
      processes, that is all processes which have been/are executing
      code in interpreted modules.</p>

    <image file="images/monitor.jpg">
      <icaption>The Monitor Window.</icaption>
    </image>

    <p>The Auto Attach buttons, Stack Trace label, Back Trace Size
      label, and Strings button show some options set, see
      <seealso marker="#options">Options Menu</seealso> for further
      information about these options.</p>

    <section>
      <title>Process Grid</title>

      <taglist>
	<tag><em>Pid</em></tag>
	<item><p>The process identifier.</p></item>
	
	<tag><em>Initial Call</em></tag>
	<item><p>The first call to an interpreted function by this
	    process. (<c>Module:Function/Arity</c>)</p></item>

	<tag><em>Name</em></tag>
	<item><p>The registered name, if any. If a registered name does
	    not show up, it may be that the Debugger received
	    information about the process before the name had been
	    registered. Try selecting <em>Edit->Refresh</em>.</p></item>

	<tag><em>Status</em></tag>
	<item><p>The current status, one of the following:</p>
	  <taglist>
	    <tag><em>idle</em></tag>
	    <item><p>The interpreted function call has returned a value,
		and the process is no longer executing interpreted code.
	      </p></item>

	    <tag><em>running</em></tag>
	    <item><p>The process is running.</p></item>
	    
	    <tag><em>waiting</em></tag>
	    <item><p>The process is waiting in a <c>receive</c>
		statement.</p></item>
	    
	    <tag><em>break</em></tag>
	    <item><p>The process is stopped at a breakpoint.</p></item>
	    
	    <tag><em>exit</em></tag>
	    <item><p>The process has terminated.</p></item>
	    
	    <tag><em>no_conn</em></tag>
	    <item><p>There is no connection to the node where
		the process is located.</p></item>
	  </taglist>
	</item>

	<tag><em>Information</em></tag>
	<item><p>Additional information, if any. If the process is
	    stopped at a breakpoint, the field contains information
	    about the location <c>{Module,Line}</c>. If the process has
	    terminated, the field contains the exit reason.</p></item>
      </taglist>
    </section>

    <section>
      <title>The File Menu</title>
      
      <taglist>
	<tag><em>Load Settings...</em></tag>
	<item>
	  <p>Try to load and restore Debugger settings from a file
	    previously saved using <em>Save Settings...</em>, see below.
	    Any errors are silently ignored.
            <em>Note:</em> Settings saved by Erlang R16B01 or later
            cannot be read by Erlang R16B or earlier.</p>
	</item>
	
	<tag><em>Save Settings...</em></tag>
	<item><p>Save Debugger settings to a file. The settings include
	    the set of interpreted files, breakpoints, and the selected
	    options. The settings can be restored in a later Debugger
	    session using <em>Load Settings...</em>, see above.
	    Any errors are silently ignored.</p>
	</item>
	
	<tag><em>Exit</em></tag>
	<item><p>Stop Debugger.</p></item>
      </taglist>
    </section>

    <section>
      <title>The Edit Menu</title>
      <taglist>
	<tag><em>Refresh</em></tag>
	<item><p>Update information about debugged processes. Removes
	    information about all terminated processes from the window,
	    and also closes all Attach Process windows for terminated
	    processes.</p></item>

	<tag><em>Kill All</em></tag>
	<item><p>Terminate all processes listed in the window using
	    <c>exit(Pid,kill)</c>.</p></item>
      </taglist>
    </section>

    <section>
      <title>The Module Menu</title>
      <taglist>
	<tag><em>Interpret...</em></tag>
	<item><p>Open the <seealso marker="#interpret">Interpret Dialog
	      window</seealso> where new modules to be interpreted can
	    be specified.</p></item>
	
	<tag><em>Delete All</em></tag>
	<item><p>Stop interpreting all modules. Processes executing in
	    interpreted modules will terminate.</p></item>
      </taglist>

      <p>For each interpreted module, a corresponding entry is added to
	the Module menu, with the following submenu:</p>
      <taglist>
	<tag><em>Delete</em></tag>
	<item><p>Stop interpreting the selected module. Processes
	    executing in this module will terminate.</p></item>
	
	<tag><em>View</em></tag>
	<item><p>Open a <seealso marker="#view">View Module
	      window</seealso> showing the contents of the selected
	    module.</p></item>
      </taglist>
    </section>

    <section>
      <title>The Process Menu</title>
      <p>The following menu items apply to the currently selected
	process, provided it is stopped at a breakpoint. See the chapter
	about the <seealso marker="#attach">Attach Process
	  window</seealso> for more information.</p>
      <taglist>
	<tag><em>Step</em></tag><item></item>
	<tag><em>Next</em></tag><item></item>
	<tag><em>Continue</em></tag><item></item>
	<tag><em>Finish</em></tag><item></item>
      </taglist>
      <p>The following menu items apply to the currently selected
	process.</p>
      <taglist>
	<tag><em>Attach</em></tag>
	<item><p>Attach to the process and open a
	    <seealso marker="#attach">Attach Process window</seealso>.
	  </p></item>
	
	<tag><em>Kill</em></tag>
	<item><p>Terminate the process using <c>exit(Pid,kill)</c>.</p>
	</item>
      </taglist>
    </section>

  <section>
    <title>The Break Menu</title>
      <p>The items in this menu are used to create and delete
	breakpoints.
	See the <seealso marker="#breakpoints">Breakpoints</seealso>
	chapter for more information.</p>
      <taglist>
	<tag><em>Line Break...</em></tag>
	<item><p>Set a line breakpoint.</p></item>

	<tag><em>Conditional Break...</em></tag>
	<item><p>Set a conditional breakpoint.</p></item>

	<tag><em>Function Break...</em></tag>
	<item><p>Set a function breakpoint.</p></item>

	<tag><em>Enable All</em></tag>
	<item><p>Enable all breakpoints.</p></item>

	<tag><em>Disable All</em></tag>
	<item><p>Disable all breakpoints.</p></item>

      <tag><em>Delete All</em></tag>
	<item><p>Remove all breakpoints.</p></item>
      </taglist>

      <p>For each breakpoint, a corresponding entry is added to
      the Break
	menu, from which it is possible to disable/enable or delete
	the breakpoint,	and to change its trigger action.</p>
    </section>

    <section>
      <marker id="options"/>
      <title>The Options Menu</title>

      <taglist>
	<tag><em>Trace Window</em></tag>
	<item><p>Set which areas should be visible in
	    an <seealso marker="#attach">Attach Process
	      window</seealso>. Does not affect already existing
	    Attach Process windows.</p>
	</item>

	<tag><em>Auto Attach</em></tag>
	<item><p>Set at which events a debugged process should be
	    automatically attached to. Affects existing debugged
	    processes.</p>
	  <list>
	    <item><em>First Call</em> - the first time a process calls a
	      function in an interpreted module.</item>
	    <item><em>On Exit</em> - at process termination.</item>
	    <item><em>On Break</em> - when a process reaches a
	      breakpoint.</item>
	  </list>
	</item>

	<tag><em>Stack Trace</em></tag>
	<item><p>Set stack trace option, see section
	  <seealso marker="#stack_trace">Stack Trace</seealso>. Does not
	    affect already existing debugged processes.</p>
	  <list>
	    <item><em>Stack On, Tail</em> - save information about all
	      current calls.</item>
	    <item><em>Stack On, No Tail</em> - save information about
	      current calls, discarding previous information when a tail
	      recursive call is made.</item>
	    <item><em>Stack Off</em> - do not save any information about
	      current calls.</item>
	  </list>
	</item>

	<tag><em>Strings</em></tag>
	<item><p>Set which integer lists should be printed as strings.
            Does not affect already existing debugged processes.</p>
	  <list>

	    <item><em>Use range of +pc flag</em> - use the printable
              character range set by the
              <seealso marker="erts:erl#printable_character_range">
              <c>erl(1)</c></seealso> flag <c>+pc</c>.
          </item>
	  </list>
	</item>

	<tag><em>Back Trace Size...</em></tag>
	<item><p>Set how many call frames should be fetched when
	    inspecting the call stack from the Attach Process window.
	    Does not affect already existing Attach Process windows.</p>
	</item>
      </taglist>
    </section>

    <section>
      <title>The Windows Menu</title>
      <p>Contains a menu item for each open Debugger window. Selecting
	one of the items will raise the corresponding window.</p>
    </section>

    <section>
      <title>The Help Menu</title>
      <taglist>
	<tag><em>Help</em></tag>
	<item><p>View the Debugger documentation. Currently this
	    function requires a web browser to be up and running.</p></item>
      </taglist>
    </section>
  </section>
    
  <section>
    <marker id="interpret"/>
    <title>The Interpret Dialog Window</title>

    <p>The interpret dialog module is used for selecting which modules
      to interpret. Initially, the window shows the modules (<c>erl</c>
      files) and subdirectories of the current working directory.</p>

    <p>Interpretable modules are modules for which a BEAM file, compiled
      with the option <c>debug_info</c> set, can be found in the same
      directory as the source code, or in an <c>ebin</c> directory next
      to it.</p>

    <p>Modules, for which the above requirements are not fulfilled, are
      not interpretable and are therefore displayed within parentheses.
    </p>

    <p>The <c>debug_info</c> option causes <em>debug information</em> or
      <em>abstract code</em> to be added to the BEAM file. This will
      increase the size of the file, and also makes it possible to
      reconstruct the source code. It is therefore recommended not to
      include debug information in code aimed for target systems.</p>

    <p>An example of how to compile code with debug information using
      <c>erlc</c>:<br/>
      <c>% erlc +debug_info module.erl</c></p>
      
    <p>An example of how to compile code with debug information from
      the Erlang shell:<br/>
      <c>4> c(module, debug_info).</c></p>
      
    <image file="images/interpret.jpg">
      <icaption>The Interpret Dialog Window.</icaption>
    </image>

    <p>Browse the file hierarchy and interpret the appropriate modules
      by selecting a module name and pressing <em>Choose</em> (or
      carriage return), or by double clicking the module name.
      Interpreted modules have the type <c>erl src</c>.
    </p>

    <p>Pressing <em>All</em> will interpret all displayed modules in
      the chosen directory.</p>

    <p>Pressing <em>Done</em> will close the window.</p>

    <note>
      <p>When the Debugger is started in global mode (which is
	the default, see
	<seealso marker="debugger:debugger#start/0">debugger:start/0</seealso>),
	modules added (or deleted) for interpretation will be added (or
	deleted) on all known Erlang nodes.</p>
    </note>
  </section>

  <section>
    <marker id="attach"/>
    <title>The Attach Process Window</title>

    <p>From an Attach Process window the user can interact with a
      debugged process. One window is opened for each process that has
      been attached to. Note that when attaching to a process, its
      execution is automatically stopped.
    </p>

    <image file="images/attach.jpg">
      <icaption>The Attach Process Window.</icaption>
    </image>

    <p>The window is divided into five parts:</p>
    <list>
      <item><p>The Code area, showing the code being executed. The code
	  is indented and each line is prefixed with its line number.
	  If the process execution is stopped, the current line is
	  marked with <em>--></em>. An existing break point at a line
	  is marked with a stop symbol. In the example above,
	  the execution has been stopped at line 6, before
	  the execution of <c>fac/1</c>.</p>
	<p>Active breakpoints are shown in red, while inactive
	  breakpoints are shown in blue.</p>
      </item>
      <item>The Button area, with buttons for quick access to frequently
	used functions in the Process menu.</item>
      <item>The Evaluator area, where the user can evaluate functions
	within the context of the debugged process, provided that
	process execution has been stopped.</item>
      <item>The Bindings area, showing all variables bindings.
	Clicking on a variable name will result in the value being
	displayed in the Evaluator area.
	Double-clicking on a variable name will open a window where
	the variable value may be edited. Note however that pid,
	reference, binary or port values can not be edited.
      </item>
      <item><p>The Trace area, showing a trace output for the process.
	</p>
	<taglist>
	  <tag><c>++ (N) &lt;L&gt;</c></tag>
	  <item>Function call, where <c>N</c> is the call level and
	    <c>L</c> the line number.</item>

	  <tag><c>-- (N)</c></tag>
	  <item>Function return value.</item>

	  <tag><c>==> Pid : Msg</c></tag>
	  <item>The message <c>Msg</c> is sent to process <c>Pid</c>.
	  </item>

	  <tag><c><![CDATA[<== Msg]]></c></tag>
	  <item>The message <c>Msg</c> is received.</item>

	  <tag><c>++ (N) receive</c></tag>
	  <item>Waiting in a <c>receive</c>.</item>

	  <tag><c>++ (N) receive with timeout</c></tag>
	  <item>Waiting in a <c>receive...after</c>.</item>
	</taglist>

	<p>Also the back trace, a summary of the current function calls
	  on the stack, is displayed in the Trace area.</p>
      </item>
    </list>

    <p>It is configurable using the Options menu which areas should
      be shown or hidden. By default, all areas except the Trace area
      are shown.</p>

    <section>
      <title>The File Menu</title>

      <taglist>
	<tag><em>Close</em></tag>
	<item><p>Close this window and detach from the process.</p>
	</item>
      </taglist>
    </section>

    <section>
      <title>The Edit Menu</title>

      <taglist>
	<tag><em>Go to line...</em></tag>
	<item><p>Go to a specified line number.</p></item>

	<tag><em>Search...</em></tag>
	<item><p>Search for a specified string.</p></item>
      </taglist>
    </section>

    <section>
      <title>The Process Menu</title>

      <taglist>
	<tag><em>Step</em></tag> 
	<item><p>Execute the current line of code, stepping into any
	    (interpreted) function calls.</p></item>

	<tag><em>Next</em></tag>
	<item><p>Execute the current line of code and stop at the next
	    line.</p></item>

	<tag><em>Continue</em></tag>
	<item><p>Continue the execution.</p></item>

	<tag><em>Finish</em></tag>
	<item><p>Continue the execution until the current function
	    returns.</p></item>

	<tag><em>Skip</em></tag>
	<item><p>Skip the current line of code and stop at the next
	    line. If used on the last line in a function body,
	    the function will return <c>skipped</c>.</p></item>

	<tag><em>Time Out</em></tag>
	<item><p>Simulate a timeout when executing a
	    <c>receive...after</c> statement.</p></item>

	<tag><em>Stop</em></tag>
	<item><p>Stop the execution of a running process, that is, make
	    the process stop as at a breakpoint. The command will take
	    effect (visibly) the next time the process receives a
	    message.</p></item>

	<tag><em>Where</em></tag>
	<item><p>Make sure the current location of the execution is
	    visible in the code area.</p></item>

	<tag><em>Kill</em></tag>
	<item><p>Terminate the process using <c>exit(Pid,kill)</c>.</p>
	</item>

	<tag><em>Messages</em></tag>
	<item><p>Inspect the message queue of the process. The queue is
	    printed in the evaluator area.</p></item>

	<tag><em>Back Trace</em></tag>
	<item><p>Display the back trace of the process, a summary of
	    the current function calls on the stack, in the trace area.
	    Requires that the Trace area is visible and that the stack
	    trace option is 'Stack On, Tail' or 'Stack On, No Tail'.</p>
	</item>

	<tag><em>Up</em></tag>
	<item><p>Inspect the previous function call on the stack,
	    showing the location and variable bindings.</p></item>

	<tag><em>Down</em></tag>
	<item><p>Inspect the next function call on the stack, showing
	    the location and variable bindings.</p></item>
      </taglist>
    </section>

    <section>
      <title>The Options Menu</title>

      <taglist>
	<tag><em>Trace Window</em></tag>
	<item><p>Set which areas should be visible. Does not affect
	    other Attach Process windows.</p>
	</item>

	<tag><em>Stack Trace</em></tag>
	<item><p>Same as in <seealso marker="#monitor">the Monitor
	      window</seealso>, but only affects the debugged
	    process the window is attached to.</p>
	</item>

	<tag><em>Strings</em></tag>
	<item><p>Same as in <seealso marker="#monitor">the Monitor
	      window</seealso>, but only affects the debugged
	    process the window is attached to.</p>
	</item>

	<tag><em>Back Trace Size...</em></tag>
	<item><p>Set how many call frames should be fetched when
	    inspecting the call stack. Does not affect other Attach
	    Process windows.</p>
	</item>
      </taglist>
    </section>

    <section>
      <title>Break, Windows and Help Menus</title>

      <p>The Break, Windows and Help menus look the same as in
	the Monitor window, see the chapter
	<seealso marker="#monitor">The Monitor Window</seealso>, except
	that the Breaks menu apply to the local breakpoints only.</p>
    </section>
  </section>

  <section>
    <marker id="view"/>
    <title>The View Module Window</title>

    <p>The View Module window shows the contents of an interpreted
      module and makes it possible to set breakpoints.</p>

    <image file="images/view.jpg">
      <icaption>The View Module Window.</icaption>
    </image>

    <p>The source code is indented and each line is prefixed with its
      line number.</p>

    <p>Clicking a line will highlight it and select it to be the target
      of the breakpoint functions available from the Break menu.
      Doubleclicking a line will set a line breakpoint on that line.
      Doubleclicking a line with an existing breakpoint will remove
      the breakpoint.</p>

    <p>Breakpoints are marked with a stop symbol.</p>

    <section>
      <title>File and Edit Menus</title>
      <p>The File and Edit menus look the same as in the Attach Process
	window, see the chapter <seealso marker="#attach">The Attach
	  Process Window</seealso>.</p>
    </section>

    <section>
      <title>Break, Windows and Help Menus</title>

      <p>The Break, Windows and Help menus look the same as in
	the Monitor window, see the chapter
	<seealso marker="#monitor">The Monitor Window</seealso>, except
	that the Breaks menu apply to the local breakpoints only.</p>
    </section>
  </section>

  <section>
    <title>Performance</title>

    <p>Execution of interpreted code is naturally slower than for
      regularly compiled modules. Using the Debugger also increases
      the number of processes in the system, as for each debugged
      process another process (the meta process) is created.</p>

    <p>It is also worth to keep in mind that programs with timers may
      behave differently when debugged. This is especially true when
      stopping the execution of a process, for example at a
      breakpoint. Timeouts can then occur in other processes that
      continue execution as normal.</p>
  </section>

  <section>
    <title>Code Loading Mechanism</title>

    <p>Code loading works almost as usual, except that interpreted
      modules are also stored in a database and debugged processes
      uses only this stored code. Re-interpreting an interpreted
      module will result in the new version being stored as well, but
      does not affect existing processes executing an older version
      of the code. This means that the code replacement mechanism of
      Erlang does not work for debugged processes.</p>
  </section>

  <section>
    <title>Debugging Remote Nodes</title>

    <p>By using <c>debugger:start/1</c>, it can be specified if Debugger
      should be started in local or global mode.</p>
    <pre>
debugger:start(local | global)</pre>
    <p>If no argument is provided, Debugger is started in global mode.
    </p>

    <p>In local mode, code is interpreted only at the current node.
      In global mode, code is interpreted at all known nodes. Processes
      at other nodes executing interpreted code will automatically be
      shown in the Monitor window and can be attached to like any other
      debugged process.</p>

    <p>It is possible, but definitely not recommended to start Debugger
      in global mode on more than one node in a network, as they will
      interfere with each other leading to inconsistent behaviour.</p>
  </section>
</chapter>


