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 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
 <a name="Convenience-Vars"></a>
 <div class="header">
 <p>
 Next: <a href="Convenience-Funs.html#Convenience-Funs" accesskey="n" rel="next">Convenience Funs</a>, Previous: <a href="Value-History.html#Value-History" accesskey="p" rel="prev">Value History</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
 </div>
 <hr>
 <a name="Convenience-Variables"></a>
 <h3 class="section">10.11 Convenience Variables</h3>

 <a name="index-convenience-variables"></a>
 <a name="index-user_002ddefined-variables"></a>
 <p><small>GDB</small> provides <em>convenience variables</em> that you can use within
 <small>GDB</small> to hold on to a value and refer to it later.  These variables
 exist entirely within <small>GDB</small>; they are not part of your program, and
 setting a convenience variable has no direct effect on further execution
 of your program.  That is why you can use them freely.
 </p>
 <p>Convenience variables are prefixed with &lsquo;<samp>$</samp>&rsquo;.  Any name preceded by
 &lsquo;<samp>$</samp>&rsquo; can be used for a convenience variable, unless it is one of
 the predefined machine-specific register names (see <a href="Registers.html#Registers">Registers</a>).
 (Value history references, in contrast, are <em>numbers</em> preceded
 by &lsquo;<samp>$</samp>&rsquo;.  See <a href="Value-History.html#Value-History">Value History</a>.)
 </p>
 <p>You can save a value in a convenience variable with an assignment
 expression, just as you would set a variable in your program.
 For example:
 </p>
 <div class="smallexample">
 <pre class="smallexample">set $foo = *object_ptr
 </pre></div>

 <p>would save in <code>$foo</code> the value contained in the object pointed to by
 <code>object_ptr</code>.
 </p>
 <p>Using a convenience variable for the first time creates it, but its
 value is <code>void</code> until you assign a new value.  You can alter the
 value with another assignment at any time.
 </p>
 <p>Convenience variables have no fixed types.  You can assign a convenience
 variable any type of value, including structures and arrays, even if
 that variable already has a value of a different type.  The convenience
 variable, when used as an expression, has the type of its current value.
 </p>
 <dl compact="compact">
 <dd><a name="index-show-convenience"></a>
 <a name="index-show-all-user-variables-and-functions"></a>
 </dd>
 <dt><code>show convenience</code></dt>
 <dd><p>Print a list of convenience variables used so far, and their values,
 as well as a list of the convenience functions.
 Abbreviated <code>show conv</code>.
 </p>
 <a name="index-init_002dif_002dundefined"></a>
 <a name="index-convenience-variables_002c-initializing"></a>
 </dd>
 <dt><code>init-if-undefined $<var>variable</var> = <var>expression</var></code></dt>
 <dd><p>Set a convenience variable if it has not already been set.  This is useful
 for user-defined commands that keep some state.  It is similar, in concept,
 to using local static variables with initializers in C (except that
 convenience variables are global).  It can also be used to allow users to
 override default values used in a command script.
 </p>
 <p>If the variable is already defined then the expression is not evaluated so
 any side-effects do not occur.
 </p></dd>
 </dl>

 <p>One of the ways to use a convenience variable is as a counter to be
 incremented or a pointer to be advanced.  For example, to print
 a field from successive elements of an array of structures:
 </p>
 <div class="smallexample">
 <pre class="smallexample">set $i = 0
 print bar[$i++]-&gt;contents
 </pre></div>

 <p>Repeat that command by typing <tt class="key">RET</tt>.
 </p>
 <p>Some convenience variables are created automatically by <small>GDB</small> and given
 values likely to be useful.
 </p>
 <dl compact="compact">
 <dd><a name="index-_0024_005f_002c-convenience-variable"></a>
 </dd>
 <dt><code>$_</code></dt>
 <dd><p>The variable <code>$_</code> is automatically set by the <code>x</code> command to
 the last address examined (see <a href="Memory.html#Memory">Examining Memory</a>).  Other
 commands which provide a default address for <code>x</code> to examine also
 set <code>$_</code> to that address; these commands include <code>info line</code>
 and <code>info breakpoint</code>.  The type of <code>$_</code> is <code>void *</code>
 except when set by the <code>x</code> command, in which case it is a pointer
 to the type of <code>$__</code>.
 </p>
 <a name="index-_0024_005f_005f_002c-convenience-variable"></a>
 </dd>
 <dt><code>$__</code></dt>
 <dd><p>The variable <code>$__</code> is automatically set by the <code>x</code> command
 to the value found in the last address examined.  Its type is chosen
 to match the format in which the data was printed.
 </p>
 </dd>
 <dt><code>$_exitcode</code></dt>
 <dd><a name="index-_0024_005fexitcode_002c-convenience-variable"></a>
 <p>When the program being debugged terminates normally, <small>GDB</small>
 automatically sets this variable to the exit code of the program, and
 resets <code>$_exitsignal</code> to <code>void</code>.
 </p>
 </dd>
 <dt><code>$_exitsignal</code></dt>
 <dd><a name="index-_0024_005fexitsignal_002c-convenience-variable"></a>
 <p>When the program being debugged dies due to an uncaught signal,
 <small>GDB</small> automatically sets this variable to that signal&rsquo;s number,
 and resets <code>$_exitcode</code> to <code>void</code>.
 </p>
 <p>To distinguish between whether the program being debugged has exited
 (i.e., <code>$_exitcode</code> is not <code>void</code>) or signalled (i.e.,
 <code>$_exitsignal</code> is not <code>void</code>), the convenience function
 <code>$_isvoid</code> can be used (see <a href="Convenience-Funs.html#Convenience-Funs">Convenience
 Functions</a>).  For example, considering the following source code:
 </p>
 <div class="smallexample">
 <pre class="smallexample">#include &lt;signal.h&gt;

 int
 main (int argc, char *argv[])
 {
   raise (SIGALRM);
   return 0;
 }
 </pre></div>

 <p>A valid way of telling whether the program being debugged has exited
 or signalled would be:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) define has_exited_or_signalled
 Type commands for definition of ``has_exited_or_signalled''.
 End with a line saying just ``end''.
 &gt;if $_isvoid ($_exitsignal)
  &gt;echo The program has exited\n
  &gt;else
  &gt;echo The program has signalled\n
  &gt;end
 &gt;end
 (gdb) run
 Starting program:

 Program terminated with signal SIGALRM, Alarm clock.
 The program no longer exists.
 (gdb) has_exited_or_signalled
 The program has signalled
 </pre></div>

 <p>As can be seen, <small>GDB</small> correctly informs that the program being
 debugged has signalled, since it calls <code>raise</code> and raises a
 <code>SIGALRM</code> signal.  If the program being debugged had not called
 <code>raise</code>, then <small>GDB</small> would report a normal exit:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(gdb) has_exited_or_signalled
 The program has exited
 </pre></div>

 </dd>
 <dt><code>$_exception</code></dt>
 <dd><p>The variable <code>$_exception</code> is set to the exception object being
 thrown at an exception-related catchpoint.  See <a href="Set-Catchpoints.html#Set-Catchpoints">Set Catchpoints</a>.
 </p>
 </dd>
 <dt><code>$_probe_argc</code></dt>
 <dt><code>$_probe_arg0&hellip;$_probe_arg11</code></dt>
 <dd><p>Arguments to a static probe.  See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.
 </p>
 </dd>
 <dt><code>$_sdata</code></dt>
 <dd><a name="index-_0024_005fsdata_002c-inspect_002c-convenience-variable"></a>
 <p>The variable <code>$_sdata</code> contains extra collected static tracepoint
 data.  See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>.  Note that
 <code>$_sdata</code> could be empty, if not inspecting a trace buffer, or
 if extra static tracepoint data has not been collected.
 </p>
 </dd>
 <dt><code>$_siginfo</code></dt>
 <dd><a name="index-_0024_005fsiginfo_002c-convenience-variable"></a>
 <p>The variable <code>$_siginfo</code> contains extra signal information
 (see <a href="Signals.html#extra-signal-information">extra signal information</a>).  Note that <code>$_siginfo</code>
 could be empty, if the application has not yet received any signals.
 For example, it will be empty before you execute the <code>run</code> command.
 </p>
 </dd>
 <dt><code>$_tlb</code></dt>
 <dd><a name="index-_0024_005ftlb_002c-convenience-variable"></a>
 <p>The variable <code>$_tlb</code> is automatically set when debugging
 applications running on MS-Windows in native mode or connected to
 gdbserver that supports the <code>qGetTIBAddr</code> request.
 See <a href="General-Query-Packets.html#General-Query-Packets">General Query Packets</a>.
 This variable contains the address of the thread information block.
 </p>
 </dd>
 </dl>

 <p>On HP-UX systems, if you refer to a function or variable name that
 begins with a dollar sign, <small>GDB</small> searches for a user or system
 name first, before it searches for a convenience variable.
 </p>
 <hr>
 <div class="header">
 <p>
 Next: <a href="Convenience-Funs.html#Convenience-Funs" accesskey="n" rel="next">Convenience Funs</a>, Previous: <a href="Value-History.html#Value-History" accesskey="p" rel="prev">Value History</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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	<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
	<a name="Convenience-Vars"></a>
	<div class="header">
	<p>
	Next: <a href="Convenience-Funs.html#Convenience-Funs" accesskey="n" rel="next">Convenience Funs</a>, Previous: <a href="Value-History.html#Value-History" accesskey="p" rel="prev">Value History</a>, Up: <a href="Data.html#Data" accesskey="u" rel="up">Data</a>   [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
	</div>
	<hr>
	<a name="Convenience-Variables"></a>
	<h3 class="section">10.11 Convenience Variables</h3>

	<a name="index-convenience-variables"></a>
	<a name="index-user_002ddefined-variables"></a>
	<p><small>GDB</small> provides <em>convenience variables</em> that you can use within
	<small>GDB</small> to hold on to a value and refer to it later. These variables
	exist entirely within <small>GDB</small>; they are not part of your program, and
	setting a convenience variable has no direct effect on further execution
	of your program. That is why you can use them freely.
	</p>
	<p>Convenience variables are prefixed with ‘<samp>$</samp>’. Any name preceded by
	‘<samp>$</samp>’ can be used for a convenience variable, unless it is one of
	the predefined machine-specific register names (see <a href="Registers.html#Registers">Registers</a>).
	(Value history references, in contrast, are <em>numbers</em> preceded
	by ‘<samp>$</samp>’. See <a href="Value-History.html#Value-History">Value History</a>.)
	</p>
	<p>You can save a value in a convenience variable with an assignment
	expression, just as you would set a variable in your program.
	For example:
	</p>
	<div class="smallexample">
	<pre class="smallexample">set $foo = *object_ptr
	</pre></div>

	<p>would save in <code>$foo</code> the value contained in the object pointed to by
	<code>object_ptr</code>.
	</p>
	<p>Using a convenience variable for the first time creates it, but its
	value is <code>void</code> until you assign a new value. You can alter the
	value with another assignment at any time.
	</p>
	<p>Convenience variables have no fixed types. You can assign a convenience
	variable any type of value, including structures and arrays, even if
	that variable already has a value of a different type. The convenience
	variable, when used as an expression, has the type of its current value.
	</p>
	<dl compact="compact">
	<dd><a name="index-show-convenience"></a>
	<a name="index-show-all-user-variables-and-functions"></a>
	</dd>
	<dt><code>show convenience</code></dt>
	<dd><p>Print a list of convenience variables used so far, and their values,
	as well as a list of the convenience functions.
	Abbreviated <code>show conv</code>.
	</p>
	<a name="index-init_002dif_002dundefined"></a>
	<a name="index-convenience-variables_002c-initializing"></a>
	</dd>
	<dt><code>init-if-undefined $<var>variable</var> = <var>expression</var></code></dt>
	<dd><p>Set a convenience variable if it has not already been set. This is useful
	for user-defined commands that keep some state. It is similar, in concept,
	to using local static variables with initializers in C (except that
	convenience variables are global). It can also be used to allow users to
	override default values used in a command script.
	</p>
	<p>If the variable is already defined then the expression is not evaluated so
	any side-effects do not occur.
	</p></dd>
	</dl>

	<p>One of the ways to use a convenience variable is as a counter to be
	incremented or a pointer to be advanced. For example, to print
	a field from successive elements of an array of structures:
	</p>
	<div class="smallexample">
	<pre class="smallexample">set $i = 0
	print bar[$i++]->contents
	</pre></div>

	<p>Repeat that command by typing <tt class="key">RET</tt>.
	</p>
	<p>Some convenience variables are created automatically by <small>GDB</small> and given
	values likely to be useful.
	</p>
	<dl compact="compact">
	<dd><a name="index-_0024_005f_002c-convenience-variable"></a>
	</dd>
	<dt><code>$_</code></dt>
	<dd><p>The variable <code>$_</code> is automatically set by the <code>x</code> command to
	the last address examined (see <a href="Memory.html#Memory">Examining Memory</a>). Other
	commands which provide a default address for <code>x</code> to examine also
	set <code>$_</code> to that address; these commands include <code>info line</code>
	and <code>info breakpoint</code>. The type of <code>$_</code> is <code>void *</code>
	except when set by the <code>x</code> command, in which case it is a pointer
	to the type of <code>$__</code>.
	</p>
	<a name="index-_0024_005f_005f_002c-convenience-variable"></a>
	</dd>
	<dt><code>$__</code></dt>
	<dd><p>The variable <code>$__</code> is automatically set by the <code>x</code> command
	to the value found in the last address examined. Its type is chosen
	to match the format in which the data was printed.
	</p>
	</dd>
	<dt><code>$_exitcode</code></dt>
	<dd><a name="index-_0024_005fexitcode_002c-convenience-variable"></a>
	<p>When the program being debugged terminates normally, <small>GDB</small>
	automatically sets this variable to the exit code of the program, and
	resets <code>$_exitsignal</code> to <code>void</code>.
	</p>
	</dd>
	<dt><code>$_exitsignal</code></dt>
	<dd><a name="index-_0024_005fexitsignal_002c-convenience-variable"></a>
	<p>When the program being debugged dies due to an uncaught signal,
	<small>GDB</small> automatically sets this variable to that signal’s number,
	and resets <code>$_exitcode</code> to <code>void</code>.
	</p>
	<p>To distinguish between whether the program being debugged has exited
	(i.e., <code>$_exitcode</code> is not <code>void</code>) or signalled (i.e.,
	<code>$_exitsignal</code> is not <code>void</code>), the convenience function
	<code>$_isvoid</code> can be used (see <a href="Convenience-Funs.html#Convenience-Funs">Convenience
	Functions</a>). For example, considering the following source code:
	</p>
	<div class="smallexample">
	<pre class="smallexample">#include <signal.h>

	int
	main (int argc, char *argv[])
	{
	raise (SIGALRM);
	return 0;
	}
	</pre></div>

	<p>A valid way of telling whether the program being debugged has exited
	or signalled would be:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) define has_exited_or_signalled
	Type commands for definition of ``has_exited_or_signalled''.
	End with a line saying just ``end''.
	>if $_isvoid ($_exitsignal)
	>echo The program has exited\n
	>else
	>echo The program has signalled\n
	>end
	>end
	(gdb) run
	Starting program:

	Program terminated with signal SIGALRM, Alarm clock.
	The program no longer exists.
	(gdb) has_exited_or_signalled
	The program has signalled
	</pre></div>

	<p>As can be seen, <small>GDB</small> correctly informs that the program being
	debugged has signalled, since it calls <code>raise</code> and raises a
	<code>SIGALRM</code> signal. If the program being debugged had not called
	<code>raise</code>, then <small>GDB</small> would report a normal exit:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(gdb) has_exited_or_signalled
	The program has exited
	</pre></div>

	</dd>
	<dt><code>$_exception</code></dt>
	<dd><p>The variable <code>$_exception</code> is set to the exception object being
	thrown at an exception-related catchpoint. See <a href="Set-Catchpoints.html#Set-Catchpoints">Set Catchpoints</a>.
	</p>
	</dd>
	<dt><code>$_probe_argc</code></dt>
	<dt><code>$_probe_arg0…$_probe_arg11</code></dt>
	<dd><p>Arguments to a static probe. See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.
	</p>
	</dd>
	<dt><code>$_sdata</code></dt>
	<dd><a name="index-_0024_005fsdata_002c-inspect_002c-convenience-variable"></a>
	<p>The variable <code>$_sdata</code> contains extra collected static tracepoint
	data. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>. Note that
	<code>$_sdata</code> could be empty, if not inspecting a trace buffer, or
	if extra static tracepoint data has not been collected.
	</p>
	</dd>
	<dt><code>$_siginfo</code></dt>
	<dd><a name="index-_0024_005fsiginfo_002c-convenience-variable"></a>
	<p>The variable <code>$_siginfo</code> contains extra signal information
	(see <a href="Signals.html#extra-signal-information">extra signal information</a>). Note that <code>$_siginfo</code>
	could be empty, if the application has not yet received any signals.
	For example, it will be empty before you execute the <code>run</code> command.
	</p>
	</dd>
	<dt><code>$_tlb</code></dt>
	<dd><a name="index-_0024_005ftlb_002c-convenience-variable"></a>
	<p>The variable <code>$_tlb</code> is automatically set when debugging
	applications running on MS-Windows in native mode or connected to
	gdbserver that supports the <code>qGetTIBAddr</code> request.
	See <a href="General-Query-Packets.html#General-Query-Packets">General Query Packets</a>.
	This variable contains the address of the thread information block.
	</p>
	</dd>
	</dl>

	<p>On HP-UX systems, if you refer to a function or variable name that
	begins with a dollar sign, <small>GDB</small> searches for a user or system
	name first, before it searches for a convenience variable.
	</p>
	<hr>
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