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 <h3 class="section">10.11 Convenience Variables</h3>

 <p><a name="index-convenience-variables-583"></a><a name="index-user_002ddefined-variables-584"></a><span class="sc">gdb</span> provides <dfn>convenience variables</dfn> that you can use within
 <span class="sc">gdb</span> to hold on to a value and refer to it later.  These variables
 exist entirely within <span class="sc">gdb</span>; 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>Convenience variables are prefixed with &lsquo;<samp><span class="samp">$</span></samp>&rsquo;.  Any name preceded by
 &lsquo;<samp><span class="samp">$</span></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><span class="samp">$</span></samp>&rsquo;.  See <a href="Value-History.html#Value-History">Value History</a>.)

    <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:

 <pre class="smallexample">     set $foo = *object_ptr
 </pre>
    <p class="noindent">would save in <code>$foo</code> the value contained in the object pointed to by
 <code>object_ptr</code>.

    <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>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.


 <a name="index-show-convenience-585"></a>
 <a name="index-show-all-user-variables-586"></a>
 <dl><dt><code>show convenience</code><dd>Print a list of convenience variables used so far, and their values.
 Abbreviated <code>show conv</code>.

      <p><a name="index-init_002dif_002dundefined-587"></a><a name="index-convenience-variables_002c-initializing-588"></a><br><dt><code>init-if-undefined $</code><var>variable</var><code> = </code><var>expression</var><dd>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>If the variable is already defined then the expression is not evaluated so
 any side-effects do not occur.
 </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:

 <pre class="smallexample">     set $i = 0
      print bar[$i++]-&gt;contents
 </pre>
    <p class="noindent">Repeat that command by typing &lt;RET&gt;.

    <p>Some convenience variables are created automatically by <span class="sc">gdb</span> and given
 values likely to be useful.


 <a name="index-g_t_0024_005f_0040r_007b_002c-convenience-variable_007d-589"></a>
 <dl><dt><code>$_</code><dd>The variable <code>$_</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-g_t_0024_005f_005f_0040r_007b_002c-convenience-variable_007d-590"></a><br><dt><code>$__</code><dd>The variable <code>$__</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.

      <br><dt><code>$_exitcode</code><dd><a name="index-g_t_0024_005fexitcode_0040r_007b_002c-convenience-variable_007d-591"></a>The variable <code>$_exitcode</code> is automatically set to the exit code when
 the program being debugged terminates.

      <br><dt><code>$_sdata</code><dd><a name="index-g_t_0024_005fsdata_0040r_007b_002c-inspect_002c-convenience-variable_007d-592"></a>The variable <code>$_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.

      <br><dt><code>$_siginfo</code><dd><a name="index-g_t_0024_005fsiginfo_0040r_007b_002c-convenience-variable_007d-593"></a>The variable <code>$_siginfo</code> contains extra signal information
 (see <a href="extra-signal-information.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.

      <br><dt><code>$_tlb</code><dd><a name="index-g_t_0024_005ftlb_0040r_007b_002c-convenience-variable_007d-594"></a>The variable <code>$_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.

    </dl>

    <p>On HP-UX systems, if you refer to a function or variable name that
 begins with a dollar sign, <span class="sc">gdb</span> searches for a user or system
 name first, before it searches for a convenience variable.

    <p><a name="index-convenience-functions-595"></a><span class="sc">gdb</span> also supplies some <dfn>convenience functions</dfn>.  These
 have a syntax similar to convenience variables.  A convenience
 function can be used in an expression just like an ordinary function;
 however, a convenience function is implemented internally to
 <span class="sc">gdb</span>.

      <dl>
 <dt><code>help function</code><dd><a name="index-help-function-596"></a><a name="index-show-all-convenience-functions-597"></a>Print a list of all convenience functions.
 </dl>

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	<h3 class="section">10.11 Convenience Variables</h3>

	<p><a name="index-convenience-variables-583"></a><a name="index-user_002ddefined-variables-584"></a><span class="sc">gdb</span> provides <dfn>convenience variables</dfn> that you can use within
	<span class="sc">gdb</span> to hold on to a value and refer to it later. These variables
	exist entirely within <span class="sc">gdb</span>; 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>Convenience variables are prefixed with ‘<samp><span class="samp">$</span></samp>’. Any name preceded by
	‘<samp><span class="samp">$</span></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><span class="samp">$</span></samp>’. See <a href="Value-History.html#Value-History">Value History</a>.)

	<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:

	<pre class="smallexample"> set $foo = *object_ptr
	</pre>
	<p class="noindent">would save in <code>$foo</code> the value contained in the object pointed to by
	<code>object_ptr</code>.

	<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>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.


	<a name="index-show-convenience-585"></a>
	<a name="index-show-all-user-variables-586"></a>
	<dl><dt><code>show convenience</code><dd>Print a list of convenience variables used so far, and their values.
	Abbreviated <code>show conv</code>.

	<p><a name="index-init_002dif_002dundefined-587"></a><a name="index-convenience-variables_002c-initializing-588"></a><br><dt><code>init-if-undefined $</code><var>variable</var><code> = </code><var>expression</var><dd>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>If the variable is already defined then the expression is not evaluated so
	any side-effects do not occur.
	</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:

	<pre class="smallexample"> set $i = 0
	print bar[$i++]->contents
	</pre>
	<p class="noindent">Repeat that command by typing <RET>.

	<p>Some convenience variables are created automatically by <span class="sc">gdb</span> and given
	values likely to be useful.


	<a name="index-g_t_0024_005f_0040r_007b_002c-convenience-variable_007d-589"></a>
	<dl><dt><code>$_</code><dd>The variable <code>$_</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-g_t_0024_005f_005f_0040r_007b_002c-convenience-variable_007d-590"></a><br><dt><code>$__</code><dd>The variable <code>$__</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.

	<br><dt><code>$_exitcode</code><dd><a name="index-g_t_0024_005fexitcode_0040r_007b_002c-convenience-variable_007d-591"></a>The variable <code>$_exitcode</code> is automatically set to the exit code when
	the program being debugged terminates.

	<br><dt><code>$_sdata</code><dd><a name="index-g_t_0024_005fsdata_0040r_007b_002c-inspect_002c-convenience-variable_007d-592"></a>The variable <code>$_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.

	<br><dt><code>$_siginfo</code><dd><a name="index-g_t_0024_005fsiginfo_0040r_007b_002c-convenience-variable_007d-593"></a>The variable <code>$_siginfo</code> contains extra signal information
	(see <a href="extra-signal-information.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.

	<br><dt><code>$_tlb</code><dd><a name="index-g_t_0024_005ftlb_0040r_007b_002c-convenience-variable_007d-594"></a>The variable <code>$_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.

	</dl>

	<p>On HP-UX systems, if you refer to a function or variable name that
	begins with a dollar sign, <span class="sc">gdb</span> searches for a user or system
	name first, before it searches for a convenience variable.

	<p><a name="index-convenience-functions-595"></a><span class="sc">gdb</span> also supplies some <dfn>convenience functions</dfn>. These
	have a syntax similar to convenience variables. A convenience
	function can be used in an expression just like an ordinary function;
	however, a convenience function is implemented internally to
	<span class="sc">gdb</span>.

	<dl>
	<dt><code>help function</code><dd><a name="index-help-function-596"></a><a name="index-show-all-convenience-functions-597"></a>Print a list of all convenience functions.
	</dl>

	</body></html>