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<a name="Convenience-Vars"></a>
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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>
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<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>$_tlb</code> is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the <code>qGetTIBAddr</code> request.
See <a href="General-Query-Packets.html#General-Query-Packets">General Query Packets</a>.
This variable contains the address of the thread information block.
</p>
</dd>
<dt><code>$_inferior</code></dt>
<dd><p>The number of the current inferior. See <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors and Programs</a>.
</p>
</dd>
<dt><code>$_thread</code></dt>
<dd><p>The thread number of the current thread. See <a href="Threads.html#thread-numbers">thread numbers</a>.
</p>
</dd>
<dt><code>$_gthread</code></dt>
<dd><p>The global number of the current thread. See <a href="Threads.html#global-thread-numbers">global thread numbers</a>.
</p>
</dd>
</dl>
<hr>
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