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<a name="Convenience-Vars"></a>
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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 &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>$_ada_exception</code></dt>
<dd><p>The variable <code>$_ada_exception</code> is set to the address of the
exception being caught or thrown at an Ada 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>$_tlb</code> is automatically set when debugging
applications running on MS-Windows in native mode or connected to
gdbserver that supports the <code>qGetTIBAddr</code> request.
See <a href="General-Query-Packets.html#General-Query-Packets">General Query Packets</a>.
This variable contains the address of the thread information block.
</p>
</dd>
<dt><code>$_inferior</code></dt>
<dd><p>The number of the current inferior.  See <a href="Inferiors-Connections-and-Programs.html#Inferiors-Connections-and-Programs">Debugging Multiple Inferiors Connections 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>
<dt><code>$_gdb_major</code></dt>
<dt><code>$_gdb_minor</code></dt>
<dd><a name="index-_0024_005fgdb_005fmajor_002c-convenience-variable"></a>
<a name="index-_0024_005fgdb_005fminor_002c-convenience-variable"></a>
<p>The major and minor version numbers of the running <small>GDB</small>.
Development snapshots and pretest versions have their minor version
incremented by one; thus, <small>GDB</small> pretest 9.11.90 will produce
the value 12 for <code>$_gdb_minor</code>.  These variables allow you to
write scripts that work with different versions of <small>GDB</small>
without errors caused by features unavailable in some of those
versions.
</p>
</dd>
<dt><code>$_shell_exitcode</code></dt>
<dt><code>$_shell_exitsignal</code></dt>
<dd><a name="index-_0024_005fshell_005fexitcode_002c-convenience-variable"></a>
<a name="index-_0024_005fshell_005fexitsignal_002c-convenience-variable"></a>
<a name="index-shell-command_002c-exit-code"></a>
<a name="index-shell-command_002c-exit-signal"></a>
<a name="index-exit-status-of-shell-commands"></a>
<p><small>GDB</small> commands such as <code>shell</code> and <code>|</code> are launching
shell commands.  When a launched command terminates, <small>GDB</small>
automatically maintains the variables <code>$_shell_exitcode</code>
and <code>$_shell_exitsignal</code> according to the exit status of the last
launched command.  These variables are set and used similarly to
the variables <code>$_exitcode</code> and <code>$_exitsignal</code>.
</p>
</dd>
</dl>

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