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

<p><a name="index-convenience-variables-723"></a><a name="index-user_002ddefined-variables-724"></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-725"></a>
<a name="index-show-all-user-variables-and-functions-726"></a>
<dl><dt><code>show convenience</code><dd>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-727"></a><a name="index-convenience-variables_002c-initializing-728"></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-729"></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-730"></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>$_exitcode</code><dd><a name="index-g_t_0024_005fexitcode_0040r_007b_002c-convenience-variable_007d-731"></a>When the program being debugged terminates normally, <span class="sc">gdb</span>
automatically sets this variable to the exit code of the program, and
resets <code>$_exitsignal</code> to <code>void</code>.

     <br><dt><code>$_exitsignal</code><dd><a name="index-g_t_0024_005fexitsignal_0040r_007b_002c-convenience-variable_007d-732"></a>When the program being debugged dies due to an uncaught signal,
<span class="sc">gdb</span> automatically sets this variable to that signal's number,
and resets <code>$_exitcode</code> to <code>void</code>.

     <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:

     <pre class="smallexample">          #include &lt;signal.h&gt;
          
          int
          main (int argc, char *argv[])
          {
            raise (SIGALRM);
            return 0;
          }
</pre>
     <p>A valid way of telling whether the program being debugged has exited
or signalled would be:

     <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>
     <p>As can be seen, <span class="sc">gdb</span> 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 <span class="sc">gdb</span> would report a normal exit:

     <pre class="smallexample">          (gdb) has_exited_or_signalled
          The program has exited
</pre>
     <br><dt><code>$_exception</code><dd>The variable <code>$_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>.

     <br><dt><code>$_probe_argc</code><dt><code>$_probe_arg0...$_probe_arg11</code><dd>Arguments to a static probe.  See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.

     <br><dt><code>$_sdata</code><dd><a name="index-g_t_0024_005fsdata_0040r_007b_002c-inspect_002c-convenience-variable_007d-733"></a>The variable <code>$_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>$_siginfo</code><dd><a name="index-g_t_0024_005fsiginfo_0040r_007b_002c-convenience-variable_007d-734"></a>The variable <code>$_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>$_tlb</code><dd><a name="index-g_t_0024_005ftlb_0040r_007b_002c-convenience-variable_007d-735"></a>The variable <code>$_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.

     <br><dt><code>$_inferior</code><dd>The number of the current inferior.  See <a href="Inferiors-and-Programs.html#Inferiors-and-Programs">Debugging Multiple Inferiors and Programs</a>.

     <br><dt><code>$_thread</code><dd>The thread number of the current thread.  See <a href="thread-numbers.html#thread-numbers">thread numbers</a>.

     <br><dt><code>$_gthread</code><dd>The global number of the current thread.  See <a href="global-thread-numbers.html#global-thread-numbers">global thread numbers</a>.

   </dl>

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