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<a name="Convenience-Vars"></a>
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Next: <a rel="next" accesskey="n" href="Convenience-Funs.html#Convenience-Funs">Convenience Funs</a>,
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Previous: <a rel="previous" accesskey="p" href="Value-History.html#Value-History">Value History</a>,
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Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
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<h3 class="section">10.11 Convenience Variables</h3>
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<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
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<span class="sc">gdb</span> to hold on to a value and refer to it later. These variables
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exist entirely within <span class="sc">gdb</span>; they are not part of your program, and
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setting a convenience variable has no direct effect on further execution
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of your program. That is why you can use them freely.
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<p>Convenience variables are prefixed with ‘<samp><span class="samp">$</span></samp>’. Any name preceded by
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‘<samp><span class="samp">$</span></samp>’ can be used for a convenience variable, unless it is one of
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the predefined machine-specific register names (see <a href="Registers.html#Registers">Registers</a>).
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(Value history references, in contrast, are <em>numbers</em> preceded
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by ‘<samp><span class="samp">$</span></samp>’. See <a href="Value-History.html#Value-History">Value History</a>.)
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<p>You can save a value in a convenience variable with an assignment
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expression, just as you would set a variable in your program.
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For example:
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<pre class="smallexample"> set $foo = *object_ptr
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</pre>
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<p class="noindent">would save in <code>$foo</code> the value contained in the object pointed to by
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<code>object_ptr</code>.
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<p>Using a convenience variable for the first time creates it, but its
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value is <code>void</code> until you assign a new value. You can alter the
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value with another assignment at any time.
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<p>Convenience variables have no fixed types. You can assign a convenience
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variable any type of value, including structures and arrays, even if
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that variable already has a value of a different type. The convenience
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variable, when used as an expression, has the type of its current value.
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<a name="index-show-convenience-725"></a>
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<a name="index-show-all-user-variables-and-functions-726"></a>
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<dl><dt><code>show convenience</code><dd>Print a list of convenience variables used so far, and their values,
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as well as a list of the convenience functions.
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Abbreviated <code>show conv</code>.
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<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
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for user-defined commands that keep some state. It is similar, in concept,
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to using local static variables with initializers in C (except that
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convenience variables are global). It can also be used to allow users to
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override default values used in a command script.
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<p>If the variable is already defined then the expression is not evaluated so
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any side-effects do not occur.
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</dl>
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<p>One of the ways to use a convenience variable is as a counter to be
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incremented or a pointer to be advanced. For example, to print
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a field from successive elements of an array of structures:
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<pre class="smallexample"> set $i = 0
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print bar[$i++]->contents
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</pre>
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<p class="noindent">Repeat that command by typing <RET>.
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<p>Some convenience variables are created automatically by <span class="sc">gdb</span> and given
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values likely to be useful.
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<a name="index-g_t_0024_005f_0040r_007b_002c-convenience-variable_007d-729"></a>
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<dl><dt><code>$_</code><dd>The variable <code>$_</code> is automatically set by the <code>x</code> command to
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the last address examined (see <a href="Memory.html#Memory">Examining Memory</a>). Other
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commands which provide a default address for <code>x</code> to examine also
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set <code>$_</code> to that address; these commands include <code>info line</code>
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and <code>info breakpoint</code>. The type of <code>$_</code> is <code>void *</code>
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except when set by the <code>x</code> command, in which case it is a pointer
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to the type of <code>$__</code>.
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<p><a name="index-g_t_0024_005f_005f_0040r_007b_002c-convenience-variable_007d-730"></a><br><dt><code>$__</code><dd>The variable <code>$__</code> is automatically set by the <code>x</code> command
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to the value found in the last address examined. Its type is chosen
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to match the format in which the data was printed.
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<br><dt><code>$_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>
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automatically sets this variable to the exit code of the program, and
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resets <code>$_exitsignal</code> to <code>void</code>.
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<br><dt><code>$_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,
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<span class="sc">gdb</span> automatically sets this variable to that signal's number,
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and resets <code>$_exitcode</code> to <code>void</code>.
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<p>To distinguish between whether the program being debugged has exited
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(i.e., <code>$_exitcode</code> is not <code>void</code>) or signalled (i.e.,
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<code>$_exitsignal</code> is not <code>void</code>), the convenience function
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<code>$_isvoid</code> can be used (see <a href="Convenience-Funs.html#Convenience-Funs">Convenience Functions</a>). For example, considering the following source code:
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<pre class="smallexample"> #include <signal.h>
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int
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main (int argc, char *argv[])
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{
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raise (SIGALRM);
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return 0;
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}
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</pre>
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<p>A valid way of telling whether the program being debugged has exited
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or signalled would be:
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<pre class="smallexample"> (gdb) define has_exited_or_signalled
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Type commands for definition of ``has_exited_or_signalled''.
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End with a line saying just ``end''.
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>if $_isvoid ($_exitsignal)
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>echo The program has exited\n
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>else
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>echo The program has signalled\n
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>end
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>end
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(gdb) run
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Starting program:
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Program terminated with signal SIGALRM, Alarm clock.
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The program no longer exists.
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(gdb) has_exited_or_signalled
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The program has signalled
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</pre>
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<p>As can be seen, <span class="sc">gdb</span> correctly informs that the program being
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debugged has signalled, since it calls <code>raise</code> and raises a
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<code>SIGALRM</code> signal. If the program being debugged had not called
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<code>raise</code>, then <span class="sc">gdb</span> would report a normal exit:
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<pre class="smallexample"> (gdb) has_exited_or_signalled
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The program has exited
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</pre>
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<br><dt><code>$_exception</code><dd>The variable <code>$_exception</code> is set to the exception object being
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thrown at an exception-related catchpoint. See <a href="Set-Catchpoints.html#Set-Catchpoints">Set Catchpoints</a>.
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<br><dt><code>$_probe_argc</code><dt><code>$_probe_arg0...$_probe_arg11</code><dd>Arguments to a static probe. See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.
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<br><dt><code>$_sdata</code><dd><a name="index-g_t_0024_005fsdata_0040r_007b_002c-inspect_002c-convenience-variable_007d-733"></a>The variable <code>$_sdata</code> contains extra collected static tracepoint
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data. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>. Note that
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<code>$_sdata</code> could be empty, if not inspecting a trace buffer, or
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if extra static tracepoint data has not been collected.
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<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
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(see <a href="extra-signal-information.html#extra-signal-information">extra signal information</a>). Note that <code>$_siginfo</code>
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could be empty, if the application has not yet received any signals.
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For example, it will be empty before you execute the <code>run</code> command.
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<br><dt><code>$_tlb</code><dd><a name="index-g_t_0024_005ftlb_0040r_007b_002c-convenience-variable_007d-735"></a>The variable <code>$_tlb</code> is automatically set when debugging
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applications running on MS-Windows in native mode or connected to
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gdbserver that supports the <code>qGetTIBAddr</code> request.
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See <a href="General-Query-Packets.html#General-Query-Packets">General Query Packets</a>.
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This variable contains the address of the thread information block.
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<br><dt><code>$_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>.
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<br><dt><code>$_thread</code><dd>The thread number of the current thread. See <a href="thread-numbers.html#thread-numbers">thread numbers</a>.
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<br><dt><code>$_gthread</code><dd>The global number of the current thread. See <a href="global-thread-numbers.html#global-thread-numbers">global thread numbers</a>.
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</dl>
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