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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 ‘<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-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++]->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-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 <signal.h> |
| |
| 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''. |
| >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> |
| <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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