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286 lines
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286 lines
12 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Convenience Vars (Debugging with GDB)</title>
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<meta name="description" content="Convenience Vars (Debugging with GDB)">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Data.html#Data" rel="up" title="Data">
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<link href="Convenience-Funs.html#Convenience-Funs" rel="next" title="Convenience Funs">
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</head>
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<body lang="en">
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<a name="Convenience-Vars"></a>
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<div class="header">
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<p>
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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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</div>
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<hr>
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<a name="Convenience-Variables"></a>
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<h3 class="section">10.11 Convenience Variables</h3>
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<a name="index-convenience-variables"></a>
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<a name="index-user_002ddefined-variables"></a>
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<p><small>GDB</small> provides <em>convenience variables</em> that you can use within
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<small>GDB</small> to hold on to a value and refer to it later. These variables
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exist entirely within <small>GDB</small>; 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>
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<p>Convenience variables are prefixed with ‘<samp>$</samp>’. Any name preceded by
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‘<samp>$</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>$</samp>’. See <a href="Value-History.html#Value-History">Value History</a>.)
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</p>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample">set $foo = *object_ptr
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</pre></div>
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<p>would save in <code>$foo</code> the value contained in the object pointed to by
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<code>object_ptr</code>.
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</p>
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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>
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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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</p>
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<dl compact="compact">
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<dd><a name="index-show-convenience"></a>
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<a name="index-show-all-user-variables-and-functions"></a>
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</dd>
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<dt><code>show convenience</code></dt>
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<dd><p>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>
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<a name="index-init_002dif_002dundefined"></a>
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<a name="index-convenience-variables_002c-initializing"></a>
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</dd>
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<dt><code>init-if-undefined $<var>variable</var> = <var>expression</var></code></dt>
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<dd><p>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>
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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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</p></dd>
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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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</p>
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<div class="smallexample">
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<pre class="smallexample">set $i = 0
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print bar[$i++]->contents
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</pre></div>
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<p>Repeat that command by typing <tt class="key">RET</tt>.
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</p>
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<p>Some convenience variables are created automatically by <small>GDB</small> and given
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values likely to be useful.
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</p>
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<dl compact="compact">
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<dd><a name="index-_0024_005f_002c-convenience-variable"></a>
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</dd>
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<dt><code>$_</code></dt>
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<dd><p>The variable <code>$_</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>$__</code>.
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</p>
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<a name="index-_0024_005f_005f_002c-convenience-variable"></a>
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</dd>
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<dt><code>$__</code></dt>
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<dd><p>The variable <code>$__</code> is automatically set by the <code>x</code> command
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to the value found in the last address examined. Its type is chosen
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to match the format in which the data was printed.
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</p>
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</dd>
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<dt><code>$_exitcode</code></dt>
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<dd><a name="index-_0024_005fexitcode_002c-convenience-variable"></a>
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<p>When the program being debugged terminates normally, <small>GDB</small>
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automatically sets this variable to the exit code of the program, and
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resets <code>$_exitsignal</code> to <code>void</code>.
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</p>
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</dd>
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<dt><code>$_exitsignal</code></dt>
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<dd><a name="index-_0024_005fexitsignal_002c-convenience-variable"></a>
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<p>When the program being debugged dies due to an uncaught signal,
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<small>GDB</small> automatically sets this variable to that signal’s number,
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and resets <code>$_exitcode</code> to <code>void</code>.
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</p>
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<p>To distinguish between whether the program being debugged has exited
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(i.e., <code>$_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
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Functions</a>). For example, considering the following source code:
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</p>
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<div class="smallexample">
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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></div>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>As can be seen, <small>GDB</small> 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 <small>GDB</small> would report a normal exit:
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</p>
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<div class="smallexample">
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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></div>
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</dd>
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<dt><code>$_exception</code></dt>
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<dd><p>The variable <code>$_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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</p>
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</dd>
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<dt><code>$_probe_argc</code></dt>
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<dt><code>$_probe_arg0…$_probe_arg11</code></dt>
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<dd><p>Arguments to a static probe. See <a href="Static-Probe-Points.html#Static-Probe-Points">Static Probe Points</a>.
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</p>
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</dd>
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<dt><code>$_sdata</code></dt>
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<dd><a name="index-_0024_005fsdata_002c-inspect_002c-convenience-variable"></a>
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<p>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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</p>
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</dd>
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<dt><code>$_siginfo</code></dt>
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<dd><a name="index-_0024_005fsiginfo_002c-convenience-variable"></a>
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<p>The variable <code>$_siginfo</code> contains extra signal information
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(see <a href="Signals.html#extra-signal-information">extra signal information</a>). Note that <code>$_siginfo</code>
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could be empty, if the application has not yet received any signals.
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For example, it will be empty before you execute the <code>run</code> command.
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</p>
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</dd>
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<dt><code>$_tlb</code></dt>
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<dd><a name="index-_0024_005ftlb_002c-convenience-variable"></a>
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<p>The variable <code>$_tlb</code> is automatically set when debugging
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applications running on MS-Windows in native mode or connected to
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gdbserver that supports the <code>qGetTIBAddr</code> request.
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See <a href="General-Query-Packets.html#General-Query-Packets">General Query Packets</a>.
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This variable contains the address of the thread information block.
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</p>
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</dd>
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<dt><code>$_inferior</code></dt>
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<dd><p>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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</p>
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</dd>
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<dt><code>$_thread</code></dt>
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<dd><p>The thread number of the current thread. See <a href="Threads.html#thread-numbers">thread numbers</a>.
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</p>
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</dd>
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<dt><code>$_gthread</code></dt>
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<dd><p>The global number of the current thread. See <a href="Threads.html#global-thread-numbers">global thread numbers</a>.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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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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</div>
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