205 lines
11 KiB
HTML
205 lines
11 KiB
HTML
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Convenience Vars - Debugging with GDB</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="Debugging with GDB">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="Data.html#Data" title="Data">
|
||
|
|
<link rel="prev" href="Value-History.html#Value-History" title="Value History">
|
||
|
|
<link rel="next" href="Convenience-Funs.html#Convenience-Funs" title="Convenience Funs">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
Copyright (C) 1988-2019 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with the
|
||
|
|
Invariant Sections being ``Free Software'' and ``Free Software Needs
|
||
|
|
Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
|
||
|
|
and with the Back-Cover Texts as in (a) below.
|
||
|
|
|
||
|
|
(a) The FSF's Back-Cover Text is: ``You are free to copy and modify
|
||
|
|
this GNU Manual. Buying copies from GNU Press supports the FSF in
|
||
|
|
developing GNU and promoting software freedom.''
|
||
|
|
-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Convenience-Vars"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Convenience-Funs.html#Convenience-Funs">Convenience Funs</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="Value-History.html#Value-History">Value History</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="Data.html#Data">Data</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<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>
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|