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<title>Hooks (Debugging with GDB)</title>
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<a name="Hooks"></a>
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<hr>
<a name="User_002ddefined-Command-Hooks"></a>
<h4 class="subsection">23.1.2 User-defined Command Hooks</h4>
<a name="index-command-hooks"></a>
<a name="index-hooks_002c-for-commands"></a>
<a name="index-hooks_002c-pre_002dcommand"></a>
<a name="index-hook"></a>
<p>You may define <em>hooks</em>, which are a special kind of user-defined
command. Whenever you run the command ‘<samp>foo</samp>’, if the user-defined
command ‘<samp>hook-foo</samp>’ exists, it is executed (with no arguments)
before that command.
</p>
<a name="index-hooks_002c-post_002dcommand"></a>
<a name="index-hookpost"></a>
<p>A hook may also be defined which is run after the command you executed.
Whenever you run the command ‘<samp>foo</samp>’, if the user-defined command
‘<samp>hookpost-foo</samp>’ exists, it is executed (with no arguments) after
that command. Post-execution hooks may exist simultaneously with
pre-execution hooks, for the same command.
</p>
<p>It is valid for a hook to call the command which it hooks. If this
occurs, the hook is not re-executed, thereby avoiding infinite recursion.
</p>
<a name="index-stop_002c-a-pseudo_002dcommand"></a>
<p>In addition, a pseudo-command, ‘<samp>stop</samp>’ exists. Defining
(‘<samp>hook-stop</samp>’) makes the associated commands execute every time
execution stops in your program: before breakpoint commands are run,
displays are printed, or the stack frame is printed.
</p>
<p>For example, to ignore <code>SIGALRM</code> signals while
single-stepping, but treat them normally during normal execution,
you could define:
</p>
<div class="smallexample">
<pre class="smallexample">define hook-stop
handle SIGALRM nopass
end
define hook-run
handle SIGALRM pass
end
define hook-continue
handle SIGALRM pass
end
</pre></div>
<p>As a further example, to hook at the beginning and end of the <code>echo</code>
command, and to add extra text to the beginning and end of the message,
you could define:
</p>
<div class="smallexample">
<pre class="smallexample">define hook-echo
echo <<<---
end
define hookpost-echo
echo --->>>\n
end
(gdb) echo Hello World
<<<---Hello World--->>>
(gdb)
</pre></div>
<p>You can define a hook for any single-word command in <small>GDB</small>, but
not for command aliases; you should define a hook for the basic command
name, e.g. <code>backtrace</code> rather than <code>bt</code>.
You can hook a multi-word command by adding <code>hook-</code> or
<code>hookpost-</code> to the last word of the command, e.g.
‘<samp>define target hook-remote</samp>’ to add a hook to ‘<samp>target remote</samp>’.
</p>
<p>If an error occurs during the execution of your hook, execution of
<small>GDB</small> commands stops and <small>GDB</small> issues a prompt
(before the command that you actually typed had a chance to run).
</p>
<p>If you try to define a hook which does not match any known command, you
get a warning from the <code>define</code> command.
</p>
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