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