124 lines
5.6 KiB
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124 lines
5.6 KiB
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<html lang="en">
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<head>
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<title>Hooks - Debugging with GDB</title>
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<meta name="description" content="Debugging with GDB">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Sequences.html#Sequences" title="Sequences">
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<link rel="prev" href="Define.html#Define" title="Define">
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<link rel="next" href="Command-Files.html#Command-Files" title="Command Files">
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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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Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,''
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<body>
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<div class="node">
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<a name="Hooks"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Command-Files.html#Command-Files">Command Files</a>,
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Previous: <a rel="previous" accesskey="p" href="Define.html#Define">Define</a>,
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Up: <a rel="up" accesskey="u" href="Sequences.html#Sequences">Sequences</a>
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<hr>
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</div>
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<h4 class="subsection">23.1.2 User-defined Command Hooks</h4>
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<p><a name="index-command-hooks-1810"></a><a name="index-hooks_002c-for-commands-1811"></a><a name="index-hooks_002c-pre_002dcommand-1812"></a>
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<a name="index-hook-1813"></a>You may define <dfn>hooks</dfn>, which are a special kind of user-defined
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command. Whenever you run the command ‘<samp><span class="samp">foo</span></samp>’, if the user-defined
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command ‘<samp><span class="samp">hook-foo</span></samp>’ exists, it is executed (with no arguments)
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before that command.
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<p><a name="index-hooks_002c-post_002dcommand-1814"></a><a name="index-hookpost-1815"></a>A hook may also be defined which is run after the command you executed.
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Whenever you run the command ‘<samp><span class="samp">foo</span></samp>’, if the user-defined command
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‘<samp><span class="samp">hookpost-foo</span></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>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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<!-- It would be nice if hookpost could be passed a parameter indicating -->
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<!-- if the command it hooks executed properly or not. FIXME! -->
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<p><a name="index-stop_0040r_007b_002c-a-pseudo_002dcommand_007d-1816"></a>In addition, a pseudo-command, ‘<samp><span class="samp">stop</span></samp>’ exists. Defining
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(‘<samp><span class="samp">hook-stop</span></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>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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<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>
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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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<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>
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<p>You can define a hook for any single-word command in <span class="sc">gdb</span>, 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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<!-- FIXME! So how does Joe User discover whether a command is an alias -->
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<!-- or not? -->
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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><span class="samp">define target hook-remote</span></samp>’ to add a hook to ‘<samp><span class="samp">target remote</span></samp>’.
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<p>If an error occurs during the execution of your hook, execution of
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<span class="sc">gdb</span> commands stops and <span class="sc">gdb</span> issues a prompt
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(before the command that you actually typed had a chance to run).
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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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</body></html>
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