Hooks.html 6.6 KB

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  58. <a name="Hooks"></a>
  59. <div class="header">
  60. <p>
  61. 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> &nbsp; [<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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  63. <hr>
  64. <a name="User_002ddefined-Command-Hooks"></a>
  65. <h4 class="subsection">23.1.2 User-defined Command Hooks</h4>
  66. <a name="index-command-hooks"></a>
  67. <a name="index-hooks_002c-for-commands"></a>
  68. <a name="index-hooks_002c-pre_002dcommand"></a>
  69. <a name="index-hook"></a>
  70. <p>You may define <em>hooks</em>, which are a special kind of user-defined
  71. command. Whenever you run the command &lsquo;<samp>foo</samp>&rsquo;, if the user-defined
  72. command &lsquo;<samp>hook-foo</samp>&rsquo; exists, it is executed (with no arguments)
  73. before that command.
  74. </p>
  75. <a name="index-hooks_002c-post_002dcommand"></a>
  76. <a name="index-hookpost"></a>
  77. <p>A hook may also be defined which is run after the command you executed.
  78. Whenever you run the command &lsquo;<samp>foo</samp>&rsquo;, if the user-defined command
  79. &lsquo;<samp>hookpost-foo</samp>&rsquo; exists, it is executed (with no arguments) after
  80. that command. Post-execution hooks may exist simultaneously with
  81. pre-execution hooks, for the same command.
  82. </p>
  83. <p>It is valid for a hook to call the command which it hooks. If this
  84. occurs, the hook is not re-executed, thereby avoiding infinite recursion.
  85. </p>
  86. <a name="index-stop_002c-a-pseudo_002dcommand"></a>
  87. <p>In addition, a pseudo-command, &lsquo;<samp>stop</samp>&rsquo; exists. Defining
  88. (&lsquo;<samp>hook-stop</samp>&rsquo;) makes the associated commands execute every time
  89. execution stops in your program: before breakpoint commands are run,
  90. displays are printed, or the stack frame is printed.
  91. </p>
  92. <p>For example, to ignore <code>SIGALRM</code> signals while
  93. single-stepping, but treat them normally during normal execution,
  94. you could define:
  95. </p>
  96. <div class="smallexample">
  97. <pre class="smallexample">define hook-stop
  98. handle SIGALRM nopass
  99. end
  100. define hook-run
  101. handle SIGALRM pass
  102. end
  103. define hook-continue
  104. handle SIGALRM pass
  105. end
  106. </pre></div>
  107. <p>As a further example, to hook at the beginning and end of the <code>echo</code>
  108. command, and to add extra text to the beginning and end of the message,
  109. you could define:
  110. </p>
  111. <div class="smallexample">
  112. <pre class="smallexample">define hook-echo
  113. echo &lt;&lt;&lt;---
  114. end
  115. define hookpost-echo
  116. echo ---&gt;&gt;&gt;\n
  117. end
  118. (gdb) echo Hello World
  119. &lt;&lt;&lt;---Hello World---&gt;&gt;&gt;
  120. (gdb)
  121. </pre></div>
  122. <p>You can define a hook for any single-word command in <small>GDB</small>, but
  123. not for command aliases; you should define a hook for the basic command
  124. name, e.g. <code>backtrace</code> rather than <code>bt</code>.
  125. You can hook a multi-word command by adding <code>hook-</code> or
  126. <code>hookpost-</code> to the last word of the command, e.g.
  127. &lsquo;<samp>define target hook-remote</samp>&rsquo; to add a hook to &lsquo;<samp>target remote</samp>&rsquo;.
  128. </p>
  129. <p>If an error occurs during the execution of your hook, execution of
  130. <small>GDB</small> commands stops and <small>GDB</small> issues a prompt
  131. (before the command that you actually typed had a chance to run).
  132. </p>
  133. <p>If you try to define a hook which does not match any known command, you
  134. get a warning from the <code>define</code> command.
  135. </p>
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