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<title>Debugging with GDB: Break Commands</title>
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<a name="Break-Commands"></a>
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<div class="header">
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<p>
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Next: <a href="Dynamic-printf.html#Dynamic-printf" accesskey="n" rel="next">Dynamic printf</a>, Previous: <a href="Conditions.html#Conditions" accesskey="p" rel="previous">Conditions</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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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<hr>
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<a name="Breakpoint-Command-Lists"></a>
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<h4 class="subsection">5.1.7 Breakpoint Command Lists</h4>
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<a name="index-breakpoint-commands"></a>
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<p>You can give any breakpoint (or watchpoint or catchpoint) a series of
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commands to execute when your program stops due to that breakpoint. For
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example, you might want to print the values of certain expressions, or
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enable other breakpoints.
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</p>
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<dl compact="compact">
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<dd><a name="index-commands"></a>
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<a name="index-end-_0028breakpoint-commands_0029"></a>
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</dd>
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<dt><code>commands <span class="roman">[</span><var>list</var>…<span class="roman">]</span></code></dt>
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<dt><code>… <var>command-list</var> …</code></dt>
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<dt><code>end</code></dt>
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<dd><p>Specify a list of commands for the given breakpoints. The commands
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themselves appear on the following lines. Type a line containing just
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<code>end</code> to terminate the commands.
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</p>
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<p>To remove all commands from a breakpoint, type <code>commands</code> and
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follow it immediately with <code>end</code>; that is, give no commands.
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</p>
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<p>With no argument, <code>commands</code> refers to the last breakpoint,
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watchpoint, or catchpoint set (not to the breakpoint most recently
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encountered). If the most recent breakpoints were set with a single
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command, then the <code>commands</code> will apply to all the breakpoints
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set by that command. This applies to breakpoints set by
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<code>rbreak</code>, and also applies when a single <code>break</code> command
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creates multiple breakpoints (see <a href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous
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Expressions</a>).
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</p></dd>
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</dl>
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<p>Pressing <tt class="key">RET</tt> as a means of repeating the last <small>GDB</small> command is
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disabled within a <var>command-list</var>.
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</p>
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<p>You can use breakpoint commands to start your program up again. Simply
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use the <code>continue</code> command, or <code>step</code>, or any other command
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that resumes execution.
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</p>
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<p>Any other commands in the command list, after a command that resumes
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execution, are ignored. This is because any time you resume execution
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(even with a simple <code>next</code> or <code>step</code>), you may encounter
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another breakpoint—which could have its own command list, leading to
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ambiguities about which list to execute.
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</p>
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<a name="index-silent"></a>
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<p>If the first command you specify in a command list is <code>silent</code>, the
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usual message about stopping at a breakpoint is not printed. This may
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be desirable for breakpoints that are to print a specific message and
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then continue. If none of the remaining commands print anything, you
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see no sign that the breakpoint was reached. <code>silent</code> is
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meaningful only at the beginning of a breakpoint command list.
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</p>
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<p>The commands <code>echo</code>, <code>output</code>, and <code>printf</code> allow you to
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print precisely controlled output, and are often useful in silent
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breakpoints. See <a href="Output.html#Output">Commands for Controlled Output</a>.
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</p>
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<p>For example, here is how you could use breakpoint commands to print the
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value of <code>x</code> at entry to <code>foo</code> whenever <code>x</code> is positive.
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</p>
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<div class="smallexample">
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<pre class="smallexample">break foo if x>0
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commands
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silent
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printf "x is %d\n",x
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cont
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end
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</pre></div>
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<p>One application for breakpoint commands is to compensate for one bug so
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you can test for another. Put a breakpoint just after the erroneous line
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of code, give it a condition to detect the case in which something
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erroneous has been done, and give it commands to assign correct values
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to any variables that need them. End with the <code>continue</code> command
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so that your program does not stop, and start with the <code>silent</code>
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command so that no output is produced. Here is an example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">break 403
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commands
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silent
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set x = y + 4
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cont
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end
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</pre></div>
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<div class="header">
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<p>
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Next: <a href="Dynamic-printf.html#Dynamic-printf" accesskey="n" rel="next">Dynamic printf</a>, Previous: <a href="Conditions.html#Conditions" accesskey="p" rel="previous">Conditions</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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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