<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2015 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Free Software" and "Free Software Needs
Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
and with the Back-Cover Texts as in (a) below.

(a) The FSF's Back-Cover Text is: "You are free to copy and modify
this GNU Manual.  Buying copies from GNU Press supports the FSF in
developing GNU and promoting software freedom." -->
<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
<head>
<title>Debugging with GDB: Break Commands</title>

<meta name="description" content="Debugging with GDB: Break Commands">
<meta name="keywords" content="Debugging with GDB: Break Commands">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<link href="index.html#Top" rel="start" title="Top">
<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Breakpoints.html#Breakpoints" rel="up" title="Breakpoints">
<link href="Dynamic-Printf.html#Dynamic-Printf" rel="next" title="Dynamic Printf">
<link href="Conditions.html#Conditions" rel="prev" title="Conditions">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.indentedblock {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smallindentedblock {margin-left: 3.2em; font-size: smaller}
div.smalllisp {margin-left: 3.2em}
kbd {font-style:oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nocodebreak {white-space:nowrap}
span.nolinebreak {white-space:nowrap}
span.roman {font-family:serif; font-weight:normal}
span.sansserif {font-family:sans-serif; font-weight:normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
<a name="Break-Commands"></a>
<div class="header">
<p>
Next: <a href="Dynamic-Printf.html#Dynamic-Printf" accesskey="n" rel="next">Dynamic Printf</a>, Previous: <a href="Conditions.html#Conditions" accesskey="p" rel="prev">Conditions</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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>
</div>
<hr>
<a name="Breakpoint-Command-Lists"></a>
<h4 class="subsection">5.1.7 Breakpoint Command Lists</h4>

<a name="index-breakpoint-commands"></a>
<p>You can give any breakpoint (or watchpoint or catchpoint) a series of
commands to execute when your program stops due to that breakpoint.  For
example, you might want to print the values of certain expressions, or
enable other breakpoints.
</p>
<dl compact="compact">
<dd><a name="index-commands"></a>
<a name="index-end-_0028breakpoint-commands_0029"></a>
</dd>
<dt><code>commands <span class="roman">[</span><var>range</var>&hellip;<span class="roman">]</span></code></dt>
<dt><code>&hellip; <var>command-list</var> &hellip;</code></dt>
<dt><code>end</code></dt>
<dd><p>Specify a list of commands for the given breakpoints.  The commands
themselves appear on the following lines.  Type a line containing just
<code>end</code> to terminate the commands.
</p>
<p>To remove all commands from a breakpoint, type <code>commands</code> and
follow it immediately with <code>end</code>; that is, give no commands.
</p>
<p>With no argument, <code>commands</code> refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered).  If the most recent breakpoints were set with a single
command, then the <code>commands</code> will apply to all the breakpoints
set by that command.  This applies to breakpoints set by
<code>rbreak</code>, and also applies when a single <code>break</code> command
creates multiple breakpoints (see <a href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous
Expressions</a>).
</p></dd>
</dl>

<p>Pressing <tt class="key">RET</tt> as a means of repeating the last <small>GDB</small> command is
disabled within a <var>command-list</var>.
</p>
<p>You can use breakpoint commands to start your program up again.  Simply
use the <code>continue</code> command, or <code>step</code>, or any other command
that resumes execution.
</p>
<p>Any other commands in the command list, after a command that resumes
execution, are ignored.  This is because any time you resume execution
(even with a simple <code>next</code> or <code>step</code>), you may encounter
another breakpoint&mdash;which could have its own command list, leading to
ambiguities about which list to execute.
</p>
<a name="index-silent"></a>
<p>If the first command you specify in a command list is <code>silent</code>, the
usual message about stopping at a breakpoint is not printed.  This may
be desirable for breakpoints that are to print a specific message and
then continue.  If none of the remaining commands print anything, you
see no sign that the breakpoint was reached.  <code>silent</code> is
meaningful only at the beginning of a breakpoint command list.
</p>
<p>The commands <code>echo</code>, <code>output</code>, and <code>printf</code> allow you to
print precisely controlled output, and are often useful in silent
breakpoints.  See <a href="Output.html#Output">Commands for Controlled Output</a>.
</p>
<p>For example, here is how you could use breakpoint commands to print the
value of <code>x</code> at entry to <code>foo</code> whenever <code>x</code> is positive.
</p>
<div class="smallexample">
<pre class="smallexample">break foo if x&gt;0
commands
silent
printf &quot;x is %d\n&quot;,x
cont
end
</pre></div>

<p>One application for breakpoint commands is to compensate for one bug so
you can test for another.  Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them.  End with the <code>continue</code> command
so that your program does not stop, and start with the <code>silent</code>
command so that no output is produced.  Here is an example:
</p>
<div class="smallexample">
<pre class="smallexample">break 403
commands
silent
set x = y + 4
cont
end
</pre></div>

<hr>
<div class="header">
<p>
Next: <a href="Dynamic-Printf.html#Dynamic-Printf" accesskey="n" rel="next">Dynamic Printf</a>, Previous: <a href="Conditions.html#Conditions" accesskey="p" rel="prev">Conditions</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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>
</div>


</body>
</html>