1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<!-- Copyright (C) 1988-2021 Free Software Foundation, Inc. 4 5Permission is granted to copy, distribute and/or modify this document 6under the terms of the GNU Free Documentation License, Version 1.3 or 7any later version published by the Free Software Foundation; with the 8Invariant Sections being "Free Software" and "Free Software Needs 9Free Documentation", with the Front-Cover Texts being "A GNU Manual," 10and with the Back-Cover Texts as in (a) below. 11 12(a) The FSF's Back-Cover Text is: "You are free to copy and modify 13this GNU Manual. Buying copies from GNU Press supports the FSF in 14developing GNU and promoting software freedom." --> 15<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> 16<head> 17<title>Debugging with GDB: Break Commands</title> 18 19<meta name="description" content="Debugging with GDB: Break Commands"> 20<meta name="keywords" content="Debugging with GDB: Break Commands"> 21<meta name="resource-type" content="document"> 22<meta name="distribution" content="global"> 23<meta name="Generator" content="makeinfo"> 24<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 25<link href="index.html#Top" rel="start" title="Top"> 26<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> 27<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> 28<link href="Breakpoints.html#Breakpoints" rel="up" title="Breakpoints"> 29<link href="Dynamic-printf.html#Dynamic-printf" rel="next" title="Dynamic printf"> 30<link href="Conditions.html#Conditions" rel="previous" title="Conditions"> 31<style type="text/css"> 32<!-- 33a.summary-letter {text-decoration: none} 34blockquote.smallquotation {font-size: smaller} 35div.display {margin-left: 3.2em} 36div.example {margin-left: 3.2em} 37div.indentedblock {margin-left: 3.2em} 38div.lisp {margin-left: 3.2em} 39div.smalldisplay {margin-left: 3.2em} 40div.smallexample {margin-left: 3.2em} 41div.smallindentedblock {margin-left: 3.2em; font-size: smaller} 42div.smalllisp {margin-left: 3.2em} 43kbd {font-style:oblique} 44pre.display {font-family: inherit} 45pre.format {font-family: inherit} 46pre.menu-comment {font-family: serif} 47pre.menu-preformatted {font-family: serif} 48pre.smalldisplay {font-family: inherit; font-size: smaller} 49pre.smallexample {font-size: smaller} 50pre.smallformat {font-family: inherit; font-size: smaller} 51pre.smalllisp {font-size: smaller} 52span.nocodebreak {white-space:nowrap} 53span.nolinebreak {white-space:nowrap} 54span.roman {font-family:serif; font-weight:normal} 55span.sansserif {font-family:sans-serif; font-weight:normal} 56ul.no-bullet {list-style: none} 57--> 58</style> 59 60 61</head> 62 63<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> 64<a name="Break-Commands"></a> 65<div class="header"> 66<p> 67Next: <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> 68</div> 69<hr> 70<a name="Breakpoint-Command-Lists"></a> 71<h4 class="subsection">5.1.7 Breakpoint Command Lists</h4> 72 73<a name="index-breakpoint-commands"></a> 74<p>You can give any breakpoint (or watchpoint or catchpoint) a series of 75commands to execute when your program stops due to that breakpoint. For 76example, you might want to print the values of certain expressions, or 77enable other breakpoints. 78</p> 79<dl compact="compact"> 80<dd><a name="index-commands"></a> 81<a name="index-end-_0028breakpoint-commands_0029"></a> 82</dd> 83<dt><code>commands <span class="roman">[</span><var>list</var>…<span class="roman">]</span></code></dt> 84<dt><code>… <var>command-list</var> …</code></dt> 85<dt><code>end</code></dt> 86<dd><p>Specify a list of commands for the given breakpoints. The commands 87themselves appear on the following lines. Type a line containing just 88<code>end</code> to terminate the commands. 89</p> 90<p>To remove all commands from a breakpoint, type <code>commands</code> and 91follow it immediately with <code>end</code>; that is, give no commands. 92</p> 93<p>With no argument, <code>commands</code> refers to the last breakpoint, 94watchpoint, or catchpoint set (not to the breakpoint most recently 95encountered). If the most recent breakpoints were set with a single 96command, then the <code>commands</code> will apply to all the breakpoints 97set by that command. This applies to breakpoints set by 98<code>rbreak</code>, and also applies when a single <code>break</code> command 99creates multiple breakpoints (see <a href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous 100Expressions</a>). 101</p></dd> 102</dl> 103 104<p>Pressing <tt class="key">RET</tt> as a means of repeating the last <small>GDB</small> command is 105disabled within a <var>command-list</var>. 106</p> 107<p>You can use breakpoint commands to start your program up again. Simply 108use the <code>continue</code> command, or <code>step</code>, or any other command 109that resumes execution. 110</p> 111<p>Any other commands in the command list, after a command that resumes 112execution, are ignored. This is because any time you resume execution 113(even with a simple <code>next</code> or <code>step</code>), you may encounter 114another breakpoint—which could have its own command list, leading to 115ambiguities about which list to execute. 116</p> 117<a name="index-silent"></a> 118<p>If the first command you specify in a command list is <code>silent</code>, the 119usual message about stopping at a breakpoint is not printed. This may 120be desirable for breakpoints that are to print a specific message and 121then continue. If none of the remaining commands print anything, you 122see no sign that the breakpoint was reached. <code>silent</code> is 123meaningful only at the beginning of a breakpoint command list. 124</p> 125<p>The commands <code>echo</code>, <code>output</code>, and <code>printf</code> allow you to 126print precisely controlled output, and are often useful in silent 127breakpoints. See <a href="Output.html#Output">Commands for Controlled Output</a>. 128</p> 129<p>For example, here is how you could use breakpoint commands to print the 130value of <code>x</code> at entry to <code>foo</code> whenever <code>x</code> is positive. 131</p> 132<div class="smallexample"> 133<pre class="smallexample">break foo if x>0 134commands 135silent 136printf "x is %d\n",x 137cont 138end 139</pre></div> 140 141<p>One application for breakpoint commands is to compensate for one bug so 142you can test for another. Put a breakpoint just after the erroneous line 143of code, give it a condition to detect the case in which something 144erroneous has been done, and give it commands to assign correct values 145to any variables that need them. End with the <code>continue</code> command 146so that your program does not stop, and start with the <code>silent</code> 147command so that no output is produced. Here is an example: 148</p> 149<div class="smallexample"> 150<pre class="smallexample">break 403 151commands 152silent 153set x = y + 4 154cont 155end 156</pre></div> 157 158<hr> 159<div class="header"> 160<p> 161Next: <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> 162</div> 163 164 165 166</body> 167</html> 168