1<html lang="en"> 2<head> 3<title>Tracepoint Conditions - Debugging with GDB</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Debugging with GDB"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="Set-Tracepoints.html#Set-Tracepoints" title="Set Tracepoints"> 9<link rel="prev" href="Tracepoint-Passcounts.html#Tracepoint-Passcounts" title="Tracepoint Passcounts"> 10<link rel="next" href="Trace-State-Variables.html#Trace-State-Variables" title="Trace State Variables"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988-2013 Free Software Foundation, Inc. 14 15Permission is granted to copy, distribute and/or modify this document 16under the terms of the GNU Free Documentation License, Version 1.3 or 17any later version published by the Free Software Foundation; with the 18Invariant Sections being ``Free Software'' and ``Free Software Needs 19Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,'' 20and with the Back-Cover Texts as in (a) below. 21 22(a) The FSF's Back-Cover Text is: ``You are free to copy and modify 23this GNU Manual. Buying copies from GNU Press supports the FSF in 24developing GNU and promoting software freedom.'' 25--> 26<meta http-equiv="Content-Style-Type" content="text/css"> 27<style type="text/css"><!-- 28 pre.display { font-family:inherit } 29 pre.format { font-family:inherit } 30 pre.smalldisplay { font-family:inherit; font-size:smaller } 31 pre.smallformat { font-family:inherit; font-size:smaller } 32 pre.smallexample { font-size:smaller } 33 pre.smalllisp { font-size:smaller } 34 span.sc { font-variant:small-caps } 35 span.roman { font-family:serif; font-weight:normal; } 36 span.sansserif { font-family:sans-serif; font-weight:normal; } 37--></style> 38<link rel="stylesheet" type="text/css" href="../cs.css"> 39</head> 40<body> 41<div class="node"> 42<a name="Tracepoint-Conditions"></a> 43<p> 44Next: <a rel="next" accesskey="n" href="Trace-State-Variables.html#Trace-State-Variables">Trace State Variables</a>, 45Previous: <a rel="previous" accesskey="p" href="Tracepoint-Passcounts.html#Tracepoint-Passcounts">Tracepoint Passcounts</a>, 46Up: <a rel="up" accesskey="u" href="Set-Tracepoints.html#Set-Tracepoints">Set Tracepoints</a> 47<hr> 48</div> 49 50<h4 class="subsection">13.1.4 Tracepoint Conditions</h4> 51 52<p><a name="index-conditional-tracepoints-781"></a><a name="index-tracepoint-conditions-782"></a> 53The simplest sort of tracepoint collects data every time your program 54reaches a specified place. You can also specify a <dfn>condition</dfn> for 55a tracepoint. A condition is just a Boolean expression in your 56programming language (see <a href="Expressions.html#Expressions">Expressions</a>). A 57tracepoint with a condition evaluates the expression each time your 58program reaches it, and data collection happens only if the condition 59is true. 60 61 <p>Tracepoint conditions can be specified when a tracepoint is set, by 62using ‘<samp><span class="samp">if</span></samp>’ in the arguments to the <code>trace</code> command. 63See <a href="Create-and-Delete-Tracepoints.html#Create-and-Delete-Tracepoints">Setting Tracepoints</a>. They can 64also be set or changed at any time with the <code>condition</code> command, 65just as with breakpoints. 66 67 <p>Unlike breakpoint conditions, <span class="sc">gdb</span> does not actually evaluate 68the conditional expression itself. Instead, <span class="sc">gdb</span> encodes the 69expression into an agent expression (see <a href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</a>) 70suitable for execution on the target, independently of <span class="sc">gdb</span>. 71Global variables become raw memory locations, locals become stack 72accesses, and so forth. 73 74 <p>For instance, suppose you have a function that is usually called 75frequently, but should not be called after an error has occurred. You 76could use the following tracepoint command to collect data about calls 77of that function that happen while the error code is propagating 78through the program; an unconditional tracepoint could end up 79collecting thousands of useless trace frames that you would have to 80search through. 81 82<pre class="smallexample"> (gdb) <kbd>trace normal_operation if errcode > 0</kbd> 83</pre> 84 </body></html> 85 86