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. 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You can also specify a <em>condition</em> for 77a tracepoint. A condition is just a Boolean expression in your 78programming language (see <a href="Expressions.html#Expressions">Expressions</a>). A 79tracepoint with a condition evaluates the expression each time your 80program reaches it, and data collection happens only if the condition 81is true. 82</p> 83<p>Tracepoint conditions can be specified when a tracepoint is set, by 84using ‘<samp>if</samp>’ in the arguments to the <code>trace</code> command. 85See <a href="Create-and-Delete-Tracepoints.html#Create-and-Delete-Tracepoints">Setting Tracepoints</a>. They can 86also be set or changed at any time with the <code>condition</code> command, 87just as with breakpoints. 88</p> 89<p>Unlike breakpoint conditions, <small>GDB</small> does not actually evaluate 90the conditional expression itself. Instead, <small>GDB</small> encodes the 91expression into an agent expression (see <a href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</a>) 92suitable for execution on the target, independently of <small>GDB</small>. 93Global variables become raw memory locations, locals become stack 94accesses, and so forth. 95</p> 96<p>For instance, suppose you have a function that is usually called 97frequently, but should not be called after an error has occurred. You 98could use the following tracepoint command to collect data about calls 99of that function that happen while the error code is propagating 100through the program; an unconditional tracepoint could end up 101collecting thousands of useless trace frames that you would have to 102search through. 103</p> 104<div class="smallexample"> 105<pre class="smallexample">(gdb) <kbd>trace normal_operation if errcode > 0</kbd> 106</pre></div> 107 108 109 110 111</body> 112</html> 113