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112 lines
5.3 KiB
HTML
112 lines
5.3 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2019 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Free Software" and "Free Software Needs
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Free Documentation", with the Front-Cover Texts being "A GNU Manual,"
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and with the Back-Cover Texts as in (a) below.
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(a) The FSF's Back-Cover Text is: "You are free to copy and modify
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this GNU Manual. Buying copies from GNU Press supports the FSF in
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developing GNU and promoting software freedom." -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Tracepoint Conditions (Debugging with GDB)</title>
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<meta name="description" content="Tracepoint Conditions (Debugging with GDB)">
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<meta name="keywords" content="Tracepoint Conditions (Debugging with GDB)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Set-Tracepoints.html#Set-Tracepoints" rel="up" title="Set Tracepoints">
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<link href="Trace-State-Variables.html#Trace-State-Variables" rel="next" title="Trace State Variables">
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<link href="Tracepoint-Passcounts.html#Tracepoint-Passcounts" rel="prev" title="Tracepoint Passcounts">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Tracepoint-Conditions"></a>
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<div class="header">
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<p>
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Next: <a href="Trace-State-Variables.html#Trace-State-Variables" accesskey="n" rel="next">Trace State Variables</a>, Previous: <a href="Tracepoint-Passcounts.html#Tracepoint-Passcounts" accesskey="p" rel="prev">Tracepoint Passcounts</a>, Up: <a href="Set-Tracepoints.html#Set-Tracepoints" accesskey="u" rel="up">Set Tracepoints</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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</div>
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<hr>
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<a name="Tracepoint-Conditions-1"></a>
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<h4 class="subsection">13.1.4 Tracepoint Conditions</h4>
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<a name="index-conditional-tracepoints"></a>
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<a name="index-tracepoint-conditions"></a>
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<p>The simplest sort of tracepoint collects data every time your program
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reaches a specified place. You can also specify a <em>condition</em> for
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a tracepoint. A condition is just a Boolean expression in your
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programming language (see <a href="Expressions.html#Expressions">Expressions</a>). A
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tracepoint with a condition evaluates the expression each time your
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program reaches it, and data collection happens only if the condition
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is true.
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</p>
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<p>Tracepoint conditions can be specified when a tracepoint is set, by
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using ‘<samp>if</samp>’ in the arguments to the <code>trace</code> command.
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See <a href="Create-and-Delete-Tracepoints.html#Create-and-Delete-Tracepoints">Setting Tracepoints</a>. They can
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also be set or changed at any time with the <code>condition</code> command,
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just as with breakpoints.
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</p>
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<p>Unlike breakpoint conditions, <small>GDB</small> does not actually evaluate
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the conditional expression itself. Instead, <small>GDB</small> encodes the
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expression into an agent expression (see <a href="Agent-Expressions.html#Agent-Expressions">Agent Expressions</a>)
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suitable for execution on the target, independently of <small>GDB</small>.
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Global variables become raw memory locations, locals become stack
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accesses, and so forth.
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</p>
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<p>For instance, suppose you have a function that is usually called
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frequently, but should not be called after an error has occurred. You
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could use the following tracepoint command to collect data about calls
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of that function that happen while the error code is propagating
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through the program; an unconditional tracepoint could end up
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collecting thousands of useless trace frames that you would have to
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search through.
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) <kbd>trace normal_operation if errcode > 0</kbd>
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</pre></div>
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</body>
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</html>
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