85 lines
4.2 KiB
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85 lines
4.2 KiB
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<title>Tracepoint Conditions - Debugging with GDB</title>
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<a name="Tracepoint-Conditions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Trace-State-Variables.html#Trace-State-Variables">Trace State Variables</a>,
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Previous: <a rel="previous" accesskey="p" href="Tracepoint-Passcounts.html#Tracepoint-Passcounts">Tracepoint Passcounts</a>,
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Up: <a rel="up" accesskey="u" href="Set-Tracepoints.html#Set-Tracepoints">Set Tracepoints</a>
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<h4 class="subsection">13.1.4 Tracepoint Conditions</h4>
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<p><a name="index-conditional-tracepoints-882"></a><a name="index-tracepoint-conditions-883"></a>
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The simplest sort of tracepoint collects data every time your program
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reaches a specified place. You can also specify a <dfn>condition</dfn> 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>Tracepoint conditions can be specified when a tracepoint is set, by
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using ‘<samp><span class="samp">if</span></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>Unlike breakpoint conditions, <span class="sc">gdb</span> does not actually evaluate
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the conditional expression itself. Instead, <span class="sc">gdb</span> 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 <span class="sc">gdb</span>.
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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>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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<pre class="smallexample"> (gdb) <kbd>trace normal_operation if errcode > 0</kbd>
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</pre>
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