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<h2 class="chapter">13 Tracepoints</h2>

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<p>In some applications, it is not feasible for the debugger to interrupt
the program&rsquo;s execution long enough for the developer to learn
anything helpful about its behavior.  If the program&rsquo;s correctness
depends on its real-time behavior, delays introduced by a debugger
might cause the program to change its behavior drastically, or perhaps
fail, even when the code itself is correct.  It is useful to be able
to observe the program&rsquo;s behavior without interrupting it.
</p>
<p>Using <small>GDB</small>&rsquo;s <code>trace</code> and <code>collect</code> commands, you can
specify locations in the program, called <em>tracepoints</em>, and
arbitrary expressions to evaluate when those tracepoints are reached.
Later, using the <code>tfind</code> command, you can examine the values
those expressions had when the program hit the tracepoints.  The
expressions may also denote objects in memory&mdash;structures or arrays,
for example&mdash;whose values <small>GDB</small> should record; while visiting
a particular tracepoint, you may inspect those objects as if they were
in memory at that moment.  However, because <small>GDB</small> records these
values without interacting with you, it can do so quickly and
unobtrusively, hopefully not disturbing the program&rsquo;s behavior.
</p>
<p>The tracepoint facility is currently available only for remote
targets.  See <a href="Targets.html#Targets">Targets</a>.  In addition, your remote target must know
how to collect trace data.  This functionality is implemented in the
remote stub; however, none of the stubs distributed with <small>GDB</small>
support tracepoints as of this writing.  The format of the remote
packets used to implement tracepoints are described in <a href="Tracepoint-Packets.html#Tracepoint-Packets">Tracepoint Packets</a>.
</p>
<p>It is also possible to get trace data from a file, in a manner reminiscent
of corefiles; you specify the filename, and use <code>tfind</code> to search
through the file.  See <a href="Trace-Files.html#Trace-Files">Trace Files</a>, for more details.
</p>
<p>This chapter describes the tracepoint commands and features.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top">&bull; <a href="Set-Tracepoints.html#Set-Tracepoints" accesskey="1">Set Tracepoints</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Analyze-Collected-Data.html#Analyze-Collected-Data" accesskey="2">Analyze Collected Data</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Tracepoint-Variables.html#Tracepoint-Variables" accesskey="3">Tracepoint Variables</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
<tr><td align="left" valign="top">&bull; <a href="Trace-Files.html#Trace-Files" accesskey="4">Trace Files</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">
</td></tr>
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