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249 lines
12 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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<!-- 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Create and Delete Tracepoints (Debugging with GDB)</title>
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<meta name="description" content="Create and Delete Tracepoints (Debugging with GDB)">
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<meta name="keywords" content="Create and Delete Tracepoints (Debugging with GDB)">
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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="Enable-and-Disable-Tracepoints.html#Enable-and-Disable-Tracepoints" rel="next" title="Enable and Disable Tracepoints">
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<link href="Set-Tracepoints.html#Set-Tracepoints" rel="prev" title="Set Tracepoints">
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</head>
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<body lang="en">
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<a name="Create-and-Delete-Tracepoints"></a>
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<div class="header">
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<p>
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Next: <a href="Enable-and-Disable-Tracepoints.html#Enable-and-Disable-Tracepoints" accesskey="n" rel="next">Enable and Disable Tracepoints</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="Create-and-Delete-Tracepoints-1"></a>
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<h4 class="subsection">13.1.1 Create and Delete Tracepoints</h4>
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<dl compact="compact">
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<dd><a name="index-set-tracepoint"></a>
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<a name="index-trace"></a>
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</dd>
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<dt><code>trace <var>location</var></code></dt>
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<dd><p>The <code>trace</code> command is very similar to the <code>break</code> command.
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Its argument <var>location</var> can be any valid location.
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See <a href="Specify-Location.html#Specify-Location">Specify Location</a>. The <code>trace</code> command defines a tracepoint,
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which is a point in the target program where the debugger will briefly stop,
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collect some data, and then allow the program to continue. Setting a tracepoint
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or changing its actions takes effect immediately if the remote stub
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supports the ‘<samp>InstallInTrace</samp>’ feature (see <a href="General-Query-Packets.html#install-tracepoint-in-tracing">install tracepoint in tracing</a>).
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If remote stub doesn’t support the ‘<samp>InstallInTrace</samp>’ feature, all
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these changes don’t take effect until the next <code>tstart</code>
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command, and once a trace experiment is running, further changes will
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not have any effect until the next trace experiment starts. In addition,
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<small>GDB</small> supports <em>pending tracepoints</em>—tracepoints whose
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address is not yet resolved. (This is similar to pending breakpoints.)
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Pending tracepoints are not downloaded to the target and not installed
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until they are resolved. The resolution of pending tracepoints requires
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<small>GDB</small> support—when debugging with the remote target, and
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<small>GDB</small> disconnects from the remote stub (see <a href="Starting-and-Stopping-Trace-Experiments.html#disconnected-tracing">disconnected tracing</a>), pending tracepoints can not be resolved (and downloaded to
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the remote stub) while <small>GDB</small> is disconnected.
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</p>
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<p>Here are some examples of using the <code>trace</code> command:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) <b>trace foo.c:121</b> // a source file and line number
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(gdb) <b>trace +2</b> // 2 lines forward
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(gdb) <b>trace my_function</b> // first source line of function
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(gdb) <b>trace *my_function</b> // EXACT start address of function
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(gdb) <b>trace *0x2117c4</b> // an address
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</pre></div>
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<p>You can abbreviate <code>trace</code> as <code>tr</code>.
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</p>
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</dd>
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<dt><code>trace <var>location</var> if <var>cond</var></code></dt>
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<dd><p>Set a tracepoint with condition <var>cond</var>; evaluate the expression
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<var>cond</var> each time the tracepoint is reached, and collect data only
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if the value is nonzero—that is, if <var>cond</var> evaluates as true.
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See <a href="Tracepoint-Conditions.html#Tracepoint-Conditions">Tracepoint Conditions</a>, for more
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information on tracepoint conditions.
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</p>
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</dd>
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<dt><code>ftrace <var>location</var> [ if <var>cond</var> ]</code></dt>
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<dd><a name="index-set-fast-tracepoint"></a>
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<a name="index-fast-tracepoints_002c-setting"></a>
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<a name="index-ftrace"></a>
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<p>The <code>ftrace</code> command sets a fast tracepoint. For targets that
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support them, fast tracepoints will use a more efficient but possibly
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less general technique to trigger data collection, such as a jump
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instruction instead of a trap, or some sort of hardware support. It
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may not be possible to create a fast tracepoint at the desired
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location, in which case the command will exit with an explanatory
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message.
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</p>
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<p><small>GDB</small> handles arguments to <code>ftrace</code> exactly as for
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<code>trace</code>.
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</p>
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<p>On 32-bit x86-architecture systems, fast tracepoints normally need to
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be placed at an instruction that is 5 bytes or longer, but can be
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placed at 4-byte instructions if the low 64K of memory of the target
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program is available to install trampolines. Some Unix-type systems,
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such as <small>GNU</small>/Linux, exclude low addresses from the program’s
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address space; but for instance with the Linux kernel it is possible
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to let <small>GDB</small> use this area by doing a <code>sysctl</code> command
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to set the <code>mmap_min_addr</code> kernel parameter, as in
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</p>
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<div class="example">
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<pre class="example">sudo sysctl -w vm.mmap_min_addr=32768
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</pre></div>
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<p>which sets the low address to 32K, which leaves plenty of room for
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trampolines. The minimum address should be set to a page boundary.
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</p>
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</dd>
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<dt><code>strace <var>location</var> [ if <var>cond</var> ]</code></dt>
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<dd><a name="index-set-static-tracepoint"></a>
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<a name="index-static-tracepoints_002c-setting"></a>
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<a name="index-probe-static-tracepoint-marker"></a>
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<a name="index-strace"></a>
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<p>The <code>strace</code> command sets a static tracepoint. For targets that
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support it, setting a static tracepoint probes a static
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instrumentation point, or marker, found at <var>location</var>. It may not
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be possible to set a static tracepoint at the desired location, in
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which case the command will exit with an explanatory message.
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</p>
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<p><small>GDB</small> handles arguments to <code>strace</code> exactly as for
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<code>trace</code>, with the addition that the user can also specify
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<code>-m <var>marker</var></code> as <var>location</var>. This probes the marker
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identified by the <var>marker</var> string identifier. This identifier
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depends on the static tracepoint backend library your program is
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using. You can find all the marker identifiers in the ‘<samp>ID</samp>’ field
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of the <code>info static-tracepoint-markers</code> command output.
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See <a href="Listing-Static-Tracepoint-Markers.html#Listing-Static-Tracepoint-Markers">Listing Static Tracepoint
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Markers</a>. For example, in the following small program using the UST
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tracing engine:
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</p>
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<div class="smallexample">
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<pre class="smallexample">main ()
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{
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trace_mark(ust, bar33, "str %s", "FOOBAZ");
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}
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</pre></div>
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<p>the marker id is composed of joining the first two arguments to the
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<code>trace_mark</code> call with a slash, which translates to:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) info static-tracepoint-markers
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Cnt Enb ID Address What
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1 n ust/bar33 0x0000000000400ddc in main at stexample.c:22
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Data: "str %s"
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[etc...]
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</pre></div>
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<p>so you may probe the marker above with:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) strace -m ust/bar33
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</pre></div>
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<p>Static tracepoints accept an extra collect action — <code>collect
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$_sdata</code>. This collects arbitrary user data passed in the probe point
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call to the tracing library. In the UST example above, you’ll see
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that the third argument to <code>trace_mark</code> is a printf-like format
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string. The user data is then the result of running that formating
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string against the following arguments. Note that <code>info
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static-tracepoint-markers</code> command output lists that format string in
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the ‘<samp>Data:</samp>’ field.
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</p>
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<p>You can inspect this data when analyzing the trace buffer, by printing
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the $_sdata variable like any other variable available to
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<small>GDB</small>. See <a href="Tracepoint-Actions.html#Tracepoint-Actions">Tracepoint Action Lists</a>.
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</p>
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<a name="index-_0024tpnum"></a>
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<a name="index-last-tracepoint-number"></a>
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<a name="index-recent-tracepoint-number"></a>
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<a name="index-tracepoint-number"></a>
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<p>The convenience variable <code>$tpnum</code> records the tracepoint number
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of the most recently set tracepoint.
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</p>
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<a name="index-delete-tracepoint"></a>
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<a name="index-tracepoint-deletion"></a>
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</dd>
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<dt><code>delete tracepoint <span class="roman">[</span><var>num</var><span class="roman">]</span></code></dt>
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<dd><p>Permanently delete one or more tracepoints. With no argument, the
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default is to delete all tracepoints. Note that the regular
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<code>delete</code> command can remove tracepoints also.
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</p>
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<p>Examples:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(gdb) <b>delete trace 1 2 3</b> // remove three tracepoints
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(gdb) <b>delete trace</b> // remove all tracepoints
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</pre></div>
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<p>You can abbreviate this command as <code>del tr</code>.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Enable-and-Disable-Tracepoints.html#Enable-and-Disable-Tracepoints" accesskey="n" rel="next">Enable and Disable Tracepoints</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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