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<title>Debugging with GDB: Static Probe Points</title>
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<a name="Static-Probe-Points"></a>
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<div class="header">
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<p>
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Next: <a href="Error-in-Breakpoints.html#Error-in-Breakpoints" accesskey="n" rel="next">Error in Breakpoints</a>, Previous: <a href="Save-Breakpoints.html#Save-Breakpoints" accesskey="p" rel="previous">Save Breakpoints</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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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<hr>
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<a name="Static-Probe-Points-1"></a>
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<h4 class="subsection">5.1.10 Static Probe Points</h4>
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<a name="index-static-probe-point_002c-SystemTap"></a>
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<a name="index-static-probe-point_002c-DTrace"></a>
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<p><small>GDB</small> supports <em>SDT</em> probes in the code. <acronym>SDT</acronym> stands
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for Statically Defined Tracing, and the probes are designed to have a tiny
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runtime code and data footprint, and no dynamic relocations.
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</p>
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<p>Currently, the following types of probes are supported on
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ELF-compatible systems:
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</p>
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<ul>
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<li> <code>SystemTap</code> (<a href="http://sourceware.org/systemtap/">http://sourceware.org/systemtap/</a>)
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<acronym>SDT</acronym> probes<a name="DOCF5" href="#FOOT5"><sup>5</sup></a>. <code>SystemTap</code> probes are usable
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from assembly, C and C<tt>++</tt> languages<a name="DOCF6" href="#FOOT6"><sup>6</sup></a>.
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</li><li> <code>DTrace</code> (<a href="http://oss.oracle.com/projects/DTrace">http://oss.oracle.com/projects/DTrace</a>)
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<acronym>USDT</acronym> probes. <code>DTrace</code> probes are usable from C and
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C<tt>++</tt> languages.
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</li></ul>
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<a name="index-semaphores-on-static-probe-points"></a>
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<p>Some <code>SystemTap</code> probes have an associated semaphore variable;
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for instance, this happens automatically if you defined your probe
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using a DTrace-style <samp>.d</samp> file. If your probe has a semaphore,
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<small>GDB</small> will automatically enable it when you specify a
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breakpoint using the ‘<samp>-probe-stap</samp>’ notation. But, if you put a
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breakpoint at a probe’s location by some other method (e.g.,
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<code>break file:line</code>), then <small>GDB</small> will not automatically set
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the semaphore. <code>DTrace</code> probes do not support semaphores.
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</p>
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<p>You can examine the available static static probes using <code>info
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probes</code>, with optional arguments:
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</p>
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<dl compact="compact">
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<dd><a name="index-info-probes"></a>
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</dd>
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<dt><code>info probes <span class="roman">[</span><var>type</var><span class="roman">]</span> <span class="roman">[</span><var>provider</var> <span class="roman">[</span><var>name</var> <span class="roman">[</span><var>objfile</var><span class="roman">]</span><span class="roman">]</span><span class="roman">]</span></code></dt>
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<dd><p>If given, <var>type</var> is either <code>stap</code> for listing
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<code>SystemTap</code> probes or <code>dtrace</code> for listing <code>DTrace</code>
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probes. If omitted all probes are listed regardless of their types.
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</p>
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<p>If given, <var>provider</var> is a regular expression used to match against provider
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names when selecting which probes to list. If omitted, probes by all
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probes from all providers are listed.
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</p>
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<p>If given, <var>name</var> is a regular expression to match against probe names
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when selecting which probes to list. If omitted, probe names are not
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considered when deciding whether to display them.
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</p>
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<p>If given, <var>objfile</var> is a regular expression used to select which
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object files (executable or shared libraries) to examine. If not
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given, all object files are considered.
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</p>
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</dd>
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<dt><code>info probes all</code></dt>
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<dd><p>List the available static probes, from all types.
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</p></dd>
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</dl>
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<a name="index-enabling-and-disabling-probes"></a>
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<p>Some probe points can be enabled and/or disabled. The effect of
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enabling or disabling a probe depends on the type of probe being
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handled. Some <code>DTrace</code> probes can be enabled or
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disabled, but <code>SystemTap</code> probes cannot be disabled.
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</p>
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<p>You can enable (or disable) one or more probes using the following
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commands, with optional arguments:
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</p>
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<dl compact="compact">
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<dd><a name="index-enable-probes"></a>
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</dd>
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<dt><code>enable probes <span class="roman">[</span><var>provider</var> <span class="roman">[</span><var>name</var> <span class="roman">[</span><var>objfile</var><span class="roman">]</span><span class="roman">]</span><span class="roman">]</span></code></dt>
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<dd><p>If given, <var>provider</var> is a regular expression used to match against
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provider names when selecting which probes to enable. If omitted,
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all probes from all providers are enabled.
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</p>
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<p>If given, <var>name</var> is a regular expression to match against probe
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names when selecting which probes to enable. If omitted, probe names
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are not considered when deciding whether to enable them.
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</p>
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<p>If given, <var>objfile</var> is a regular expression used to select which
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object files (executable or shared libraries) to examine. If not
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given, all object files are considered.
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</p>
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<a name="index-disable-probes"></a>
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</dd>
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<dt><code>disable probes <span class="roman">[</span><var>provider</var> <span class="roman">[</span><var>name</var> <span class="roman">[</span><var>objfile</var><span class="roman">]</span><span class="roman">]</span><span class="roman">]</span></code></dt>
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<dd><p>See the <code>enable probes</code> command above for a description of the
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optional arguments accepted by this command.
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</p></dd>
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</dl>
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<a name="index-_0024_005fprobe_005farg_002c-convenience-variable"></a>
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<p>A probe may specify up to twelve arguments. These are available at the
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point at which the probe is defined—that is, when the current PC is
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at the probe’s location. The arguments are available using the
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convenience variables (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>)
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<code>$_probe_arg0</code>…<code>$_probe_arg11</code>. In <code>SystemTap</code>
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probes each probe argument is an integer of the appropriate size;
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types are not preserved. In <code>DTrace</code> probes types are preserved
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provided that they are recognized as such by <small>GDB</small>; otherwise
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the value of the probe argument will be a long integer. The
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convenience variable <code>$_probe_argc</code> holds the number of arguments
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at the current probe point.
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</p>
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<p>These variables are always available, but attempts to access them at
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any location other than a probe point will cause <small>GDB</small> to give
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an error message.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT5" href="#DOCF5">(5)</a></h3>
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<p>See
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<a href="http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps">http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps</a>
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for more information on how to add <code>SystemTap</code> <acronym>SDT</acronym>
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probes in your applications.</p>
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<h3><a name="FOOT6" href="#DOCF6">(6)</a></h3>
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<p>See
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<a href="http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation">http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation</a>
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for a good reference on how the <acronym>SDT</acronym> probes are implemented.</p>
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</div>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Error-in-Breakpoints.html#Error-in-Breakpoints" accesskey="n" rel="next">Error in Breakpoints</a>, Previous: <a href="Save-Breakpoints.html#Save-Breakpoints" accesskey="p" rel="previous">Save Breakpoints</a>, Up: <a href="Breakpoints.html#Breakpoints" accesskey="u" rel="up">Breakpoints</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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