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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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<hr>
<a name="Static-Probe-Points-1"></a>
<h4 class="subsection">5.1.10 Static Probe Points</h4>

<a name="index-static-probe-point_002c-SystemTap"></a>
<a name="index-static-probe-point_002c-DTrace"></a>
<p><small>GDB</small> supports <em>SDT</em> probes in the code.  <acronym>SDT</acronym> stands
for Statically Defined Tracing, and the probes are designed to have a tiny
runtime code and data footprint, and no dynamic relocations.
</p>
<p>Currently, the following types of probes are supported on
ELF-compatible systems:
</p>
<ul>
<li> <code>SystemTap</code> (<a href="http://sourceware.org/systemtap/">http://sourceware.org/systemtap/</a>)
<acronym>SDT</acronym> probes<a name="DOCF5" href="#FOOT5"><sup>5</sup></a>.  <code>SystemTap</code> probes are usable
from assembly, C and C<tt>++</tt> languages<a name="DOCF6" href="#FOOT6"><sup>6</sup></a>.  

</li><li> <code>DTrace</code> (<a href="http://oss.oracle.com/projects/DTrace">http://oss.oracle.com/projects/DTrace</a>)
<acronym>USDT</acronym> probes.  <code>DTrace</code> probes are usable from C and
C<tt>++</tt> languages.
</li></ul>

<a name="index-semaphores-on-static-probe-points"></a>
<p>Some <code>SystemTap</code> probes have an associated semaphore variable;
for instance, this happens automatically if you defined your probe
using a DTrace-style <samp>.d</samp> file.  If your probe has a semaphore,
<small>GDB</small> will automatically enable it when you specify a
breakpoint using the &lsquo;<samp>-probe-stap</samp>&rsquo; notation.  But, if you put a
breakpoint at a probe&rsquo;s location by some other method (e.g.,
<code>break file:line</code>), then <small>GDB</small> will not automatically set
the semaphore.  <code>DTrace</code> probes do not support semaphores.
</p>
<p>You can examine the available static static probes using <code>info
probes</code>, with optional arguments:
</p>
<dl compact="compact">
<dd><a name="index-info-probes"></a>
</dd>
<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>
<dd><p>If given, <var>type</var> is either <code>stap</code> for listing
<code>SystemTap</code> probes or <code>dtrace</code> for listing <code>DTrace</code>
probes.  If omitted all probes are listed regardless of their types.
</p>
<p>If given, <var>provider</var> is a regular expression used to match against provider
names when selecting which probes to list.  If omitted, probes by all
probes from all providers are listed.
</p>
<p>If given, <var>name</var> is a regular expression to match against probe names
when selecting which probes to list.  If omitted, probe names are not
considered when deciding whether to display them.
</p>
<p>If given, <var>objfile</var> is a regular expression used to select which
object files (executable or shared libraries) to examine.  If not
given, all object files are considered.
</p>
</dd>
<dt><code>info probes all</code></dt>
<dd><p>List the available static probes, from all types.
</p></dd>
</dl>

<a name="index-enabling-and-disabling-probes"></a>
<p>Some probe points can be enabled and/or disabled.  The effect of
enabling or disabling a probe depends on the type of probe being
handled.  Some <code>DTrace</code> probes can be enabled or
disabled, but <code>SystemTap</code> probes cannot be disabled.
</p>
<p>You can enable (or disable) one or more probes using the following
commands, with optional arguments:
</p>
<dl compact="compact">
<dd><a name="index-enable-probes"></a>
</dd>
<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>
<dd><p>If given, <var>provider</var> is a regular expression used to match against
provider names when selecting which probes to enable.  If omitted,
all probes from all providers are enabled.
</p>
<p>If given, <var>name</var> is a regular expression to match against probe
names when selecting which probes to enable.  If omitted, probe names
are not considered when deciding whether to enable them.
</p>
<p>If given, <var>objfile</var> is a regular expression used to select which
object files (executable or shared libraries) to examine.  If not
given, all object files are considered.
</p>
<a name="index-disable-probes"></a>
</dd>
<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>
<dd><p>See the <code>enable probes</code> command above for a description of the
optional arguments accepted by this command.
</p></dd>
</dl>

<a name="index-_0024_005fprobe_005farg_002c-convenience-variable"></a>
<p>A probe may specify up to twelve arguments.  These are available at the
point at which the probe is defined&mdash;that is, when the current PC is
at the probe&rsquo;s location.  The arguments are available using the
convenience variables (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>)
<code>$_probe_arg0</code>&hellip;<code>$_probe_arg11</code>.  In <code>SystemTap</code>
probes each probe argument is an integer of the appropriate size;
types are not preserved.  In <code>DTrace</code> probes types are preserved
provided that they are recognized as such by <small>GDB</small>; otherwise
the value of the probe argument will be a long integer.  The
convenience variable <code>$_probe_argc</code> holds the number of arguments
at the current probe point.
</p>
<p>These variables are always available, but attempts to access them at
any location other than a probe point will cause <small>GDB</small> to give
an error message.
</p>

<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT5" href="#DOCF5">(5)</a></h3>
<p>See
<a href="http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps">http://sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps</a>
for more information on how to add <code>SystemTap</code> <acronym>SDT</acronym>
probes in your applications.</p>
<h3><a name="FOOT6" href="#DOCF6">(6)</a></h3>
<p>See
<a href="http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation">http://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation</a>
for a good reference on how the <acronym>SDT</acronym> probes are implemented.</p>
</div>
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