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| <a name="Static-Probe-Points"></a> |
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| <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="DOCF4" href="#FOOT4"><sup>4</sup></a>. <code>SystemTap</code> probes are usable |
| from assembly, C and C<tt>++</tt> languages<a name="DOCF5" href="#FOOT5"><sup>5</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 ‘<samp>-probe-stap</samp>’ notation. But, if you put a |
| breakpoint at a probe’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—that is, when the current PC is |
| at the probe’s location. The arguments are available using the |
| convenience variables (see <a href="Convenience-Vars.html#Convenience-Vars">Convenience Vars</a>) |
| <code>$_probe_arg0</code>…<code>$_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="FOOT4" href="#DOCF4">(4)</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="FOOT5" href="#DOCF5">(5)</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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