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<a name="Interacting-with-the-GCC-Garbage-Collector"></a>
<h3 class="section">24.4 Interacting with the GCC Garbage Collector</h3>

<p>Some plugins may want to be informed when GGC (the GCC Garbage
Collector) is running. They can register callbacks for the
<code>PLUGIN_GGC_START</code> and <code>PLUGIN_GGC_END</code> events (for which
the callback is called with a null <code>gcc_data</code>) to be notified of
the start or end of the GCC garbage collection.
</p>
<p>Some plugins may need to have GGC mark additional data. This can be
done by registering a callback (called with a null <code>gcc_data</code>)
for the <code>PLUGIN_GGC_MARKING</code> event. Such callbacks can call the
<code>ggc_set_mark</code> routine, preferably through the <code>ggc_mark</code> macro
(and conversely, these routines should usually not be used in plugins
outside of the <code>PLUGIN_GGC_MARKING</code> event).  Plugins that wish to hold
weak references to gc data may also use this event to drop weak references when
the object is about to be collected.  The <code>ggc_marked_p</code> function can be
used to tell if an object is marked, or is about to  be collected.  The
<code>gt_clear_cache</code> overloads which some types define may also be of use in
managing weak references.
</p>
<p>Some plugins may need to add extra GGC root tables, e.g. to handle their own
<code>GTY</code>-ed data. This can be done with the <code>PLUGIN_REGISTER_GGC_ROOTS</code>
pseudo-event with a null callback and the extra root table (of type <code>struct
ggc_root_tab*</code>) as <code>user_data</code>.  Running the
 <code>gengtype -p <var>source-dir</var> <var>file-list</var> <var>plugin*.c</var> ...</code>
utility generates these extra root tables.
</p>
<p>You should understand the details of memory management inside GCC
before using <code>PLUGIN_GGC_MARKING</code> or <code>PLUGIN_REGISTER_GGC_ROOTS</code>.
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