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23.4 Interacting with the GCC Garbage Collector

Some plugins may want to be informed when GGC (the GCC Garbage Collector) is running. They can register callbacks for the PLUGIN_GGC_START and PLUGIN_GGC_END events (for which the callback is called with a null gcc_data) to be notified of the start or end of the GCC garbage collection.

Some plugins may need to have GGC mark additional data. This can be done by registering a callback (called with a null gcc_data) for the PLUGIN_GGC_MARKING event. Such callbacks can call the ggc_set_mark routine, preferably through the ggc_mark macro (and conversely, these routines should usually not be used in plugins outside of the PLUGIN_GGC_MARKING 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 ggc_marked_p function can be used to tell if an object is marked, or is about to be collected. The gt_clear_cache overloads which some types define may also be of use in managing weak references.

Some plugins may need to add extra GGC root tables, e.g. to handle their own GTY-ed data. This can be done with the PLUGIN_REGISTER_GGC_ROOTS pseudo-event with a null callback and the extra root table (of type struct ggc_root_tab*) as user_data. Running the gengtype -p source-dir file-list plugin*.c ... utility generates these extra root tables.

You should understand the details of memory management inside GCC before using PLUGIN_GGC_MARKING or PLUGIN_REGISTER_GGC_ROOTS.