### 24.4 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`.