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### 23.6 How to invoke the garbage collector

The GCC garbage collector GGC is only invoked explicitly. In contrast with many other garbage collectors, it is not implicitly invoked by allocation routines when a lot of memory has been consumed. So the only way to have GGC reclaim storage is to call the `ggc_collect` function explicitly. With mode `GGC_COLLECT_FORCE` or otherwise (default `GGC_COLLECT_HEURISTIC`) when the internal heuristic decides to collect, this call is potentially an expensive operation, as it may have to scan the entire heap. Beware that local variables (on the GCC call stack) are not followed by such an invocation (as many other garbage collectors do): you should reference all your data from static or external `GTY`-ed variables, and it is advised to call `ggc_collect` with a shallow call stack. The GGC is an exact mark and sweep garbage collector (so it does not scan the call stack for pointers). In practice GCC passes don’t often call `ggc_collect` themselves, because it is called by the pass manager between passes.

At the time of the `ggc_collect` call all pointers in the GC-marked structures must be valid or `NULL`. In practice this means that there should not be uninitialized pointer fields in the structures even if your code never reads or writes those fields at a particular instance. One way to ensure this is to use cleared versions of allocators unless all the fields are initialized manually immediately after allocation.