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| <title>GNU Compiler Collection (GCC) Internals: Invoking the garbage collector</title> |
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| <h3 class="section">22.6 How to invoke the garbage collector</h3> |
| <a name="index-garbage-collector_002c-invocation"></a> |
| <a name="index-ggc_005fcollect"></a> |
| |
| <p>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 <code>ggc_collect</code> |
| function explicitly. This call is 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 <code>GTY</code>-ed variables, and it is advised to call |
| <code>ggc_collect</code> 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 <code>ggc_collect</code> |
| themselves, because it is called by the pass manager between passes. |
| </p> |
| <p>At the time of the <code>ggc_collect</code> call all pointers in the GC-marked |
| structures must be valid or <code>NULL</code>. 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. |
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