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  69. <h3 class="section">22.6 How to invoke the garbage collector</h3>
  70. <a name="index-garbage-collector_002c-invocation"></a>
  71. <a name="index-ggc_005fcollect"></a>
  72. <p>The GCC garbage collector GGC is only invoked explicitly. In contrast
  73. with many other garbage collectors, it is not implicitly invoked by
  74. allocation routines when a lot of memory has been consumed. So the
  75. only way to have GGC reclaim storage is to call the <code>ggc_collect</code>
  76. function explicitly. This call is an expensive operation, as it may
  77. have to scan the entire heap. Beware that local variables (on the GCC
  78. call stack) are not followed by such an invocation (as many other
  79. garbage collectors do): you should reference all your data from static
  80. or external <code>GTY</code>-ed variables, and it is advised to call
  81. <code>ggc_collect</code> with a shallow call stack. The GGC is an exact mark
  82. and sweep garbage collector (so it does not scan the call stack for
  83. pointers). In practice GCC passes don&rsquo;t often call <code>ggc_collect</code>
  84. themselves, because it is called by the pass manager between passes.
  85. </p>
  86. <p>At the time of the <code>ggc_collect</code> call all pointers in the GC-marked
  87. structures must be valid or <code>NULL</code>. In practice this means that
  88. there should not be uninitialized pointer fields in the structures even
  89. if your code never reads or writes those fields at a particular
  90. instance. One way to ensure this is to use cleared versions of
  91. allocators unless all the fields are initialized manually immediately
  92. after allocation.
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