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11.2 Garbage Collection

Garbage collection is simple in theory but tricky to implement. Emacs Lisp uses the oldest garbage collection method, called mark and sweep. Garbage collection begins by starting with all accessible locations (i.e. all variables and other slots where Lisp objects might occur) and recursively traversing all objects accessible from those slots, marking each one that is found. We then go through all of memory and free each object that is not marked, and unmarking each object that is marked. Note that “all of memory” means all currently allocated objects. Traversing all these objects means traversing all frob blocks, all vectors (which are chained in one big list), and all lcrecords (which are likewise chained).

Garbage collection can be invoked explicitly by calling garbage-collect but is also called automatically by eval, once a certain amount of memory has been allocated since the last garbage collection (according to gc-cons-threshold).