garbage collection
<programming> (GC) The process by which dynamically allocated storage is
reclaimed during the execution of a program. The term usually refers to
automatic periodic storage reclamation by the garbage collector (part of the
run-time system), as opposed to explicit code to free specific blocks of memory.
Automatic garbage collection is usually triggered during memory allocation when
the amount free memory falls below some threshold or after a certain number of
allocations. Normal execution is suspended and the garbage collector is run.
There are many variations on this basic scheme.
Languages like Lisp represent expressions as graphs built from cells which
contain pointers and data. These languages use automatic dynamic storage
allocation to build expressions. During the evaluation of an expression it is
necessary to reclaim space which is used by subexpressions but which is no
longer pointed to by anything. This reclaimed memory is returned to the free
memory pool for subsequent reallocation.
Without garbage collection the program's memory requirements would increase
monotonically throughout execution, possibly exceeding system limits on virtual
memory size.
The three main methods are mark-sweep garbage collection, reference counting and
copying garbage collection.
See also the AI koan about garbage collection.
(1997-08-25)
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