GC_MALLOC(1L) GC_MALLOC(1L)
12 February 1996
NAME
GC_malloc, GC_malloc_atomic, GC_free, GC_realloc,
GC_enable_incremental, GC_register_finalizer,
GC_malloc_ignore_off_page, GC_malloc_atomic_ignore_off_page,
GC_set_warn_proc - Garbage collecting malloc replacement
SYNOPSIS
#include "gc.h"
# define malloc(n) GC_malloc(n)
cc ... gc.a
DESCRIPTION
GC_malloc and GC_free are plug-in replacements for standard malloc and
free. However, GC_malloc will attempt to reclaim inaccessible space
automatically by invoking a conservative garbage collector at
appropriate points. The collector traverses all data structures
accessible by following pointers from the machines registers,
stack(s), data, and bss segments. Inaccessible structures will be
reclaimed. A machine word is considered to be a valid pointer if it
is an address inside an object allocated by GC_malloc or friends. See
the documentation in the include file gc_cpp.h for an alternate, C++
specific interface to the garbage collector. Unlike the standard
implementations of malloc, GC_malloc clears the newly allocated
storage. GC_malloc_atomic does not. Furthermore, it informs the
collector that the resulting object will never contain any pointers,
and should therefore not be scanned by the collector. GC_free can be
used to deallocate objects, but its use is optional, and generally
discouraged. GC_realloc has the standard realloc semantics. It
preserves pointer-free-ness. GC_register_finalizer allows for
registration of functions that are invoked when an object becomes
inaccessible. The garbage collector tries to avoid allocating memory
at locations that already appear to be referenced before allocation.
(Such apparent ``pointers'' are usually large integers and the like
that just happen to look like an address.) This may make it hard to
allocate very large objects. An attempt to do so may generate a
warning. GC_malloc_ignore_off_page and
GC_malloc_atomic_ignore_off_page inform the collector that the client
code will always maintain a pointer to near the beginning of the
object (within the first 512 bytes), and that pointers beyond that can
be ignored by the collector. This makes it much easier for the
collector to place large objects. These are recommended for large
object allocation. (Objects expected to be larger than about
100KBytes should be allocated this way.) It is also possible to use
the collector to find storage leaks in programs destined to be run
with standard malloc/free. The collector can be compiled for thread-
safe operation. Unlike standard malloc, it is safe to call malloc
after a previous malloc call was interrupted by a signal, provided the
original malloc call is not resumed. The collector may, on rare
occasion produce warning messages. On UNIX machines these appear on
stderr. Warning messages can be filtered, redirected, or ignored with
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GC_MALLOC(1L) GC_MALLOC(1L)
12 February 1996
GC_set_warn_proc. This is recommended for production code. See gc.h
for details. Debugging versions of many of the above routines are
provided as macros. Their names are identical to the above, but
consist of all capital letters. If GC_DEBUG is defined before gc.h is
included, these routines do additional checking, and allow the leak
detecting version of the collector to produce slightly more useful
output. Without GC_DEBUG defined, they behave exactly like the
lower-case versions. On some machines, collection will be performed
incrementally after a call to GC_enable_incremental. This may
temporarily write protect pages in the heap. See the README file for
more information on how this interacts with system calls that write to
the heap. Other facilities not discussed here include limited
facilities to support incremental collection on machines without
appropriate VM support, provisions for providing more explicit object
layout information to the garbage collector, more direct support for
``weak'' pointers, support for ``abortable'' garbage collections
during idle time, etc.
SEE ALSO
The README and gc.h files in the distribution. More detailed
definitions of the functions exported by the collector are given
there. (The above list is not complete.) Boehm, H., and M. Weiser,
"Garbage Collection in an Uncooperative Environment", Software
Practice & Experience, September 1988, pp. 807-820. The malloc(3) man
page.
AUTHOR
Hans-J. Boehm (boehm@parc.xerox.com). Some of the code was written by
others, most notably Alan Demers.
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