GC_MALLOC(1L) GC_MALLOC(1L)
2 October 2003
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"
void * GC_malloc(size_t size);
void GC_free(void *ptr);
void * GC_realloc(void *ptr, size_t size);
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. In
most cases it is preferable to call the macros GC_MALLOC, GC_FREE,
etc. instead of calling GC_malloc and friends directly. This allows
debugging versions of the routines to be substituted by defining
GC_DEBUG before including gc.h. See the documentation in the include
files gc_cpp.h and gc_allocator.h, as well as the gcinterface.html
file in the distribution, for an alternate, C++ specific interface to
the garbage collector. Note that C++ programs generally need to be
careful to ensure that all allocated memory (whether via new, malloc,
or STL allocators) that may point to garbage collected memory is
either itself garbage collected, or at least traced by the 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
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GC_MALLOC(1L) GC_MALLOC(1L)
2 October 2003
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
GC_set_warn_proc This is recommended for production code. See gc.h
for details. Fully portable code should call GC_INIT from the main
program before making any other GC calls. On most platforms this does
nothing and the collector is initialized on first use. On a few
platforms explicit initialization is necessary. And it can never
hurt. 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.) The web site at
http://www.hboehm.info/gc/ . 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@acm.org). Some of the code was written by
others, most notably Alan Demers.
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