GC_malloc, GC_malloc_atomic, GC_free, GC_realloc, GC_enable_incremen‐
tal, GC_register_finalizer, GC_malloc_ignore_off_page, GC_mal‐
loc_atomic_ignore_off_page, GC_set_warn_proc - Garbage collecting mal‐
void * GC_malloc(size_t size);
void GC_free(void *ptr);
void * GC_realloc(void *ptr, size_t size);
cc ... gc.a
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 appropri‐
ate 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 alter‐
nate, C++ specific interface to the garbage collector. Note that C++
programs generally need to be careful to ensure that all allocated mem‐
ory (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 reg‐
istration of functions that are invoked when an object becomes inacces‐
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 recom‐
mended 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 pro‐
grams 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 sup‐
port 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.
The README and gc.h files in the distribution. More detailed defini‐
tions of the functions exported by the collector are given there. (The
above list is not complete.)
The web site at http://www.hpl.hp.com/personal/Hans_Boehm/gc .
Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Envi‐
ronment", Software Practice & Experience, September 1988, pp. 807-820.
The malloc(3) man page.
Hans-J. Boehm (Hans.Boehm@hp.com). Some of the code was written by
others, most notably Alan Demers.
2 October 2003 gc(3)