GC_malloc, GC_malloc_atomic, GC_free, GC_realloc,
GC_set_warn_proc - Garbage collecting malloc replacement
void * GC_malloc(size_t size);
void GC_free(void *ptr);
void * GC_realloc(void *ptr, size_t size);
+ cc ... -I/usr/include/gc -lgc
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 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
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 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
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.hpl.hp.com/personal/Hans_Boehm/gc .
Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative
Environment", Software Practice & Experience, September 1988, pp.
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_MALLOC(3)