Boehm-Demers-Weiser Garbage Collector
This is version 8.3.0 (next release development) of a conservative garbage collector for C and C++.
Also, the latest bug fixes and new features are available in the development repository.
This is intended to be a general purpose, garbage collecting storage allocator. The algorithms used are described in:
Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment", Software Practice & Experience, September 1988, pp. 807-820.
Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection", Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164.
Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design and Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206.
Boehm H., "Reducing Garbage Collector Cache Misses", Proceedings of the 2000 International Symposium on Memory Management.
Possible interactions between the collector and optimizing compilers are discussed in
Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation", The Journal of C Language Translation 4, 2 (December 1992).
Boehm H., "Simple GC-safe Compilation", Proceedings of the ACM SIGPLAN '96 Conference on Programming Language Design and Implementation.
Unlike the collector described in the second reference, this collector operates either with the mutator stopped during the entire collection (default) or incrementally during allocations. (The latter is supported on fewer machines.) On the most common platforms, it can be built with or without thread support. On some platforms, it can take advantage of a multiprocessor to speed up garbage collection.
Many of the ideas underlying the collector have previously been explored by others. Notably, some of the run-time systems developed at Xerox PARC in the early 1980s conservatively scanned thread stacks to locate possible pointers (cf. Paul Rovner, "On Adding Garbage Collection and Runtime Types to a Strongly-Typed Statically Checked, Concurrent Language" Xerox PARC CSL 84-7). Doug McIlroy wrote a simpler fully conservative collector that was part of version 8 UNIX (tm), but appears to not have received widespread use.
Rudimentary tools for use of the collector as a leak detector are included, as is a fairly sophisticated string package "cord" that makes use of the collector. (See README.cords and H.-J. Boehm, R. Atkinson, and M. Plass, "Ropes: An Alternative to Strings", Software Practice and Experience 25, 12 (December 1995), pp. 1315-1330. This is very similar to the "rope" package in Xerox Cedar, or the "rope" package in the SGI STL or the g++ distribution.)
Further collector documentation can be found in the overview.
Some of the known uses of the collector are listed on the GitHub Known-clients page.
This is a garbage collecting storage allocator that is intended to be used as a plug-in replacement for C's malloc.
Since the collector does not require pointers to be tagged, it does not attempt to ensure that all inaccessible storage is reclaimed. However, in our experience, it is typically more successful at reclaiming unused memory than most C programs using explicit deallocation. Unlike manually introduced leaks, the amount of unreclaimed memory typically stays bounded.
In the following, an "object" is defined to be a region of memory allocated by the routines described below.
Any objects not intended to be collected must be pointed to either
from other such accessible objects, or from the registers,
stack, data, or statically allocated bss segments. Pointers from
the stack or registers may point to anywhere inside an object.
The same is true for heap pointers if the collector is compiled with
ALL_INTERIOR_POINTERS defined, or
GC_all_interior_pointers is otherwise
set, as is now the default.
ALL_INTERIOR_POINTERS may reduce accidental retention
of garbage objects, by requiring pointers from the heap to the beginning
of an object. But this no longer appears to be a significant
issue for most programs occupying a small fraction of the possible
There are a number of routines which modify the pointer recognition
GC_register_displacement allows certain interior pointers
to be recognized even if
ALL_INTERIOR_POINTERS is not defined.
GC_malloc_ignore_off_page allows some pointers into the middle of
large objects to be disregarded, greatly reducing the probability of
accidental retention of large objects. For most purposes it seems
best to compile with
ALL_INTERIOR_POINTERS and to use
GC_malloc_ignore_off_page if you get collector warnings from
allocations of very large objects. See here for details.
WARNING: pointers inside memory allocated by the standard (system)
are not seen by the garbage collector. Thus objects pointed to only from such
a region may be prematurely deallocated. It is thus suggested that the
malloc be used only for memory regions, such as I/O buffers, that
are guaranteed not to contain pointers to garbage collectible memory.
Pointers in C language automatic, static, or register variables,
are correctly recognized. (Note that
semantics similar to standard malloc, but allocates objects that are
traced by the collector.)
WARNING: the collector does not always know how to find pointers in data
areas that are associated with dynamic libraries. This is easy to remedy
if you know how to find those data areas on your operating system (see
GC_add_roots). Code for doing this under SunOS, IRIX 5.X and 6.X, HP/UX,
Alpha OSF/1, Linux, and Win32 is included and used by default.
(See README.win32 and
README.win64 for Windows details.) On other
systems, pointers from dynamic library data areas may not be considered by the
collector. If you're writing a program that depends on the collector scanning
dynamic library data areas, it may be a good idea to include at least one call
GC_is_visible to ensure that those areas are visible to the collector.
Note that the garbage collector does not need to be informed of shared read-only data. However, if the shared library mechanism can introduce discontiguous data areas that may contain pointers then the collector does need to be informed.
Signal processing for most signals may be deferred during collection, and during uninterruptible parts of the allocation process. Like standard ANSI C mallocs, by default it is unsafe to invoke malloc (and other GC routines) from a signal handler while another malloc call may be in progress.
The allocator/collector can also be configured for thread-safe operation. (Full signal safety can also be achieved, but only at the cost of two system calls per malloc, which is usually unacceptable.)
WARNING: the collector does not guarantee to scan thread-local storage
(e.g. of the kind accessed with
pthread_getspecific). The collector
does scan thread stacks, though, so generally the best solution is to
ensure that any pointers stored in thread-local storage are also
stored on the thread's stack for the duration of their lifetime.
(This is arguably a longstanding bug, but it hasn't been fixed yet.)
Installation and Portability
The collector operates silently in the default configuration.
In the event of issues, this can usually be changed by defining the
GC_PRINT_VERBOSE_STATS environment variables. This
will result in a few lines of descriptive output for each collection.
(The given statistics exhibit a few peculiarities.
Things don't appear to add up for a variety of reasons, most notably
fragmentation losses. These are probably much more significant for the
gctest than for your application.)
On most Unix-like platforms, the collector can be built either using a
GNU autoconf-based build infrastructure (type
./configure; make in the
simplest case), or using CMake (see the sample below), or with a classic
makefile by itself (type
make -f Makefile.direct).
Please note that the collector source repository does not contain configure
and similar auto-generated files, thus the full procedure of autoconf-based
master branch of the collector could look like:
git clone https://github.com/ivmai/bdwgc cd bdwgc git clone https://github.com/ivmai/libatomic_ops ./autogen.sh ./configure make -j make check
libatomic_ops is now optional provided the compiler supports
atomic intrinsics. See README.autoconf for details.
As noted above, alternatively, the collector could be built with CMake, like this:
mkdir out cd out cmake -Dbuild_tests=ON .. cmake --build . ctest
See README.cmake for details.
Finally, on most targets, the collector could be built and tested directly with a single compiler invocation, like this:
gcc -I include -o gctest tests/gctest.c extra/gc.c && ./gctest
On Windows, CMake could be used to build the library as described above or
nmake -f NT_MAKEFILE, this assumes you have Microsoft command-line
tools installed and suitably configured. See
README.win32 for details.
The library is built with threads support on (i.e. for thread-safe operation)
by default, unless
--disable-threads is passed to
-Denable_threads=OFF is passed to
The library could be configured more precisely during the build by defining the macros listed in README.macros file.
Below we focus on the collector build using classic makefile. For the
Makefile.direct-based process, typing
make check instead of
automatically build the collector and then run
setjmp_test will give you information about configuring the collector, which
is useful primarily if you have a machine that's not already supported.
gctest is a somewhat superficial test of collector functionality. Failure is
indicated by a core dump or a message to the effect that the collector is
broken. gctest takes about a second to two to run on reasonable 2007 vintage
desktops. It may use up to about 30 MB of memory. (The multi-threaded
version will use more. 64-bit versions may use more.)
make check will also,
as its last step, attempt to build and test the "cord" string library.)
Makefile.direct will generate a library libgc.a which you should link against.
make -f Makefile.direct cords will build the cord library (libcord.a)
The GNU style build process understands the usual targets.
runs a number of tests.
make install installs at least libgc, and libcord.
./configure --help to see the configuration options. It is currently
not possible to exercise all combinations of build options this way.
All include files that need to be used by clients will be put in the
include subdirectory. (Normally this is just gc.h.
make cords adds
"cord.h" and "ec.h".)
The collector currently is designed to run essentially unmodified on machines that use a flat 32-bit or 64-bit address space. That includes the vast majority of Workstations and x86 (i386 or later) PCs.
In a few cases (e.g., OS/2, Win32) a separate makefile is supplied; these have a separate host-specific docs/platforms/README.* file.
Dynamic libraries are completely supported only under SunOS/Solaris, (and even that support is not functional on the last Sun 3 release), Linux, FreeBSD, NetBSD, IRIX, HP/UX, Win32 (not win32s) and OSF/1 on DEC AXP machines plus perhaps a few others listed near the top of dyn_load.c. On other machines we recommend that you do one of the following:
- Add dynamic library support (and send us the code).
- Use static versions of the libraries.
- Arrange for dynamic libraries to use the standard malloc. This is still
dangerous if the library stores a pointer to a garbage collected object.
But nearly all standard interfaces prohibit this, because they deal
correctly with pointers to stack allocated objects. (
strtokis an exception. Don't use it.)
In all cases we assume that pointer alignment is consistent with that enforced by the standard C compilers. If you use a nonstandard compiler you may have to adjust the alignment parameters defined in gc_priv.h. Note that this may also be an issue with packed records/structs, if those enforce less alignment for pointers.
A port to a machine that is not byte addressed, or does not use 32 bit or 64 bit addresses will require a major effort. A port to plain MSDOS or win16 is hard.
For machines not already mentioned, or for nonstandard compilers, some porting suggestions are provided here.
The C Interface to the Allocator
The following routines are intended to be directly called by the user.
Note that usually only
GC_malloc is necessary.
GC_add_roots calls may be required if the collector has to trace
from nonstandard places (e.g. from dynamic library data areas on a
machine on which the collector doesn't already understand them.) On
some machines, it may be desirable to set
GC_stackbottom to a good
approximation of the stack base (bottom).
Client code may include "gc.h", which defines all of the following, plus many others.
GC_malloc(bytes)- Allocate an object of a given size. Unlike malloc, the object is cleared before being returned to the user.
GC_mallocwill invoke the garbage collector when it determines this to be appropriate. GC_malloc may return 0 if it is unable to acquire sufficient space from the operating system. This is the most probable consequence of running out of space. Other possible consequences are that a function call will fail due to lack of stack space, or that the collector will fail in other ways because it cannot maintain its internal data structures, or that a crucial system process will fail and take down the machine. Most of these possibilities are independent of the malloc implementation.
GC_malloc_atomic(bytes)- Allocate an object of a given size that is guaranteed not to contain any pointers. The returned object is not guaranteed to be cleared. (Can always be replaced by
GC_malloc, but results in faster collection times. The collector will probably run faster if large character arrays, etc. are allocated with
GC_malloc_atomicthan if they are statically allocated.)
GC_realloc(object, new_bytes)- Change the size of object to be of a given size. Returns a pointer to the new object, which may, or may not, be the same as the pointer to the old object. The new object is taken to be atomic if and only if the old one was. If the new object is composite and larger than the original object then the newly added bytes are cleared. This is very likely to allocate a new object.
GC_free(object)- Explicitly deallocate an object returned by
GC_malloc_atomic, or friends. Not necessary, but can be used to minimize collections if performance is critical. Probably a performance loss for very small objects (<= 8 bytes).
GC_expand_hp(bytes)- Explicitly increase the heap size. (This is normally done automatically if a garbage collection failed to reclaim enough memory. Explicit calls to
GC_expand_hpmay prevent unnecessarily frequent collections at program startup.)
GC_malloc_ignore_off_page(bytes)- Identical to
GC_malloc, but the client promises to keep a pointer to the somewhere within the first 256 bytes of the object while it is live. (This pointer should normally be declared volatile to prevent interference from compiler optimizations.) This is the recommended way to allocate anything that is likely to be larger than 100 KB or so. (
GC_mallocmay result in a failure to reclaim such objects.)
GC_set_warn_proc(proc)- Can be used to redirect warnings from the collector. Such warnings should be rare, and should not be ignored during code development.
GC_enable_incremental()- Enables generational and incremental collection. Useful for large heaps on machines that provide access to page dirty information. Some dirty bit implementations may interfere with debugging (by catching address faults) and place restrictions on heap arguments to system calls (since write faults inside a system call may not be handled well).
GC_register_finalizer(object, proc, data, 0, 0)and friends - Allow for registration of finalization code. User supplied finalization code (
(*proc)(object, data)) is invoked after object becomes unreachable. For more sophisticated uses, and for finalization ordering issues, see gc.h.
The global variable
GC_free_space_divisor may be adjusted up from it
default value of 3 to use less space and more collection time, or down for
the opposite effect. Setting it to 1 will almost disable collections
and cause all allocations to simply grow the heap.
GC_non_gc_bytes, which is normally 0, may be changed to reflect
the amount of memory allocated by the above routines that should not be
considered as a candidate for collection. Careless use may, of course, result
in excessive memory consumption.
Some additional tuning is possible through the parameters defined near the top of gc_priv.h.
GC_malloc is intended to be used, it might be appropriate to define:
#define malloc(n) GC_malloc(n) #define calloc(m,n) GC_malloc((m)*(n))
For small pieces of VERY allocation intensive code,
some allocation macros that may be used in place of
All externally visible names in the garbage collector start with
To avoid name conflicts, client code should avoid this prefix, except when
accessing garbage collector routines.
There are provisions for allocation with explicit type information.
This is rarely necessary. Details can be found in
The C++ Interface to the Allocator
The Ellis-Hull C++ interface to the collector is included in the collector
distribution. If you intend to use this, type
./configure --enable-cplusplus && make (or
cmake -Denable_cplusplus=ON . && cmake --build ., or
make -f Makefile.direct c++ depending on the build system you use).
This creates libgccpp.a and libgctba.a files, or their shared library
equivalents (libgccpp.so and libgctba.so). You should link with either the
first (gccpp) or the second one (gctba), but not both. See gc_cpp.h and
here for the definition of the interface.
This interface tries to approximate the Ellis-Detlefs C++ garbage collection
proposal without compiler changes.
Very often it will also be necessary to use
gc_allocator.h and the
allocator declared there to construct STL data structures. Otherwise
subobjects of STL data structures will be allocated using a system
allocator, and objects they refer to may be prematurely collected.
Use as Leak Detector
The collector may be used to track down leaks in C programs that are
intended to run with malloc/free (e.g. code with extreme real-time or
portability constraints). To do so define
FIND_LEAK in Makefile.
This will cause the collector to print a human-readable object description
whenever an inaccessible object is found that has not been explicitly freed.
Such objects will also be automatically reclaimed.
If all objects are allocated with
GC_DEBUG_MALLOC (see the next section)
then, by default, the human-readable object description will at least contain
the source file and the line number at which the leaked object was allocated.
This may sometimes be sufficient. (On a few machines, it will also report
a cryptic stack trace. If this is not symbolic, it can sometimes be called
into a symbolic stack trace by invoking program "foo" with
tools/callprocs.sh foo. It is a short shell script that invokes adb to
expand program counter values to symbolic addresses. It was largely supplied
by Scott Schwartz.)
Note that the debugging facilities described in the next section can
sometimes be slightly LESS effective in leak finding mode, since in the latter
GC_debug_free actually results in reuse of the object. (Otherwise the
object is simply marked invalid.) Also, note that most GC tests are not
designed to run meaningfully in
GC_debug_free provide an alternate interface to the collector, which
provides some help with memory overwrite errors, and the like.
Objects allocated in this way are annotated with additional
information. Some of this information is checked during garbage
collections, and detected inconsistencies are reported to stderr.
Simple cases of writing past the end of an allocated object should
be caught if the object is explicitly deallocated, or if the
collector is invoked while the object is live. The first deallocation
of an object will clear the debugging info associated with an
object, so accidentally repeated calls to
GC_debug_free will report the
deallocation of an object without debugging information. Out of
memory errors will be reported to stderr, in addition to returning
GC_debug_malloc checking during garbage collection is enabled
with the first call to this function. This will result in some
slowdown during collections. If frequent heap checks are desired,
this can be achieved by explicitly invoking
GC_gcollect, e.g. from
GC_debug_malloc allocated objects should not be passed to
GC_free, and conversely. It is however acceptable to allocate only
some objects with
GC_debug_malloc, and to use
GC_malloc for other objects,
provided the two pools are kept distinct. In this case, there is a very
low probability that
GC_malloc allocated objects may be misidentified as
having been overwritten. This should happen with probability at most
one in 2**32. This probability is zero if
GC_debug_malloc is never called.
GC_debug_realloc take two
additional trailing arguments, a string and an integer. These are not
interpreted by the allocator. They are stored in the object (the string is
not copied). If an error involving the object is detected, they are printed.
GC_REGISTER_FINALIZER and friends are also provided. These require the same
arguments as the corresponding (nondebugging) routines. If gc.h is included
GC_DEBUG defined, they call the debugging versions of these
functions, passing the current file name and line number as the two
extra arguments, where appropriate. If gc.h is included without
defined then all these macros will instead be defined to their nondebugging
GC_REGISTER_FINALIZER is necessary, since pointers to
objects with debugging information are really pointers to a displacement
of 16 bytes from the object beginning, and some translation is necessary
when finalization routines are invoked. For details, about what's stored
in the header, see the definition of the type oh in dbg_mlc.c file.)
The collector normally interrupts client code for the duration of
a garbage collection mark phase. This may be unacceptable if interactive
response is needed for programs with large heaps. The collector
can also run in a "generational" mode, in which it usually attempts to
collect only objects allocated since the last garbage collection.
Furthermore, in this mode, garbage collections run mostly incrementally,
with a small amount of work performed in response to each of a large number of
This mode is enabled by a call to
Incremental and generational collection is effective in reducing pause times only if the collector has some way to tell which objects or pages have been recently modified. The collector uses two sources of information:
Information provided by the VM system. This may be provided in one of several forms. Under Solaris 2.X (and potentially under other similar systems) information on dirty pages can be read from the /proc file system. Under other systems (e.g. SunOS4.X) it is possible to write-protect the heap, and catch the resulting faults. On these systems we require that system calls writing to the heap (other than read) be handled specially by client code. See
Information supplied by the programmer. The object is considered dirty after a call to
GC_end_stubborn_changeprovided the library has been compiled suitably. It is typically not worth using for short-lived objects. Note that bugs caused by a missing
GC_reachable_herecall are likely to be observed very infrequently and hard to trace.
Any memory that does not have a recognizable pointer to it will be reclaimed. Exclusive-or'ing forward and backward links in a list doesn't cut it.
Some C optimizers may lose the last undisguised pointer to a memory object as a consequence of clever optimizations. This has almost never been observed in practice.
This is not a real-time collector. In the standard configuration, percentage of time required for collection should be constant across heap sizes. But collection pauses will increase for larger heaps. They will decrease with the number of processors if parallel marking is enabled.
(On 2007 vintage machines, GC times may be on the order of 5 ms per MB of accessible memory that needs to be scanned and processed. Your mileage may vary.) The incremental/generational collection facility may help in some cases.
Feedback, Contribution, Questions and Notifications
Please address bug reports and new feature ideas to GitHub issues. Before the submission please check that it has not been done yet by someone else.
If you want to contribute, submit a pull request to GitHub.
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Copyright & Warranty, Contributors
Our intent is to make it easy to use bdwgc (libgc), in both free and proprietary software. Hence, the Boehm-Demers-Weiser conservative garbage collector code that we expect to be linked dynamically or statically into a client application is covered by own license, which is similar in spirit to an MIT-style one.
The exact licensing information is provided in LICENSE file.
All the contributors are listed in AUTHORS file.