bdwgc/bdwgc

Boehm-Demers-Weiser Garbage Collector

This is version 8.3.0 (next release development) of a conservative garbage collector for C and C++.

License: MIT-style

Download

You might find a more recent/stable version on the Download page, or BDWGC site.

Also, the latest bug fixes and new features are available in the development repository.

Overview

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 multi-threading 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 cords.md 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.

General Description

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.

Compiling without 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 address space.

There are a number of routines which modify the pointer recognition algorithm. 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 the debugging documentation for details.

Warning: pointers inside memory allocated by the standard (system) malloc 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 standard 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 GC_malloc_uncollectable has 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 (Tru64 UNIX), 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 are 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 to 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 has not been fixed yet.)

Building and Installing

There are multiple ways to build the collector:

• CMake (it is the recommended way)
• GNU autoconf/automake
• Zig (experimental)
• MS nmake (directly)
• Makefile.direct
• Manual C compilation

CMake

The simplest way to build gc library (as well as cord library) and run the tests using CMake:

mkdir build
cd build
cmake ..
cmake --build .
ctest

This is the most cross-platform way of building the library. See cmake.md for details.

GNU Autoconf/Automake

Please note that the collector source repository does not contain configure and similar auto-generated files, thus the full procedure of autoconf-based build of the collector from the source repository could look like:

./autogen.sh
./configure
make check

The GNU style build process understands the usual targets and options. make install installs gc and cord libraries. Try ./configure --help to see all the configuration options. It is currently not possible to exercise all combinations of build options this way.

See autoconf.md for details.

Zig

Building and testing the collector using zig is straight forward in its simplest form:

zig build test

It is possible to configure the build through the use of variables, e.g. zig build -Denable_redirect_malloc -Denable_threads=false. Zig offers excellent cross-compilation functionality, it is configurable like this:

zig build -Dtarget=riscv64-linux-musl

The appropriate Zig binary package file could be downloaded from the official Zig releases page.

MS nmake

On Windows, assuming the Microsoft build tools are installed and suitably configured, it is possible to build the library and run the tests using nmake directly, e.g. by by typing nmake -f NT_MAKEFILE check. However, the recommended way is to use cmake as described above.

See README.win32 for details.

Makefile.direct

For the old-style (classic) makefile-based build process, typing make -f Makefile.direct check will automatically build gc, cord libraries, then run a number of tests such as gctest. The test 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 may take a dozen of seconds to run on reasonable 2023 vintage 64-bit desktops. It may use up to about 30 MB of memory.

Makefile.direct file generates a libgc.a file which you should link against.

Manual C Compilation

Finally, on most targets, the collector could be built and tested directly with a single compiler invocation, like this (the sample lacks multi-threading support):

cc -I include -o gctest tests/gctest.c extra/gc.c && ./gctest

E.g., this could be convenient for a debugging purpose.

Configurable Macros

The library can be configured more precisely during the build by defining the macros listed in macros.md file.

The library is built with multi-threading support enabled (i.e. for thread-safe operation) by default, unless explicitly disabled by:

• -Denable_threads=false option passed to cmake or zig build
• --disable-threads option passed to ./configure

The collector operates silently in the default configuration. In the event of issues, this can usually be changed by defining the GC_PRINT_STATS or 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 do not appear to add up for a variety of reasons, most notably fragmentation losses. These are probably much more significant for the contrived program gctest than for your application.)

Atomic_ops

Use (cloning) of libatomic_ops is now optional provided the compiler supports atomic intrinsics. Most modern compilers do. The notable exception is the MS compiler (as of Visual Studio 2022).

If needed, most OS distributes have libatomic_ops package; alternatively, you can download or clone it from libatomic_ops repository on GitHub.

Portability

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 Tru64 UNIX 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:

1. Add dynamic library support (and send us the code).

2. Use static versions of the libraries.

3. 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. (strtok is an exception. Do not 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 include/private/gc_priv.h file. 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- 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 suggestions are provided in the porting instructions.

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_clear_roots and 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 does not 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 file, which defines all of the following, plus many others.

1. GC_malloc(bytes) - Allocate an object of a given size. Unlike malloc, the object is cleared before being returned to the user. GC_malloc will 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.

2. 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_atomic than if they are statically allocated.)

3. GC_realloc(object, new_bytes) and GC_reallocf(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.

4. GC_free(object), GC_freezero(object, bytes_to_clear) - Explicitly deallocate an object returned by GC_malloc or GC_malloc_atomic, or friends. Not necessary, but can be used to minimize collections if performance is critical. Probably a performance loss could occur for very small objects (not greater than 8 bytes). GC_freezero ensures the object is zero-filled before its deallocation.

5. 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_hp may prevent unnecessarily frequent collections at program startup.)

6. GC_malloc_ignore_off_page(bytes) - Identical to GC_malloc, but the client promises to keep a pointer to the somewhere within the first GC heap block (512 .. 4096 bytes or even more, depending on the configuration) 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_malloc may result in a failure to reclaim such objects.)

7. 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.

8. 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).

9. 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 file.

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.

The variable 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 include/private/gc_priv.h file.

If only GC_malloc is intended to be used, it might be appropriate to define like:

#define malloc(n) GC_malloc(n)
define calloc(m, n) GC_malloc((m) * (n))

For small pieces of very allocation-intensive code, gc_inline.h file includes some allocation macros that may be used in place of GC_malloc and friends.

All externally visible names in the garbage collector start with GC_. To avoid name conflicts, client code should avoid this prefix, except when accessing garbage col