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

Terms

Roots

Generally roots mean pointers stored in statically allocated or stack allocated program variables. A mark-sweep garbage collector traverses all reachable objects in the heap (more accurately it's GC heap in our case) by following pointers beginning with the roots.

Coding guide

Interaction with the system malloc

It is usually best not to mix garbage-collected allocation with the system malloc-free. If you do, you need to be careful not to store pointers to the garbage-collected heap in memory allocated with the system malloc.

Using std::vector (1)

You need to be careful using std::vector since it allocates memory for its elements internally. It allocates based on the allocator you pass in at construction. If you didn't specify one, you get the default allocator and it will allocate on the native heap! So it is needed to specify GC allocator for the std::vector with elements that may have GC pointers.

To elaborate, at mark phase, GC start to scan pointer-like values and checks if the values belong to the GC heap. But if we allocate on the native heap using std allocator, the ranges are out of GC heap range, so GC ignores the values.

Using std::vector (2)

It is not recommended to use std::vector, because its std::vector::end points next space of the last element which makes GC think it as a non-collectible space! Please refer here.

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 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 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 here 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, 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 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 hasn't 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 libgc (as well as libcord) and run the tests using cmake:

mkdir out
cd out
cmake -Dbuild_tests=ON ..
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 libgc and libcord. Try ./configure --help to see all the configuration options. It is curre