Boehm garbage collector

Written in	C and C++
Type	garbage collector
License	similar to X11 (free software)
Website	www.hpl.hp.com/personal/Hans_Boehm/gc

In computer science, the Boehm-Demers-Weiser garbage collector, often simply known as Boehm GC, is a conservative garbage collector for C and C++, which is used by many projects that are implemented in C or C++, as well as by runtime environments for a number of other languages, including the GNU Compiler for Java runtime environment, the Portable.NET project, LLVM, GNU D Compiler and the Mono implementation of the Microsoft .NET platform (the last is going to change garbage collector to precise compacting GC in version 2.6). It supports numerous operating systems, including many Unix variants (such as Mac OS X) and Microsoft Windows, and provides a number of advanced features including incremental collection, parallel collection and a variety of finalizer semantics. Boehm GC was ported with small changes to the D programming language and is part of Digital Mars D compiler's standard runtime library called Phobos (this differs to other usage, because other runtimes use an unmodified C version).

Boehm GC can also run in test mode in which memory management is still done manually, but the Boehm GC can check if it is done properly. In this way a programmer can find memory leaks and double deallocations.

Boehm GC is also distributed with a C string handling library called cords. This is similar to ropes in C++ – strings are trees of small arrays, and they never change – but instead of using reference counting for proper deallocation, it relies on garbage collection to free objects. Cords are good at handling very large texts, modifications to them in the middle, slicing, concatenating, and keeping history of changes (undo/redo functionality).

Boehm GC is free software distributed under a permissive free software licence similar to the X11 license.

Example

The garbage collector works with most unmodified C programs, simply by replacing malloc with GC_malloc calls, replacing realloc with GC_realloc calls, and removing free calls. The code piece below shows how one can use Boehm instead of traditional malloc and free in C.

#include <assert.h>
#include <stdio.h>
#include <gc.h>

int main(void)
{
    int i;

    GC_INIT();
    for (i = 0; i < 10000000; ++i)
    {
        int **p = GC_MALLOC(sizeof(int *));
        int *q = GC_MALLOC_ATOMIC(sizeof(int));

        assert(*p == 0);
        *p = GC_REALLOC(q, 2 * sizeof(int));
        if (i % 100000 == 0)
            printf("Heap size = %d\n", GC_get_heap_size());
    }

    return 0;
}

Method of Operation

The developer describes the operation of the collector as follows:

"The collector uses a mark-sweep algorithm. It provides incremental and generational collection under operating systems which provide the right kind of virtual memory support. (Currently this includes SunOS[45], IRIX, OSF/1, Linux, and Windows, with varying restrictions.) It allows finalization code to be invoked when an object is collected. It can take advantage of type information to locate pointers if such information is provided, but it is usually used without such information."

External links

• Homepage
  • Implementation overview
• Sourceforge website
• Transparent Programmer-Directed Garbage Collection for C++, Hans-J. Boehm and Michael Spertus
• Using the C/C++ Garbage Collection Library
• TinyGC - an alternative lightweight implementation of BoehmGC API