The Boehm–Demers–Weiser garbage collector, often simply known as the Boehm GC or Boehm collector, is a conservative garbage collector for C and C++ developed by Hans Boehm, Alan Demers, and Mark Weiser.

Boehm GC is free software distributed under a permissive free software licence similar to the X11 license. The first paper introducing this collector appeared in 1992.

Hans Boehm 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 Linux, *BSD, recent Windows versions, MacOS X, HP/UX, Solaris, Tru64, Irix and a few other operating systems, 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.

— http://www.hboehm.info/gc/#details ^{[bare URL]}

Boehm GC can also run in leak detection 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++ (trees of constant small arrays), 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).

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 garbage collection features can be accessed from a header <gc.h>. The code piece below shows how one can use Boehm instead of traditional malloc() and free() in C.