Boehm Garbage Collector
November 10th, 2007
Toying with the Nokia 770 has been teaching me more than I ever wanted to know about C, gcc, and the other things we do to write software for handhelds. I had known about the existence of garbage collectors for C and C++ but had always assumed they worked similar to GObject. A GObject programmer must call g_object_ref() and g_object_unref() to manipulate the reference count of each object on the heap. When the count reaches zero the space is reclaimed. A Boehm GC, rather, can function as a drop-in replacement for malloc() and free(). To my amazement the free() procedure isn’t required at all and can be replaced with a noop. The word “automagically” doesn’t even annoy me in this case since the Boehm GC is quite a trick. It can determine when an object on the heap is no longer required by comparing places where pointers usually live with it’s internal book. For example, a function that calls malloc usually sets a pointer on the stack to the allocated memory. When the stack shrinks the reference is gone and the dead memory can be automatically freed.
The main benefit, for me anyway, is that I can write consing functions in C. Any linked list implementation has a foreach method. Foreach is easy because it doesn’t have to keep the return value of its lambda argument. Map, filter, and almost every other good thing that comes from functional programming requires consing up a new list. Allocating the right amount of memory for each return value is doable. The difficult part is the bookeeping required to later free all the unreferenced lists. A functional program might map, filter, then reduce in a single line as it takes the GC for granted. Manual allocation is a better fit for building fewer, less disposable structures. I’m not sure I’ll use map that much with C but it would be a good way to test-drive the Boehm GC.
There has to be a catch. I’m still wide eyed that the Boehm GC even works. It might be a sign that Mozilla and X, the two biggest hogs on my system, use it.
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