How can I provide garbage collection for an interpreted language implemented in C?

Question

If I were to implement a garbage collected interpreted language in C, how can I go about providing precise (i.e. not conservative) garbage collection without writing my own

Answer 1

For C programs, there are 2 options: the Boehm GC which replaces malloc (it is a conservative GC, so perhaps not exactly what you're looking for but it's either that or...), or write your own.

But writing your own isn't that hard. Do the mark-sweep algorithm. The root set for marking will be your symbol table. And you'll need another table or linked-list to track all of the allocated memory that can be freed. When you sweep through the list of allocations, free anything without a mark.

The actual coding will of course be more complicated because you have to iterate through these two kinds of data structures, but the algorithm itself is very simple. You can do it.

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A few years ago, I found myself on the same search and these were (and AFAIK still are) the results. Writing your own is tremendously rewarding and worthwhile.

In practice, a great many other issues will arise as Basile's answer touches upon.

If the garbage collector is called from deep in the call-stack (by an allocation routine that needs more memory, perhaps), then care must be taken about any allocations whose handles are still held in local variables of C functions in the call stack, and not saved-out to their symbol-table or database locations. In my postscript interpreter I dealt with this by using a temporary stack which all allocators pushed to. This stack was cleared by the main loop after all subroutines had returned, and it was considered part of the root set during marking. In my APL interpreter, I call the GC everytime around the mainloop. For the little programs in little languages, the speed issues are less vital than the more-dreaded memory leakage, at least among the circles which have influenced me.