l-potter / ogc

A minimal mark-and-sweep garbage collector

OGC: Ordinary Garbage Collector

About

OGC implements the typical "mark and sweep" algorithm. It works as follows:

• there is an internal struct representing a block of memory
• there is a list of blocks representing the allocated memory, and another list representing the local stack of memory
• once a new object is created,
  • its memory is allocated and its meta data is added to both lists
  • there is a threshold for the number of objects to allocate before cleaning up the memory
  • when the threshold is met:
    • call "mark" with the local stack of memory ("reachable" memory)
    • mark recurses through the list and set a "mark" flag for every block reachable from the inital list of reachable memory
    • we then call "sweep" with the global allocated memory list
    • every block that is not marked is not used, and can be freed and popped from given list

How it works

For a basic mark and sweep garbage collector, two things are required:

• a list of all of the allocations made by the program;
• a list of all the allocations in use by the program at any given time;

With these two things the algorithm is simple - compare the two lists and free any allocations which are in the first list, but not in the second - exactly those allocations which are no longer in use.

To get a list of all the allocations made by the progam is relatively simple. We make the programmer use a special function we have prepared (in this case gc_alloc) which allocates memory, and then adds a pointer to that memory to an internal list. If at any point this allocation is freed (such as by gc_free), it is removed from the list.

Function list

• gc_alloc()

  • Allocates a space in memory of the size which the user requests
  • Ensures the user entered a number > 0
  • Attempts to find a freed block of memory to reuse.
  • On success, returns the address of the newly allocated memory. Otherwise, returns NULL.

• gc_free()

  • Marks the object as free in the list of objects
  • Ensures the pointer isn't NULL or already free before marking it

• gc_run()

  • Recursively marks all reachable objects in the list of objects
  • Recursively sweeps (frees) all unmarked (unreachable) objects