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gc: mark & sweep garbage collection for C

gc is an implementation of a conservative, thread-local, mark-and-sweep garbage collector. The implementation provides a fully functional replacement for the standard POSIX malloc(), calloc(), realloc(), and free() calls.

The focus of gc is to provide a conceptually clean implementation of a mark-and-sweep GC, without delving into the depths of architecture-specific optimization (see e.g. the Boehm GC for such an undertaking). It should be particularly suitable for learning purposes and is open for all kinds of optimization (PRs welcome!).

The original motivation for gc is my desire to write my own LISP in C, entirely from scratch - and that required garbage collection.

Acknowledgements

This work would not have been possible without the ability to read the work of others, most notably the Boehm GC, orangeduck's tgc (which also follows the ideals of being tiny and simple), and The Garbage Collection Handbook.

Table of contents

Documentation Overview

Quickstart

Download, compile and test

$ git clone git@github.com:mkirchner/gc.git
$ cd gc

To compile using the clang compiler:

$ make test

To use the GNU Compiler Collection (GCC):

$ make test CC=gcc

The tests should complete successfully. To create the current coverage report:

$ make coverage

Basic usage

...
#include "gc.h"
...


void some_fun() {
    ...
    int* my_array = gc_calloc(&gc, 1024, sizeof(int));
    for (size_t i=0; i<1024; ++i) {
        my_array[i] = 42;
    }
    ...
    // look ma, no free!
}

int main(int argc, char* argv[]) {
    gc_start(&gc, &argc);
    ...
    some_fun();
    ...
    gc_stop(&gc);
    return 0;
}

Core API

This describes the core API, see gc.h for more details and the low-level API.

Starting, stopping, pausing, resuming and running GC

In order to initialize and start garbage collection, use the gc_start() function and pass a bottom-of-stack address:

void gc_start(GarbageCollector* gc, void* bos);

The bottom-of-stack parameter bos needs to point to a stack-allocated variable and marks the low end of the stack from where root finding (scanning) starts.

Garbage collection can be stopped, paused and resumed with

void gc_stop(GarbageCollector* gc);
void gc_pause(GarbageCollector* gc);
void gc_resume(GarbageCollector* gc);

and manual garbage collection can be triggered with

size_t gc_run(GarbageCollector* gc);

Memory allocation and deallocation

gc supports malloc(), calloc()and realloc()-style memory allocation. The respective function signatures mimick the POSIX functions (with the exception that we need to pass the garbage collector along as the first argument):

void* gc_malloc(GarbageCollector* gc, size_t size);
void* gc_calloc(GarbageCollector* gc, size_t count, size_t size);
void* gc_realloc(GarbageCollector* gc, void* ptr, size_t size);

It is possible to pass a pointer to a destructor function through the extended interface:

void* dtor(void* obj) {
   // do some cleanup work
   obj->parent->deregister();
   obj->db->disconnect()
   ...
   // no need to free obj
}
...
SomeObject* obj = gc_malloc_ext(gc, sizeof(SomeObject), dtor);
...

gc supports static allocations that are garbage collected only when the GC shuts down via gc_stop(). Just use the appropriate helper function:

void* gc_malloc_static(GarbageCollector* gc, size_t size, void (*dtor)(void*));

Static allocation expects a pointer to a finalization function; just set to NULL if finalization is not required.

Note that gc currently does not guarantee a specific ordering when it collects static variables, If static vars need to be deallocated in a particular order, the user should call gc_free() on them in the desired sequence prior to calling gc_stop(), see below.

It is also possible to trigger explicit memory deallocation using

void gc_free(GarbageCollector* gc, void* ptr);

Calling gc_free() is guaranteed to (a) finalize/destruct on the object pointed to by ptr if applicable and (b) to free the memory that ptr points to irrespective of the current scheduling for garbage collection and will also work if GC has been paused using gc_pause() above.

Helper functions

gc also offers a strdup() implementation that returns a garbage-collected copy:

char* gc_strdup (GarbageCollector* gc, const char* s);

Basic Concepts

The fundamental idea behind garbage collection is to automate the memory allocation/deallocation cycle. This is accomplished by keeping track of all allocated memory and periodically triggering deallocation for memory that is still allocated but unreachable.

Many advanced garbage collectors also implement their own approach to memory allocation (i.e. replace malloc()). This often enables them to layout memory in a more space-efficient manner or for faster access but comes at the price of architecture-specific implementations and increased complexity. gc sidesteps these issues by falling back on the POSIX *alloc() implementations and keeping memory management and garbage collection metadata separate. This makes gc much simpler to understand but, of course, also less space- and time-efficient than more optimized approaches.

Data Structures

The core data structure inside gc is a hash map that maps the address of allocated memory to the garbage collection metadata of that memory:

The items in the hash map are allocations, modeled with the Allocation struct:

typedef struct Allocation {
    void* ptr;                // mem pointer
    size_t size;              // allocated size in bytes
    char tag;                 // the tag for mark-and-sweep
    void (*dtor)(void*);      // destructor
    struct Allocation* next;  // separate chaining
} Allocation;

Each Allocation instance holds a pointer to the allocated memory, the size of the allocated memory at that location, a tag for mark-and-sweep (see below), an optional pointer to the destructor function and a pointer to the next Allocation instance (for separate chaining, see below).

The allocations are collected in an AllocationMap

typedef struct AllocationMap {
    size_t capacity;
    size_t min_capacity;
    double downsize_factor;
    double upsize_factor;
    double sweep_factor;
    size_t sweep_limit;
    size_t size;
    Allocation** allocs;
} AllocationMap;

that, together with a set of static functions inside gc.c, provides hash map semantics for the implementation of the public API.

The AllocationMap is the central data structure in the GarbageCollector struct which is part of the public API:

typedef struct GarbageCollector {
    struct AllocationMap* allocs;
    bool paused;
    void *bos;
    size_t min_size;
} GarbageCollector;

With the basic data structures in place, any gc_*alloc() memory allocation request is a two-step procedure: first, allocate the memory through system (i.e. standard malloc()) functionality and second, add or update the associated metadata to the hash map.

For gc_free(), use the pointer to locate the metadata in the hash map, determine if the deallocation requires a destructor call, call if required, free the managed memory and delete the metadata entry from the hash map.

These data structures and the associated interfaces enable the management of the metadata required to build a garbage collector.

Garbage collection

gc triggers collection under two circumstances: (a) when any of the calls to the system allocation fail (in the hope to deallocate sufficient memory to fulfill the current request); and (b) when the number of entries in the hash map passes a dynamically adjusted high water mark.

If either of these cases occurs, gc stops the world and starts a mark-and-sweep garbage collection run over all current allocations. This functionality is implemented in the gc_run() function which is part of the public API and delegates all work to the gc_mark() and gc_sweep() functions that are part of the private API.

gc_mark() has the task of finding roots and tagging all known allocations that are referenced from a root (or from an allocation that is referenced from a root, i.e. transitively) as "used". Once the marking of is completed, gc_sweep() iterates over all known allocations and deallocates all unused (i.e. unmarked) allocations, returns to gc_run() and the world continues to run.

Reachability

gc will keep memory allocations that are reachable and collect everything else. An allocation is considered reachable if any of the following is true:

  1. There is a pointer on the stack that points to the allocation content. The pointer must reside in a stack frame that is at least as deep in the call stack as the bottom-of-stack variable passed to gc_start() (i.e. bos is the smallest stack address considered during the mark phase).
  2. There is a pointer inside gc_*alloc()-allocated content that points to the allocation content.
  3. The allocation is tagged with GC_TAG_ROOT.

The Mark-and-Sweep Algorithm

The naïve mark-and-sweep algorithm runs in two stages. First, in a mark stage, the algorithm finds and marks all root allocations and all allocations that are reachable from the roots. Second, in the sweep stage, the algorithm passes over all known allocations, collecting all allocations that were not marked and are therefore deemed unreachable.

Finding roots

At the beginning of the mark stage, we first sweep across all known allocations and find explicit roots with the GC_TAG_ROOT tag set. Each of these roots is a starting point for depth-first recursive marking.

gc subsequently detects all roots in the stack (starting from the bottom-of-stack pointer bos that is passed to gc_start()) and the registers (by dumping them on the stack prior to the mark phase) and uses these as starting points for marking as well.

Depth-first recursive marking

Given a root allocation, marking consists of (1) setting the tag field in an Allocation object to GC_TAG_MARK and (2) scanning the allocated memory for pointers to known allocations, recursively repeating the process.

The underlying implementation is a simple, recursive depth-first search that scans over all memory content to find potential references:

void gc_mark_alloc(GarbageCollector* gc, void* ptr)
{
    Allocation* alloc = gc_allocation_map_get(gc->allocs, ptr);
    if (alloc && !(alloc->tag & GC_TAG_MARK)) {
        alloc->tag |= GC_TAG_MARK;
        for (char* p = (char*) alloc->ptr;
             p < (char*) alloc->ptr + alloc->size;
             ++p) {
            gc_mark_alloc(gc, *(void**)p);
        }
    }
}

In gc.c, gc_mark() starts the marking process by marking the known roots on the stack via a call to gc_mark_roots(). To mark the roots we do one full pass through all known allocations. We then proceed to dump the registers on the stack.

Dumping registers on the stack

In order to make the CPU register contents available for root finding, gc dumps them on the stack. This is implemented in a somewhat portable way using setjmp(), which stores them in a jmp_buf variable right before we mark the stack:

...
/* Dump registers onto stack and scan the stack */
void (*volatile _mark_stack)(GarbageCollector*) = gc_mark_stack;
jmp_buf ctx;
memset(&ctx, 0, sizeof(jmp_buf));
setjmp(ctx);
_mark_stack(gc);
...

The detour using the volatile function pointer _mark_stack to the gc_mark_stack() function is necessary to avoid the inlining of the call to gc_mark_stack().

Sweeping

After marking all memory that is reachable and therefore potentially still in use, collecting the unreachable allocations is trivial. Here is the implementation from gc_sweep():

size_t gc_sweep(GarbageCollector* gc)
{
    size_t total = 0;
    for (size_t i = 0; i < gc->allocs->capacity; ++i) {
        Allocation* chunk = gc->allocs->allocs[i];
        Allocation* next = NULL;
        while (chunk) {
            if (chunk->tag & GC_TAG_MARK) {
                /* unmark */
                chunk->tag &= ~GC_TAG_MARK;
                chunk = chunk->next;
            } else {
                total += chunk->size;
                if (chunk->dtor) {
                    chunk->dtor(chunk->ptr);
                }
                free(chunk->ptr);
                next = chunk->next;
                gc_allocation_map_remove(gc->allocs, chunk->ptr, false);
                chunk = next;
            }
        }
    }
    gc_allocation_map_resize_to_fit(gc->allocs);
    return total;
}

We iterate over all allocations in the hash map (the for loop), following every chain (the while loop with the chunk = chunk->next update) and either (1) unmark the chunk if it was marked; or (2) call the destructor on the chunk and free the memory if it was not marked, keeping a running total of the amount of memory we free.

That concludes the mark & sweep run. The stopped world is resumed and we're ready for the next run!

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