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
- Read the quickstart below to see how to get started quickly
- The concepts section describes the basic concepts and design
decisions that went into the implementation of
gc. - Interleaved with the concepts, there are implementation sections that detail the implementation of the core components, see hash map implementation, dumping registers on the stack, finding roots, and depth-first, recursive marking.
Quickstart
Download, compile and test
$ git clone [email protected]: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:
- 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.bosis the smallest stack address considered during the mark phase). - There is a pointer inside
gc_*alloc()-allocated content that points to the allocation content. - 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!