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Custom allocators and memory functions

Arena Allocator

A simple arena-style allocator

Structs

ArenaAllocator

Represents an arena allocator. ArenaAllocator holds its own buffer. The handle may be stack- or heap-allocated.

typedef struct {
    unsigned char* buf;
    size_t buf_sz;
    size_t offset_cur;
    size_t offset_prev;

    size_t save_stack[LF_ARENA_MAX_SAVE_POINTS];
    size_t save_count;
    int owns_buf;
} ArenaAllocator;

owns_buf is 1 when the arena malloc'd the slab (arena_init) and 0 when the caller provided it (arena_init_buf).

Functions

arena_init

Initializes the ArenaAllocator. buf_sz is the size of the underlying buffer in bytes. Returns 0 on success, -1 on error (NULL allocator or malloc failure).

int arena_init(ArenaAllocator *allocator, size_t buf_sz);

/* Usage */
ArenaAllocator a;
arena_init(&a, 1024);

arena_init_buf

Binds the arena to caller-owned storage (stack, static, or an existing mapping). The arena does not take ownership: arena_free detaches without freeing storage, and arena_resize_buf is rejected. storage must be non-NULL and n must be non-zero. Allocations are still aligned from the storage address. Returns 0 on success, -1 on error.

int arena_init_buf(ArenaAllocator *allocator, void *storage, size_t n);

/* Usage */
unsigned char buf[256];
ArenaAllocator a;
arena_init_buf(&a, buf, sizeof buf);
int *i = arena_malloc(&a, sizeof(int));
arena_free(&a); /* does not free buf */

arena_free

If the arena owns its slab, frees it. Always resets the handle. Does not free the allocator struct itself, since the user is responsible for managing its lifetime (matching the behavior of arena_init()).

void arena_free(ArenaAllocator *allocator);

/* Usage */
arena_free(a);
free(a); // if heap-allocated
a = NULL;

arena_clear

Resets the offset markers of the arena to 0 and clears all save points, but does not wipe the underlying buffer.

void arena_clear(ArenaAllocator *allocator);

arena_malloc

Request memory of size bytes in length from the arena. Returns NULL if the assignment failed.

void *arena_malloc(ArenaAllocator* allocator, size_t size);

arena_resize_buf

Reallocates the underlying buffer in the arena to new_sz. You can grow or shrink the arena using this function. Rejects shrinking below the current allocation offset, and rejects arenas created with arena_init_buf (the slab is not owned). Any pointers allocated out of the arena are invalid after using this function. Returns 0 on success, -1 on error (external buffer, shrink below offset, or realloc failure).

int arena_resize_buf(ArenaAllocator *allocator, const size_t new_sz);

arena_resize

Resize an allocated pointer from the arena. Returns NULL if the arena doesn't have enough space or if mem doesn't belong to the arena. When growing, the new bytes are zeroed. See the example below for a simple use case.

void *arena_resize(ArenaAllocator *allocator, void *mem, size_t old_sz, size_t new_sz);

/* Usage */
int *i = arena_malloc(a, sizeof(int));
*i = 1;
long *l = arena_resize(a, i, sizeof(int), sizeof(long));
assert(*l == 1);

arena_save

Saves the current allocation offset onto an internal stack, allowing temporary allocations to be made and later discarded with arena_restore(). Supports nesting up to LF_ARENA_MAX_SAVE_POINTS (default 32). Returns 0 on success, -1 if the save stack is full.

int arena_save(ArenaAllocator *allocator);

arena_restore

Restores the arena offset to the most recent save point, effectively freeing all allocations made since the last arena_save() call. Does nothing if no save points exist. Permanent allocations made before the save point are unaffected.

void arena_restore(ArenaAllocator *allocator);

/* Usage */
int *permanent = arena_malloc(a, sizeof(int));
*permanent = 42;

arena_save(a);
int *tmp = arena_malloc(a, sizeof(int));
*tmp = 999;  // temporary work
arena_restore(a);  // tmp is now invalid, permanent is intact

int *next = arena_malloc(a, sizeof(int));  // reuses tmp's space

Macros

LF_ARENA_MAX_SAVE_POINTS

Maximum number of nested save points for arena_save()/arena_restore(). Can be overridden by defining before including the header. Defaults to 32.

#ifndef LF_ARENA_MAX_SAVE_POINTS
#define LF_ARENA_MAX_SAVE_POINTS 32
#endif

Pool Allocator

A pool of same-sized chunks carved from one buffer. Free chunks keep a next pointer in their first word, so pool_alloc and pool_free do not call the system allocator.

The handle may be stack- or heap-allocated. chunk_align must be a non-zero power of two. A second pool_free of the same live chunk is ignored.

Structs

PoolAllocator

typedef struct {
    unsigned char *buf;
    size_t buf_sz;
    size_t chunk_size;
    size_t aligned_start;
    size_t free_count;
    void *free_head;
} PoolAllocator;

Functions

pool_init

Initializes the PoolAllocator. Returns 0 on success, -1 on error (NULL allocator, zero sizes, non-power-of-two or unsatisfiable chunk_align, malloc failure, or buffer too small for one aligned chunk).

  • buf_sz: Size of the underlying buffer in bytes
  • chunk_sz: Size of each chunk in bytes (rounded up to chunk_align; must fit a void *)
  • chunk_align: Alignment of the chunks. LF_DEFAULT_ALIGNMENT is a good default for basic types
int pool_init(PoolAllocator *allocator, size_t buf_sz, size_t chunk_sz, size_t chunk_align);

/* Usage */
PoolAllocator pool;
pool_init(&pool, 64, 16, LF_DEFAULT_ALIGNMENT);

pool_free

Return a single chunk back to the pool. ptr is a pointer to the allocated chunk. Pointers outside the pool, misaligned pointers, NULL, and a chunk already on the free list are ignored.

void pool_free(PoolAllocator *allocator, void *ptr);

pool_free_all

Returns all chunks back to the pool. Any pointers received before this call are now invalid and must be reassigned with pool_alloc or set to NULL.

void pool_free_all(PoolAllocator *allocator);

pool_alloc

Allocate a chunk from the pool. Returns NULL on failure. The chunk is zeroed.

void *pool_alloc(PoolAllocator *allocator);

pool_destroy

Frees the underlying buffer and zeros the handle. Does not free the allocator struct itself.

void pool_destroy(PoolAllocator *allocator);

Macros

pool_count_available

Returns the number of chunks left available in the pool.

#define pool_count_available(x) ((x)->free_count)

LF_DEFAULT_ALIGNMENT

The default alignment for allocators. Use this as a simple default (defaults to 16 bytes on amd64 systems) when storing basic types

#ifndef LF_DEFAULT_ALIGNMENT
#define LF_DEFAULT_ALIGNMENT (2*sizeof(void*))
#endif // LF_DEFAULT_ALIGNMENT

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