Arenas and Pools

Arena (region) allocation

Allocate a big block; hand out consecutive slices; free them all at once by discarding the block. An arena is a bump pointer plus a capacity:

void *arena_alloc(arena_t *a, size_t n) {
    if (a->used + n > a->cap) return NULL;
    void *p = a->mem + a->used;
    a->used += n;
    return p;
}

No per-object free, no fragmentation, near-zero overhead per allocation. The discipline: every object in the arena must share one lifetime. Compilers arena per-function IR; servers arena per-request.

Pool allocation

Same block size everywhere: keep a free list of slots. get() pops a slot; put() pushes it back. O(1) with zero searching, and no size-class machinery at all.

realloc growth policies

Growing a buffer by doubling gives amortized O(1) per element; growing by +1 gives O(n^2) total copies. The policy lives in your code — realloc just moves bytes when told.

size_t newcap = cap ? cap * 2 : 1;