Simple arena allocation

arena.c

#include <stdint.h>
#include <stdlib.h>

#include "arena.h"

struct block {
    union {
        struct block *prev;
        max_align_t   nope;
    };
    char              data[];
};

void
arena_create (struct arena *arena, size_t size)
{
    arena->size = size;
    arena->used = size;
    arena->curr = NULL;
}

void *
arena_allocate (struct arena *arena, size_t size, size_t align)
{
    /*  Preconditions:
     *   - "align" is a power of 2
     *   - "size" is a multiple of "align"
     *   - "size" is not greater than "arena->size"
     */
    
    size_t curr = (arena->used + align - 1) & -align;
    size_t next = curr + size;
    
    /*  If the current "struct block" does not have enough remaining space to
     *  store the requested amount of memory, allocate a new "struct block".
     */
    
    if (next > arena->size) {
        struct block *block = malloc (sizeof (struct block) + arena->size);
        block->prev = arena->curr;
        arena->curr = block;
        
        curr = 0;
        next = size;
    }
    
    arena->used = next;
    return ((struct block *) arena->curr)->data + curr;
}

void
arena_destroy (struct arena *arena)
{
    struct block *curr = arena->curr;
    while (curr) {
        struct block *prev = curr->prev;
        free(curr);
        curr = prev;
    }
}

arena.h

#ifndef PYON_ARENA_H
#define PYON_ARENA_H

#include <stddef.h>

struct arena {
    size_t  size;
    size_t  used;
    void   *curr;
};

void
arena_create (struct arena *arena, size_t size);

void *
arena_allocate (struct arena *arena, size_t size, size_t align);

void
arena_destroy (struct arena *arena);

#endif