Writing a Small Allocator

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typedef struct { char *base, *cur, *end; } Arena;

void *arena_alloc(Arena *a, size_t n, size_t align) {
    uintptr_t p = (uintptr_t)a->cur;
    p = (p + align - 1) & ~(uintptr_t)(align - 1);   // round up
    if ((char*)p + n > a->end) return NULL;
    a->cur = (char*)p + n;
    return (void*)p;
}

void arena_reset(Arena *a) { a->cur = a->base; }

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typedef struct Block { struct Block *next; } Block;

void *pool_alloc(Block **free_list) {
    Block *b = *free_list;
    if (b) *free_list = b->next;
    return b;
}

void pool_free(Block **free_list, void *p) {
    Block *b = p; b->next = *free_list; *free_list = b;
}

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#include <stdalign.h>
size_t align = alignof(max_align_t);   // usually 16 on x86-64