Minus One Header

typedef struct { size_t size; unsigned magic; } header_t;

void *my_malloc(size_t n) {
    header_t *h = carve_chunk(n + sizeof(header_t));
    h->size  = n;
    h->magic = 0x1234567;
    return (void *)(h + 1);
}

void my_free(void *ptr) {
    header_t *h = /* ??? */;
    assert(h->magic == 0x1234567);
    return_to_free_list(h, h->size + sizeof(header_t));
}

sizeof(header_t) is 16 on this platform (8-byte size_t, 4-byte unsigned, 4 bytes of padding). What belongs at ????