mm_init(p);
  if (!mm_init_lock(p->lock)) {
    mm_destroy_shm(p);
    return NULL;
  }
  return p;
}

void mm_destroy(MM *mm) {
  if (mm != NULL) {
    mm_destroy_lock(mm->lock);
    mm_destroy_shm(mm);
  }
}

size_t mm_size(MM *mm) {
  if (mm != NULL) {
    return mm->size;
  }
  return 0;
}

size_t mm_sizeof(MM *mm, void *x) {
  mm_mem_head *p;
  size_t ret;

  if (mm == NULL || x == NULL || !mm_lock(mm)) {
    return 0;
  }
  p = PTR_TO_HEAD(x);
  ret = p->size;
  mm_unlock(mm);
  return ret;
}

size_t mm_available(MM *mm) {
  size_t available;

  if (mm != NULL && mm_lock(mm)) {
    available = mm->available;
    mm_unlock(mm);
    return available;
  }
  return 0;