malloc wrapper that tracks size of allocated data

qmalloc.c

#define find_chunk(ptr) (struct mchunk*)((char*)(ptr) - offsetof(struct mchunk, data))

struct mchunk {
  size_t sz;
  char   data[];
};

void *qmalloc(size_t sz) {
  struct mchunk *chunk = malloc(sizeof *chunk + sz);
  
  if(chunk == NULL)
    return NULL;
  
  chunk->sz = sz;
  return (void*)chunk->data;
}

void *qcalloc(size_t nmemb, size_t sz) {
  void *ptr = qmalloc(nmemb * sz);
  
  if(ptr == NULL)
    return NULL;
  
  return memset(ptr, '\0', nmemb * sz);
}

void *qrealloc(void *ptr, size_t sz) {
  struct mchunk *chunk;
  
  if(ptr == NULL)
    return qmalloc(sz);
  
  chunk = find_chunk(ptr);
  chunk = realloc(chunk, sizeof *chunk + sz);
  
  if(chunk == NULL)
    return NULL;
  
  chunk->sz = sz;
  return chunk->data;
}

size_t qsize(void *ptr) {
  struct mchunk *chunk = find_chunk(ptr);
  return chunk->sz;
}

void qfree(void *ptr) {
  struct mchunk *chunk;
  
  if(ptr == NULL)
    return;
  
  chunk = find_chunk(ptr);
  free(chunk);
}