c - realloc:释放对象的无效校验和

Question

我在使用 realloc 替换 malloc 时出错。

下面这段代码在我的电脑上运行正常。

int vector_grow(Vector* vec) {
  unsigned long newcap;
  int * newarr;

  if (0 == vec->cap) {
    vec->arr = (int*)malloc(START_CAPACITY * sizeof(*vec->arr));
    if (NULL == vec->arr)
      return -1;
    vec->cap = START_CAPACITY;
    return 0;
  }
  newarr = malloc (newcap * sizeof(*vec->arr));
  if (NULL == newarr)
    return -1;

  memcpy (newarr, vec->arr, vec->len * sizeof(*vec->arr));
  free (vec->arr);
  vec->arr = newarr;
  vec->cap = newcap;

  return 0;
}

我想把malloc改成realloc，但是报错了。

int vector_grow(Vector* vec) {
  unsigned long newcap;

  if (0 == vec->cap) {
    vec->arr = (int*)malloc(START_CAPACITY * sizeof(*vec->arr));
    if (NULL == vec->arr)
      return -1;
    vec->cap = START_CAPACITY;
    return 0;
  }
  newcap = 2 * vec->cap;
  if ((vec->arr = (int*)realloc(vec->arr, newcap * sizeof(int))) == NULL)
    return -1;
return 0;
}

它说

malloc: *** error for object 0x7fca64c02598: incorrect checksum for freed object - object was probably modified after being freed.

我不知道这两段代码之间有什么区别，如果您知道导致错误的原因，请告诉我!非常感谢!

Answer 1

更新代码中缺少 vec->cap = 的错误肯定会导致对 malloc() 的各种调用以及调用代码对数据的滥用。

int vector_grow(Vector* vec) {
  unsigned long newcap;

  if (0 == vec->cap) {
    ... // not important to show the bug
  }
  newcap = 2 * vec->cap;
  if ((vec->arr = (int*)realloc(vec->arr, newcap * sizeof(int))) == NULL)
    return -1;

  // Add missing update  
  vec->cap = newcap;

  return 0;
}

也更好地测试分配是否成功

  void *p = realloc(vec->arr, sizeof *(vec->arr) * newcap);
  if (p == NULL) {
    return -1;
  }

  vec->arr = p;
  vec->cap = newcap;