c - malloc 和 free 的实现

Question

我对 malloc 后跟 free 有疑问。

char *p = (char *) malloc(100);
p++;
free(p);

在上面的场景中，free(p) 会释放多少字节的内存？是 99 还是 100？

Answer 1

这是未定义的行为。 free() 的联机帮助页说(强调我的):

The free() function frees the memory space pointed to by ptr, which must have been returned by a previous call to malloc(), calloc() or realloc(). Otherwise, or if free(ptr) has already been called before, undefined behavior occurs. If ptr is NULL, no operation is performed.