先看man 3 free的原话
The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().
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.
总结就是：
- malloc(0)和free(NULL)合法
- malloc的结果可以作为free的参数，但只能free一次
- malloc(0)可能返回非NULL指针，使用这个非NULL指针会有溢出问题
