Freeing allocated memory: realloc() vs. free()

Question

so I have a piece of memory allocated with malloc() and changed later with realloc().

At some point in my code I want to empty it, by this

Answer 1

I don't think you mean "empty"; that would mean "set it to some particular value that I consider to be empty" (often all bits zero). You mean free, or de-allocate.

The manual page says:

If ptr is NULL, then the call is equivalent to malloc(size), for all values of size; if size is equal to zero, and ptr is not NULL, then the call is equivalent to free(ptr).

Traditionally you could use realloc(ptr, 0); as a synonym for free(ptr);, just as you can use realloc(NULL, size); as a synonym for malloc(size);. I wouldn't recommend it though, it's a bit confusing and not the way people expect it to be used.

However, nowadays in modern C the definition has changed: now realloc(ptr, 0); will free the old memory, but it's not well-defined what will be done next: it's implementation-defined.

So: don't do this: use free() to de-allocate memory, and let realloc() be used only for changing the size to something non-zero.