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

void* realloc (void* ptr, size_t size);

In C90 :

if size is zero, the memory previously allocated at ptr is deallocated as if a call to free was made, and a null pointer is returned.

In C99:

If size is zero, the return value depends on the particular library implementation: it may either be a null pointer or some other location that shall not be dereferenced.