[spc476]

And he didn't get all the details right.

    realloc(valid_pointer,0);

IS defined (in C89 no less) to act like free(). It is NOT operating system dependent.

[pfacka]

True, but return type of free() is void, thus it says nothing about return value. The POSIX says: "If size is 0, either a null pointer or a unique pointer that can be successfully passed to free() shall be returned."