Using memory that has already been free()-d invokes undefined behavior. Don't do that.
Quoting C11, annex §J.2, Undefined behavior
The value of a pointer that refers to space deallocated by a call to the free or
realloc function is used.
To elaborate, calling free() on a pointer does not set the pointer to NULL or anything else. It just marks the memory as re-usable by the memory manager. That memory location (pointer) is not valid anymore for the program, so you should not be using it.
As a preventive measure to the above problem, it's considered a good coding practice to set the pointer to NULL explicitly after calling free(). However, FWIW, this is not mandatory.
Quoting C11, chapter 7.22.3.3
The free function causes the space pointed to by ptr to be deallocated, that is, made
available for further allocation. [...]
Also, just to add, partial free() is not possible by calling free(), anyway. You can refer to this previous answer for clarification.
