Rule:  For every call to malloc(…) you must have one call to free(…).

Rule:  Do not free(…) the same buffer more than once.

malloc(…) returns the address of a buffer on the heap.

free(…) takes the address of a buffer on the heap.
free(…) takes the address that malloc(…) returned.

Rule:  Do not access memory in the heap buffer after it has been freed.