What happens when freeing a pointer which has already been freed?

The allocator will set a special double$-$free bit in the status buffer which can be checked by the program

The pointer may or may not have been re$-$initialized, but that doesn't matter, it'll still try to free it The pointer may have been re$-$initialized, in which case it dogs nothing since it wouldn't want to remove memory in use Nothing, C keeps track of which pointers have been freed already