memory leak in malloc?

Question

I've got the following faulty piece of code in C and I am wondering if there will be a memory leak or if there will be a pointer which is pointing to a free memory location.

int* p = (int*) malloc(sizeof(int));
p = NULL;
free(p);

Answer 1

Yes it will leak memory. You assign p to NULL before freeing the contents it's pointing to. One quick change will fix it:

int* p = malloc(sizeof(int));
free(p);
p = NULL;

The difference here is we give free the address allocated by malloc before setting p to NULL. In general setting a pointer to NULL will not free the contents, but will allow you to check if the pointer is valid or not which can have a lot of practical applications.

Answer 2

You will have a memory leak.

After you assign NULL to p, you no longer have a way to refer to the memory you allocated with malloc.

Your call to free will try to free NULL, doing nothing.

The following will correctly free the memory:

int *p = malloc(sizeof(int));
free(p);
p = NULL;

Note that you don't need to set p to NULL after freeing, you only really need the first two lines.