Question

Following is the implementation of a stack struct in C. I need to write a function (not a method) to dispose off an existing stack and ofcourse free any memory allocated for it on the heap. the dispose (stack *s) function takes in a pointer to a stack that is to be disposed off.

typedef struct {
    int allocatedLength;
    int logicalLength;
    int elementSize;
    void *elems;
} stack;

If I implement the dispose() function as follows, will it lead to some memory never being freed?

void dispose (stack *s) {
    free (s->elems);
}

In the above implementation, I am only freeing the memory allocated to elems, which is not even on the heap. We need to remember that elems is a pointer to something. Shouldn’t we be freeing that something, instead of freeing the memory allocated to the pointer?

Answer 1

You misunderstand: free(p) does not free the memory for p, but the memory for *p — that is, the memory pointed to by p. Thus, you are correctly freeing the allocated memory to which you stored a pointer in s->elems, and all is well.

Feel free to blame everyone who talks about “freeing a pointer” for this misnomer, when really they should be saying “freeing the memory to which I have a pointer”.