I can't see that working out the way they expect.

Well, on one hand, writing correct albeit slow memcmp() is easy, on the other hand, it has some gotchas...

    int my_memcmp(const void * ptr1, const void * ptr2, size_t num) {
        const unsigned char * p1 = ptr1;
        const unsigned char * p2 = ptr2;
        for (size_t i = 0; i < num; i++) {
            if p1[i] < p2[i] {
                return -1;
            }
            if p1[i] > p2[i] {
                return 1;
            }
        }
        return 0;
    }

For example, technically speaking, unsigned char can be as wide as an int, so "p1[i] - p2[i]" may actually evaluate to unsigned int which is not what you want.

Nitpick: those if statements need parentheses.

> unsigned char can be as wide as an int, so "p1[i] - p2[i]" may actually evaluate to unsigned int which is not what you want

Would this matter?

I don't think it would; `<` and `>` aren't syntactic sugar in C.

int is guaranteed to be at least as wide as char.

This exactly what I was saying, just made a wrong choice of words.

"Int can be as wide as char" is exactly the same as "char can be as wide as int".

I meant "int can be as narrow as char".