using qsort() to sort pointers to structs containing strings

Question

I\'m not sure if this is possible to do with qsort because what I want to sort (array of pointers to struct) is not what I am comparing (strings).

Here is an abridge

Answer 1

The other answers are correct in everything but one little detail. I've just hit this, so i'll leave it here in case someone else finds themselves struggling with an extremely pedantic compiler.

qsort() comparator accepts two const void * parameters. Which means that when you dereference them to get pointers to your actual structs, you have to keep that const-ness. So, if you were to follow every single rule of C language, your code would look like this:

int
cmp(const void *p0, const void *p1)
{
    const struct student* ps0 = *(const struct student* const*) p0;
    const struct student* ps1 = *(const struct student* const*) p1;

    return strcmp(ps0->lname, ps1->lname);
}

Note the "const pointer to const" construct - that's you telling the compiler that at no point your original pointers (p0 and p1) lose their const-ness - you first dereference them to a const * const *, and then dereference that to const *. If you simply use const **, that's a const * to * (non-const) pointer, which discards the original const-ness of const void *.