Qsort not working the way I expect it to with void pointers

Question

So I am trying to use qsort to sort a generic array list of void pointers in C.

When I actually use the array list I am storing Queue pointers.

I am pretty sure I am calling qsort correctly

qsort(al->list, al->size, sizeof(void *), al->cf );

In my comparator function, I want to do this

int compareQCtgry(const void * queueA, const void * queueB)
{
    Queue * q1 = (Queue *) queueA;
    Queue * q2 = (Queue *) queueB;
    return strcmp(q1->category, q2->category);
}

However, I get a segfault.

But when I change it to

int compareQCtgry(const void * queueA, const void * queueB)
{
    Queue * q1 = *(Queue **) queueA;
    Queue * q2 = *(Queue **) queueB;
    return strcmp(q1->category, q2->category);
}

It works, yet it makes no sense to me. Why cast the void pointer to a Queue ** and then dereference it when it seems like you should just be able to cast it with Queue *?

Answer 1

qsort passes pointers to the elements of the list to the comparison routine as it sorts. Since your list is already a list of pointers-to-Queue, pointers to those are pointers-to-pointers-to-Queue.