How the compare function calls the array element and what is the order of the calls

Question

I have a problem in qsort(arr, n, sizeof(int), compare) here how the compare calls its parameter. As I known its a void pointer and it calls the address of the array element and then it compares the value. But how it known the order of array element.

I also attached the whole code and function definition.

#include <stdio.h>
#include <stdlib.h>

int compare(const void * a, const void * b)
{
    return ( *(int*)a - *(int*)b );
}

int main ()
{
    int arr[] = {10, 5, 15, 12, 90, 80};
    int n = sizeof(arr)/sizeof(arr[0]), i;
    qsort (arr, n, sizeof(int), compare);
    for (i=0; i<n; i++)
        printf ("%d ", arr[i]);
    return 0;
}

Answer 1

something like this

void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void*)){

    char *ptr1, *ptr2;
    int j, i;

    for (j = 0 ; j < nitems ; j++){
        ptr1 = ((char*) base) + (j * size);
        for( i = 0 ; i < nitems ; i++){
           ptr2 = ((char*) base) + (i * size);
           if( compar(ptr1, ptr2)) {
               ...
           }
       }
    }
    ...
}