qsort not working properly with long long

Question

I am sorting a two dimensional array a[n][2], with respect to a[i][0],a[i+1][0] breaking ties with non-decreasing a[i][1],a[i+1][1]. qsort is working fine with integer array but not with long long array.

Integer array code

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <limits.h>

int cmpfunc(const void* a, const void* b)
{
    int x = ((int*)a)[0] - ((int*)b)[0];
    if (x != 0) {
        return x;
    }
    return ((int*)a)[1] - ((int*)b)[1];
}

int main(int argc, char const* argv[])
{
    int n, i, j;
    scanf("%d", &n);
    int a[n][2];
    for (i = 0; i < n; i = i + 1) {
        scanf("%d %d", &a[i][0], &a[i][1]);
    }
    qsort(a, n, sizeof(a[0]), cmpfunc);
    for (i = 0; i < n; i = i + 1) {
        printf("%d %d\n", a[i][0], a[i][1]);
    }
    return 0;
}

long long array code

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <limits.h>

int cmpfunc(const void* a, const void* b)
{
    int x = ((int*)a)[0] - ((int*)b)[0];
    if (x != 0) {
        return x;
    }
    return ((int*)a)[1] - ((int*)b)[1];
}

int main(int argc, char const* argv[])
{
    int n, i, j;
    scanf("%d", &n);
    long long a[n][2];
    for (i = 0; i < n; i = i + 1) {
        scanf("%I64d %I64d", &a[i][0], &a[i][1]);
    }
    qsort(a, n, sizeof(a[0]), cmpfunc);
    for (i = 0; i < n; i = i + 1) {
        printf("%I64d %I64d\n", a[i][0], a[i][1]);
    }
    return 0;
}

Input:

5
4 3
4 2
4 1
4 1
4 1

Output for first code:

4 1
4 1
4 1
4 2
4 3

Output for second code:

4 2
4 1
4 1
4 1
4 3

Answer 1

You actually have two issues: The first is the one with the invalid casting. The second is also about the invalid casting but for another reason.

As stated in one of my comments the qsort function passes pointers to the elements to the comparison function. If you haven an array arr then qsort will use something like &arr[0] for the arguments. That means if the data of the array is itself arrays then the arguments will be pointers to arrays. In your specific case the argument types are really long long (*)[2], not only long long *.

So the comparison function should look something like this instead:

int cmpfunc(const void* a_, const void* b_)
{
    long long (*a)[2] = (long long (*)[2]) a_;
    long long (*b)[2] = (long long (*)[2]) b_;

    long long result;

    if ((*a)[0] - (*b)[0] != 0)
        result = (*a)[0] - (*b)[0];
    else
        result = (*a)[1] - (*b)[1];

    if (result < 0)
        return -1;
    else if (result > 0)
        return 1;
    else
        return 0;
}