indexes of sorted array

#include <stdlib.h>

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

int main()
{
    const int N = 4;
    int i;
    int input[] = { 4, 3, 2, 1 };

    int aux[2*N];
    for(i = 0; i < N; ++i) { aux[2*i] = input[i]; aux[2*i+1] = i; }
    qsort(aux, N, sizeof(int)*2, cmp);
    for(i = 0; i < N; ++i) { printf("%d ", aux[2*i + 1]); }
    return 0;
}