Indices after sorting in C

isort.h

/* Index sort. A sort function which returns indices as well as the
 * sorted values. Tested with gcc only.  T. Giorgino 2014
 */

#include <stdlib.h>
#ifndef _ISORT
#define _ISORT

const int descend = 1;

/* Somebody left this out of stdlib */
struct sort_pair { 
     double x;
     size_t i;
};

int sort_cmp (const struct sort_pair *a, const struct sort_pair *b)
{
     int r= (a->x > b->x) - (a->x < b->x);
     if(descend) 
      return -r;
     else 
      return r;
}

/* Return sorted values AND indices. 
   Inputs:  double *in - input values
            size_t N   - length of the input
   Outputs: double *out - sorted values
            size_t *idx - indices after sorting
   Caller is responsible for allocating memory
*/
void isort(const double *in, size_t N, 
       double *out, size_t *idx) {
     struct sort_pair *tosort=malloc(sizeof(struct sort_pair)*N);

     for(size_t i=0; i<N; i++) {
      tosort[i].x=in[i];
      tosort[i].i=i;
     }
     qsort(tosort,N,sizeof(struct sort_pair), (__compar_fn_t) sort_cmp);
     for(size_t i=0; i<N; i++) {
      out[i]=tosort[i].x;
      idx[i]=tosort[i].i;
     }
     free(tosort);
}

#endif