runner% cat sort_select.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define M 100
void inita(void);
void check_sorted(void);
void sort_select(int n);
int maxind(int);
void swap(double *, double *);
double a[M];

void main(void)
{
   int starttime, stoptime; /* elapsed time */
   int startclock, stopclock; /* CPU time */
   srand48((long)time(NULL));
   inita();
   starttime = time(NULL); startclock = clock();
   sort_select(M-1);
   stoptime = time(NULL); stopclock = clock();
   printf("Sorting %i numbers\n", M);
   printf("Elapsed time: %ld seconds\n",
       stoptime - starttime);
   printf("CPU time: %.3f seconds\n",
       (double)(stopclock - startclock)/1000000.0);
   check_sorted();
}

void sort_select(int n)
{
   int maxi;
   if (n > 0) {
      maxi = maxind(n);
      swap(&a[maxi], &a[n]);
      sort_select(n-1);
   }
}

int maxind(int m)
{
   int maxi;
   if (m == 0) return m;
   maxi = maxind(m-1);
   if (a[m] > a[maxi]) return m;
   return maxi;
}
void swap(double *x, double *y)
{
   double z = *x;
   *x = *y;
   *y = z;
}

void inita(void)
{
   int i;
   for (i = 0; i < M; i++)
      a[i] = drand48();
}

void check_sorted(void)
{
   int sorted = 1;
   int i;
   for (i = 0; i < M-1; i++)
      if (a[i] > a[i+1]) sorted = 0;
   if(sorted) printf("Array sorted\n");
   else printf("*** Array NOT sorted\n");
}

runner% sort_select
Sorting 100 numbers
Elapsed time: 0 seconds
CPU time: 0.010 seconds
Array sorted 
runner% sort_select
Sorting 500 numbers
Elapsed time: 0 seconds
CPU time: 0.340 seconds
Array sorted 
runner% sort_select
Sorting 1000 numbers
Elapsed time: 2 seconds
CPU time: 1.520 seconds
Array sorted 
runner% sort_select
Sorting 5000 numbers
Elapsed time: 41 seconds
CPU time: 40.560 seconds
Array sorted 
runner% sort_select
Sorting 10000 numbers
Elapsed time: 167 seconds
CPU time: 165.650 seconds
Array sorted
runner% sort_select
Sorting 20000 numbers
Elapsed time: 889 seconds
CPU time: 868.100 seconds
Array sorted