Измерить время выполнения программы

#include <stdio.h>
#include <math.h>
#include <time.h>
main ()
{ FILE *in;
  FILE *out;
  float x,y;
  clock_t start, end;
 
start = clock();
in=fopen("acc.txt","r");
out=fopen("out.txt", "w");
fscanf(in, "%f %f", &x, &y);
if ((x<=0||x>=1000000)||(y<=0||y>=1000000))
   {fprintf (out, "Nedopustimie znacheniya\n");
   end = clock();
   fprintf(out, "%.4lf\n", (end - start) / (CLOCKS_PER_SEC));
   }
else
   { 
   fprintf(out, "P vpisannogo kvadrata vpisannogo v okrujnost raven %.3f; S ravna %.3f; \nP opisanogo okolo okrujnosti kvadrata raven %.3f; S ravna %.3f\n", (sqrt(2)*4*x), (x*x*2), (8*x), (4*x*x)); 
   fprintf(out, "P orrujnosti vpisanoi v kvadrat raven %.3f; S ravna %.3f;\nP orrujnosti opisannoi okolo kvadrata ravna %.3f; S ravna %.3f\n", (3.14*y), (3.14*y*y/4), (3.14*y*sqrt(2)), (3.14*2*y*y));
   fprintf(out, "P ravnostoronnego treugolnika vpisannogo v okrujnost raven %.3f; S ravna %.3f;\nP ravnostoronnego treugolnika opisannogo okolo okrujnosi raven %.3f; S ravna %.3f\n", (0.87*x*6), (x*x*0.87*0.87*2*0.87), (6*sqrt(3)*x), (3*sqrt(3)*x*x));
   fprintf(out, "P okrujnosti vpisannoi v ravnostoronnii treugolnik %.3f; S ravna %.3f;\nP okrujnosti opisannoi okolo ravnostoronnego treugolnika %.3f; S ravna %.3f;\n",   (y*3.14*sqrt(3)/3.) ,(3.14*y*y/12),(2*sqrt(3)/3*3.14*y), (3.14*y/3));
   end = clock();
   printf("%lf\n",(end - start) / (CLOCKS_PER_SEC));
   }
}

#include  <time.h>
double difftime(time_t time2, time_t time1);

#include <time.h>
#include <stdio.h>
 
int main(void)
{
  time_t start,end;
  volatile long unsigned t;
 
  start = time(NULL);
  for(t=0; t<5000000; t++) ;
  end = time(NULL);
  printf("Цикл использовал %f секунд.\n", difftime(end, start));
 
  return 0;
}

else
   { 
   ...
   end = clock();
   printf("%lf\n",(end - start) / (CLOCKS_PER_SEC));
   }

fprintf(out, "%.4lf\n", (end - start) / (CLOCKS_PER_SEC));