C語言裡計算程式的執行時間

#include <stdio.h>
#include <time.h>
int main(void)
{
      time_t start_tm, finish_tm;   time(&start_tm);

/* Do something here */
  time(&finish_tm);   double elapsed_tm=difftime(finish_tm,start_tm);
  printf("nnn for %5.3f seconds.n", elapsed_tm);  return 0;

}