// determine actual wall clock time
double elapsed_time = (end_sort_time.tv_sec - start_sort_time.tv_sec);
elapsed_time += (end_sort_time.tv_nsec - start_sort_time.tv_nsec) / 1e9;
// determine actual wall clock time
double elapsed_time = (end_sort_time.tv_sec - start_sort_time.tv_sec);
elapsed_time += (end_sort_time.tv_nsec - start_sort_time.tv_nsec) / 1e9;