end_us = (int64_t) end_t.tv_sec * 1000000 + end_t.tv_usec;
elapsed_us = end_us - start_us;
printf ("elapsed time: %d.%02d seconds\n",
end_us = (int64_t) end_t.tv_sec * 1000000 + end_t.tv_usec;
elapsed_us = end_us - start_us;
printf ("elapsed time: %d.%02d seconds\n",