tv_diff
struct timeval tv_start, tv_end, tv_diff;
timersub(&tv_end, &tv_start, &tv_diff);
printf("time = %.6f\n", tv_diff.tv_sec + (tv_diff.tv_usec) * 1e-6);
d1 = tv_diff(tv_other, tv_sys1);
d2 = tv_diff(tv_sys2, tv_other);