calc_interval
double calc_interval(struct timeval *, struct timeval *);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);
sec = calc_interval(&cur_time, &last_time);