get difference between timespecs

timespecs.cpp

/**
 * Subtract timespec `b` from `a`. timespec `a` must be greater than `b`.
 * @param res Where we put the result
 * @param a timespec pointer for end time
 * @param b timespec pointer for start time
 */
void get_difference(timespec *res, timespec *a, timespec *b) {
  if (a->tv_nsec < b->tv_nsec) {
    res->tv_sec = a->tv_sec - b->tv_sec - 1;
    res->tv_nsec = a->tv_nsec - b->tv_nsec + 1000000000;
  } else {
    res->tv_sec = a->tv_sec - b->tv_sec;
    res->tv_nsec = a->tv_nsec - b->tv_nsec;
  }
}

/**
 * Convert a timespec into a long represending the number of milliseconds in
 * that interval.
 * @param interval
 * @return The number of milliseconds in the interval that the timespec
 * represents, as a double.
 */
long timespec_to_ms(timespec *interval) {
  return (long)interval->tv_sec * 1000 + (long)interval->tv_nsec / 1e6;
}