darwin.c

uint64_t uv__hrtime(void) {
    mach_timebase_info_data_t info;

    if (mach_timebase_info(&info) != KERN_SUCCESS)
      abort();

    return mach_absolute_time() * info.numer / info.denom;
}

linux.c

uint64_t uv__hrtime(void) {
  struct timespec ts;
  clock_gettime(CLOCK_MONOTONIC, &ts);
  return (((uint64_t) ts.tv_sec) * NANOSEC + ts.tv_nsec);
}