def test_clock_settime(self):
        t = time.clock_gettime(time.CLOCK_REALTIME)
        try:
            time.clock_settime(time.CLOCK_REALTIME, t)
        except PermissionError:
            pass

        if hasattr(time, "CLOCK_MONOTONIC"):
            self.assertRaises(OSError, time.clock_settime, time.CLOCK_MONOTONIC, 0)

 def test_monotonic_settime(self):
     t1 = time.monotonic()
     realtime = time.clock_gettime(time.CLOCK_REALTIME)
     # jump backward with an offset of 1 hour
     try:
         time.clock_settime(time.CLOCK_REALTIME, realtime - 3600)
     except PermissionError as err:
         self.skipTest(err)
     t2 = time.monotonic()
     time.clock_settime(time.CLOCK_REALTIME, realtime)
     # monotonic must not be affected by system clock updates
     self.assertGreaterEqual(t2, t1)

def main(argv):
    """
    Entry point for ntpclient.py.

    Arguments:
        argv: command line arguments
    """
    res = None
    quiet = False
    args = set(argv)
    if {'-q', '--quiet'}.intersection(args):
        quiet = True
    if '-s' in argv:
        server = argv[argv.index('-s')+1]
    elif '--server' in argv:
        server = argv[argv.index('--server')+1]
    elif 'NTPSERVER' in os.environ:
        server = os.environ['NTPSERVER']
    else:
        server = 'pool.ntp.org'
    if {'-h', '--help'}.intersection(args):
        print('Usage: ntpclient [-h|--help] [-q|--quiet] [-s|--server timeserver]')
        print(f'Using time server {server}.')
        sys.exit(0)
    t1 = time.clock_gettime(time.CLOCK_REALTIME)
    ntptime = get_ntp_time(server)
    t4 = time.clock_gettime(time.CLOCK_REALTIME)
    # It is not guaranteed that the NTP time is *exactly* in the middle of both
    # local times. But it is a reasonable simplification.
    roundtrip = round(t4 - t1, 4)
    localtime = (t1 + t4) / 2
    diff = localtime - ntptime
    if os.geteuid() == 0:
        time.clock_settime(time.CLOCK_REALTIME, ntptime)
        res = 'Time set to NTP time.'
    localtime = datetime.fromtimestamp(localtime)
    ntptime = datetime.fromtimestamp(ntptime)
    if not quiet:
        print('Using server {}.'.format(server))
        print('NTP call took approximately', roundtrip, 's.')
        print('Local time value:', localtime.strftime('%a %b %d %H:%M:%S.%f %Y.'))
        print('NTP time value:', ntptime.strftime('%a %b %d %H:%M:%S.%f %Y.'),
              '±', roundtrip/2, 's.')
        print('Local time - ntp time: {:.6f} s.'.format(diff))
        if res:
            print(res)