[ntp:questions] use ntpd, how to know unsync status on linuxsystem

赵颖科 zhaoyingke at sf-auto.com
Mon Aug 16 07:40:15 UTC 2010

>>         I`m using ntpd(ntp4.2.6p1) as client in linux(2.6.26). I have some questions about my user app how to know the ntp client sync status.
>>         1. I use ntp_gettime, is there other way?
>That depends on what you want to know about the state of ntpd.  You
>probably should want to know the error bound, in which case it is an
>appropriate method.  If you really want to know whether it is synched,
>you need to use the NTP protocol to either check the stratum (note that
>this may take a long time to go to 16, when the last server stops
>responding, or more detailed state variables.
           Thank you, David.
                I just forcus on the return value of ntp_getime, but don`t know the stratum. I think the stratum means the time quality, is right?

>>         2. If there is no server alive, when I start ntp client, why ntptime show :ntp_gettime() returns code 0, but not code 5?
>I believe there was a policy decision some time ago never to declare a
>failure to this interface.
>>          I trace the  function loop_config in the file ntpd/ntp_loopfilter.c :
>>         I think it should be :
>>                 ntv.maxerror = (u_int32)(MAXDISPERSE * 1e6);
>>                 ntv.esterror = (u_int32)(MAXDISPERSE * 1e6);
>I tend to agree.
>>         or not?
>>         3. If ntp server no answer for 1h or shortter time, ntpd can mark unsync stat? how could I config ntpd?
>Why is this important to you?  The time should still be very accurate at
>that point.

     My project is about receive sample value for the merge unit, by IEC61850-9-2. The merge unit (MU) only show the sample count in one second. eg: 0 -- 3999. So, my PC must get the SOC  and associated with sample value.
     When the ntpd, which run on my PC as client, cann`t receive the reply from ntp server, the SOC got from my PC clock will offset about 10s in 24h. So, I`m  apt to mark the unsync stat for the sample value so qickly.

        this is app map:  ntpd  --->  linux system  ---->  my app <---  MU

    So, I realy want to know the unsync, as fast as I can, by some API. By your answer, I`ll try to check the stratum, and use local server 127...    stratum 16. When remote server no reply received, can ntpd use the local server`s stratum?

        Maybe it help to  change the stratum so qickly as I wash?

     When I waited the help, I try to write a simple sntp client for my project. When I use ntp_adjtime to adjust the offset, I see ntptime show:

ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset -535.000 us, frequency -500.000 ppm, interval 1 s,
  maximum error 42516 us, estimated error 16 us,
  status 0x1 (PLL),
  time constant 2, precision 1.000 us, tolerance 500 ppm,

next to change :

ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 0.000 us, frequency -500.000 ppm, interval 1 s,
  maximum error 454516 us, estimated error 16 us,
  status 0x1 (PLL),
  time constant 2, precision 1.000 us, tolerance 500 ppm,

my function is :

static inline __attribute__((always_inline))
int sntp_adjtime(double offset)
    struct timex ntpx;

    ntpx.modes = 0;

    if ((ntpx.status&STA_UNSYNC) || (fabs(offset) >= 0.128)){
        ntpx.status = STA_PLL;
        ntpx.maxerror = MAXDISPERSE;
        ntpx.esterror = MAXDISPERSE;
        ntpx.offset = (uint32_t)(offset*1e6);

        printf("sntp adjtime %f\n", offset);

    return 0;

    I don`t know the particular about how to use the ntp_adjtime.

    I want to know how the ntpd adjust the local clock, or where can I get the doc about ntpd work logic?

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