[ntp:questions] NTP Shared Memory Driver

Damien Dusha d.dusha at gmail.com
Mon Jun 25 22:43:03 UTC 2012

Dear NTP list,

I would like to use the shared memory driver to push time from a GPS
receiver into NTP (Note 1) - host OS is Linux, using NTP 4.2.6p5.
According to documentation at
http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver28.html, I
need to populate the following structure in shared memory:

struct shmTime {
  int    mode; /* 0 - if valid set
                *       use values,
                *       clear valid
                * 1 - if valid set
                *       if count before and after read of
                *       values is equal,
                *         use values
                *       clear valid
  int    count;
  time_t clockTimeStampSec;      /* external clock */
  int    clockTimeStampUSec;     /* external clock */
  time_t receiveTimeStampSec;    /* internal clock, when external
value was received */
  int    receiveTimeStampUSec;   /* internal clock, when external
value was received */
  int    leap;
  int    precision;
  int    nsamples;
  int    valid;
  int    dummy[10];

Whilst "mode" is clearly documented in the designated HTML page, the
remaining fields are not and it's not obvious as to how they should be

For poking around ntpd and gpsd, I have made guesses that:
  * The timestamps are unix timestamps in UTC
  * "leap" is an enumeration 0 - 3, though I am not entirely sure of
its meaning, (3 is unknown leap seconds applied?)
  * "Precision" is 2^(precision) seconds estimated accuracy
  * "count" and "valid" are populated according to the "mode" selected

But this is little more than a stab in the dark. Is there a better
place where these fields are documented that I have missed?


(Note 1: In this project, we are processing our own GPS solution and
time based on raw observables and are hence are not using NMEA or
other GPS protocol. PPS is supplied to us via a GPIO line and we are
using the ATOM driver to capture this via linuxpps).

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