[ntp:questions] NTP shared memory driver

Martin Burnicki martin.burnicki at meinberg.de
Mon Sep 15 12:58:10 UTC 2014

Claudio Persico wrote:
> Thanks a lot for the explanation.
> I've just modified my test application. Now it works till 10000 seconds of difference (4 hours is too much even for the first big time jump allowed by -g option).

As far as I know there is no limit for the -g option *except* a possible 
range overflow of the 32 bit number of seconds in NTP time stamps.

The 4 hours limit is only a limitation of the SHM refclock driver, not 
of ntpd in general.

> My question now is: since my system is a battery powered one, and the batteries are very often removed so the time always starts at the epoch time (1970) or something very old, is there a way to make NTP work with a so big "time jump"?

ntpdate or ntpd -g will only help if you can get the initial time via 

If you have *only* the SHM refclock then this will bite you if the 
initial system time is off by more than 4 hours.

If the clock you have available to feed the SHM driver provides the full 
date and time the software which feeds the SHM segment could just set 
the system date/time at startup, *before* ntpd is started.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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