Question

I am in a weird situation and I need some direction with NTP time adjustments.

I have a PC (Red Hat) that runs NTP daemon and this PC adjusts its time with a Stratum 2 time server on my LAN.

My PC is also connected to a DVR over serial port (RS-232). This device and my PC time needs to in synchronization.

However after some time the clocks of my PC and DVR begin to drift away, so I need a way of detecting time adjustment on my PC and apply same adjustment to DVR as well.

Is there any way of doing this ?

I am hoping to find a way of subscribing to some kind of event at OS level for system clock changes on Red Hat. (If this is possible at all for RedHat)

It seems it is possible on Windows with SystemEvents.TimeChanged event but I could not find a counterpart on RedHat using C++.

Any help is appreciated.

Answer 1

Most of the time the NTP server does not make discrete adjustments to the system clock, it only slows it down or speeds it up using adjtime, in an effort to keep the drift rate under control. The NTP server might be doing this fairly continuously, and it will not tell you every time it makes an adjustment.

Even if the NTP server told you whenever it made adjustments, that information is not useful for you. Your DVR’s clock is driven by different hardware and therefore has a different drift rate and would require a different set of adjustments at different times. Ideally this would be done by an NTP daemon on the DVR!

The NTP daemon does under some circumstances cause a jump in the clock. It may happen at startup or if the clock gets way off, but this should be a very rare event. It probably emits a log message when it does this, so one possibility would be to watch the logs. Another possibility would be to compare the results from clock_gettime(CLOCK_REALTIME) and clock_gettime(CLOCK_MONOTONIC) from time to time. When you notice that the delta between these two clocks has changed, it must be because someone made the system time jump. But beware: the results will be jittery because an unpredictable and variable amount of time elapses from the moment you fetch one of the clocks until the moment you fetch the other one.

Depending on your needs, you might be able to achieve what you need by ignoring the system time and using only clock_gettime(CLOCK_MONOTONIC) for synchronizing with the DVR. That clock is guaranteed not to jump. But beware! (again?! haha!) I believe CLOCK_MONOTONIC may still slow down and speed up under the direction of the NTP daemon.