How do I check whether my system's clock is synchronized to a NTP server?

I have an application on a Linux system (Ubuntu Server) that needs to know whether the current system clock has been synchronized to a NTP server. While I could check timedatectl's output for System clock synchronized: yes, this seems very brittle, especially since timedatectl's human readable output might change in the future.

However, systemd seems to be full of DBus interfaces, so I suspect that there might be a way to check there. Either way, I'm looking for a bool is_ntp_synchronized().

Is there any way to simply check whether the system clock is synchronized without starting another process?

Solution

Linux provides adjtimex, which also gets used by systemd. You can check various fields to determine if you're still synchronized. A non-negative return value unequal to TIME_ERROR might be your forte, although you might use the maxerror or other fields to check the clock's quality instead.

#include <stdio.h>
#include <sys/timex.h>

int main()
{
    struct timex timex_info = {};
    timex_info.modes = 0;         /* explicitly don't adjust any time parameters */

    int ntp_result = ntp_adjtime(&timex_info);

    printf("Max       error: %9ld (us)\n", timex_info.maxerror);
    printf("Estimated error: %9ld (us)\n", timex_info.esterror);
    printf("Clock precision: %9ld (us)\n", timex_info.precision);
    printf("Jitter:          %9ld (%s)\n", timex_info.jitter,
           (timex_info.status & STA_NANO) ? "ns" : "us");
    printf("Synchronized:    %9s\n", 
           (ntp_result >= 0 && ntp_result != TIME_ERROR) ? "yes" : "no");
    return 0;
}

Note that systemd explicitly ignores the reported result (except for errors) and instead checks that the timex_info.maxerror value hasn't become more than 16 seconds.

This interface has also been provided since the pre-git times. As such, it's guaranteed to be stable, as it might otherwise break Linux's don't-break-userspace-policy.