How to detect change of system time in Linux?

Question

Is there a way to get notified when there is update to the system time from a time-server or due to DST change? I am after an API/system call or equivalent.

It is part of my effort to optimise generating a value for something similar to SQL NOW() to an hour granularity, without using SQL.

Answer 1

You can use timerfd_create(2) to create a timer, then mark it with the TFD_TIMER_CANCEL_ON_SET option when setting it. Set it for an implausible time in the future and then block on it (with poll/select etc.) - if the system time changes then the timer will be cancelled, which you can detect.

(this is how systemd does it)

e.g.:

#include <sys/timerfd.h>
#include <limits.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>

int main(void) {
        int fd = timerfd_create(CLOCK_REALTIME, 0);
        timerfd_settime(fd, TFD_TIMER_ABSTIME | TFD_TIMER_CANCEL_ON_SET,
                        &(struct itimerspec){ .it_value = { .tv_sec = INT_MAX } },
                        NULL);
        printf("Waiting\n");
        char buffer[10];
        if (-1 == read(fd, &buffer, 10)) {
                if (errno == ECANCELED)
                        printf("Timer cancelled - system clock changed\n");
                else
                        perror("error");
        }
        close(fd);
        return 0;
}