Why is the time reported by time() sometimes 1 second behind the seconds component of timespec_get() in C code?

Question

The following snippet of code:

struct timespec ts;
for (int x = 0; x < 100000000; x++) {
    timespec_get(&ts, TIME_UTC);
    long cTime = (long) time(NULL);
    if (cTime != ts.tv_sec && ts.tv_nsec < 3000000) {
        printf("cTime: %ld\n", cTime);
        printf("ts.tv_sec: %ld\n", ts.tv_sec);
        printf("ts.tv_nsec: %ld\n", ts.tv_nsec);
    }
}

produces this output:

...
cTime: 1579268059
ts.tv_sec: 1579268060
ts.tv_nsec: 2527419
cTime: 1579268059
ts.tv_sec: 1579268060
ts.tv_nsec: 2534036
cTime: 1579268059
ts.tv_sec: 1579268060
ts.tv_nsec: 2540359
cTime: 1579268059
ts.tv_sec: 1579268060
ts.tv_nsec: 2547039
...

Why the discrepancy between cTime and ts.tv_sec? Note that the problem does not occur if the conditional is changed to ts.tv_nsec >= 3000000. The problem relies on nanoseconds being smaller than 3000000.

Answer 1

The reason is, that you (implicitly) use different system clocks. timespec_get() uses the high resolution system-wide realtime clock, while time() uses the coarse realtime clock.

Try to use

clock_gettime(CLOCK_REALTIME_COARSE, &ts);

instead of your timespec_get(), then the difference should vanish.

Edit:

This can be seen in the Linux Kernel Source, vclock_gettime.c

Indeed the issue is a bit subtle to see here. The seconds-part of the structure members used by CLOCK_REALTIME_COARSE and CLOCK_REALTIME contain identical values, but the nanoseconds-part is different; with CLOCK_REALTIME it can be larger than 1000000000 (which is one second). In this case, it is fixed up on the call:

ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
ts->tv_nsec = ns;

This correction is neither performed with CLOCK_REALTIME_COARSE, nor with time(). This explains the difference between CLOCK_REALTIME and time().