clock_getres() problem

["aaron.gage" <aaron dot gage at oceanoptics dot com>]

Greetings.  I am trying to figure out a discrepancy between what is returned
by clock_getres() and repeated calls to clock_gettime().  I was directed
here from the glibc-bugs list, so hopefully this is the appropriate place to
ask.

On a Fedora Core 3 machine (Intel Celeron) with glibc package
glibc-2.3.5-0.fc3.1, I have run this code:

struct timespec ts;
clock_getres(CLOCK_REALTIME, &ts);
printf("Clock resolution: %d.%09d\n", ts.tv_sec, ts.tv_nsec);

The result is "Clock resolution: 0.000999848" or approximately 1
millisecond.

However, if I do the following:

   long lastns = 0;
   int i = 0;
   while(i < 50) {
      clock_gettime(CLOCK_REALTIME, &ts);
      ns = ts.tv_nsec;
      if(ns != lastns) {
            printf("%d ", ns - lastns);
            lastns = ns;
            i++;
      }
   }

then I get back values ranging from 3000-5000 nanoseconds (3-5
microseconds).

 

That is, clock_getres() is reporting 1ms resolution, but the clock is really
able to provide 3-5us resolution.  Why doesn't clock_getres() report this?

Is there any way to get these to agree?

Thanks.

--Aaron