monotonic clock going backwards?

[David Hill]

Hello -

I noticed some negative roundtrip times when running traceroute, which 
uses the monotonic clock to calculate the RTT.

If I run the following code, it eventually bombs.  It bombs quick if I 
launch Firefox.

timespeccmp failed
tp1 s:103780 n:63101042
tp2 s:103779 n:761117849


----


#include <sys/time.h>
#include <time.h>
#include <stdlib.h>
#include <stdio.h>

int
main(void)
{
        int r;
        struct timespec tp1, tp2, tout;

        tout.tv_sec = 0;
        tout.tv_nsec = 100000;

        for (;;) {
                r = clock_gettime(CLOCK_MONOTONIC, &tp1);
                if (r == -1) {
                        perror("clock_gettime");
                        exit(-1);
                }

                nanosleep(&tout, NULL);

                r = clock_gettime(CLOCK_MONOTONIC, &tp2);
                if (r == -1) {
                        perror("clock_gettime");
                        exit(-1);
                }

                // tp1 should never be larger than tp2
                r = timespeccmp(&tp1, &tp2, >);
                if (r == 1) {
                        printf("timespeccmp failed\n");
                        printf("tp1 s:%lld n:%ld\n", tp1.tv_sec, 
tp1.tv_nsec);
                        printf("tp2 s:%lld n:%ld\n", tp2.tv_sec, 
tp2.tv_nsec);
                        exit(-1);
                }
        }

        return 0;
}