Check the actual granularity of a clock, specifically CLOCK_MONOTONIC_COARSE.

clock-gran.c

#include <time.h>
#include <stdio.h>

#define CLOCK CLOCK_MONOTONIC_COARSE

int main() {
    struct timespec t;
    clock_getres(CLOCK, &t);
    fprintf(stderr, "Res: %lu %06lu\n", t.tv_sec, t.tv_nsec);

    long last = 0;
    long nsecs[10];
    for (int i = 0; i < 10; ++i) {
        do
            clock_gettime(CLOCK, &t);
        while (t.tv_nsec == last);
        nsecs[i] = t.tv_nsec;
        last = t.tv_nsec;
    }
    for (int i = 0; i < 10; ++i) {
        fprintf(stderr, "%9lu\n", nsecs[i]);
    }
    return 0;
}