Sanity-checking CLOCK_MONOTONIC (v1).

gistfile1.c

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

// compile with:
// $ gcc -Wall -o time_loop time_loop.c -lrt
// make sure -lrt comes after specifying the .c file or it won't link

int rv;
struct timespec current;
struct timespec last;

void print_ts() {
    //printf("got: %lu.%lu\n", current.tv_sec, current.tv_nsec);
    if (current.tv_nsec < last.tv_nsec) {
        printf("current: %lu.%lu\n", current.tv_sec, current.tv_nsec);
        printf("last: %lu.%lu\n", last.tv_sec, last.tv_nsec);
        printf("tv_nsec went backwards");
        if (current.tv_sec > last.tv_sec) {
            printf("... but tv_sec went forwards so it's okay.");
        }
        printf("\n");
    }
    if (current.tv_sec < last.tv_sec) {
        printf("tv_sec went backwards\n");
    }
}

int main(int argc, char** argv) {
    printf("Hello, world!\n");
    while (1) {
        rv = clock_gettime(CLOCK_MONOTONIC, &current);
        if (rv < 0) {
            printf("clock_gettime failed!\n");
        }
        print_ts();
        last = current;
    }
    return 0;
}