#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <sys/time.h>

/* calculate the time-difference between t1 and t2, assuming t2 comes no earlier
 * then t1.
 * stores the time difference in 'diff'.
 * returns 0 on success, -1 if t1 comes after t2.
 */
int time_diff(struct timeval *t1, struct timeval *t2, struct timeval *diff)
{
    /* sanity check. */
    if (t1->tv_sec > t2->tv_sec ||
        (t1->tv_sec == t2->tv_sec && t1->tv_usec > t2->tv_usec))
        return -1;

    if (t2->tv_sec == t1->tv_sec) {
        diff->tv_sec = 0;
        diff->tv_usec = t2->tv_usec - t2->tv_usec;
    }
    else {
        diff->tv_sec = t2->tv_sec - t1->tv_sec;
        if (t2->tv_usec >= t1->tv_usec)
            diff->tv_usec = t2->tv_usec - t1->tv_usec;
        else {
            diff->tv_usec = 1000000 + t2->tv_usec - t1->tv_usec;
            diff->tv_sec -= 1;
        }
    }

    return 0;
}

/* perform a busy-wait for a given number of seconds (3 in our example).
 * if there's an error (e.g. time went backwards), terminate immediately.
 */
int main()
{
    struct timeval start;
    struct timeval now;
    struct timeval diff;
    int delay = 3;

    if (gettimeofday(&start, NULL)) {
        fprintf(stderr, "Error getting start time - %s\n", strerror(errno));
        return 1;
    }

    printf("Busy-waiting %d seconds. time before: %lu:%lu\n",
           delay, start.tv_sec, start.tv_usec);

    /* perform the delay. */
    while (1) {
        /* get the time, calculate diff from start, terminate if it is time. */
        if (gettimeofday(&now, NULL)) {
            fprintf(stderr, "Error getting 'now' time - %s\n", strerror(errno));
            return 1;
        }
        if (time_diff(&start, &now, &diff)) {
            fprintf(stderr,
                    "Error - time just sliped backwards, now - %lu:%lu\n",
                    now.tv_sec, now.tv_usec);
            return 1;
        }
        if (diff.tv_sec >= delay)
            break;
    }

    printf("time after: %lu:%lu\n", now.tv_sec, now.tv_usec);
    printf("diff: %lu:%lu\n", diff.tv_sec, diff.tv_usec);

    return 0;
}