A loop that executes an action a certain number of times per second.

fixed_frequency_loop.c

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

/* How many times per second do we want the code to run? */
const int frequency = 2;

/* Pretend to do something useful. */
void do_work() {
    volatile int i;
    for (i = 0; i < 10000000; ++i)
        ;
}

int main(int argc, char *argv[]) {
    /* How long should each work unit take? */
    long slice = (long)(1.0 / frequency * 1000000);

    struct timeval beginning;
    gettimeofday(&beginning, NULL);
    struct timeval last_tick = beginning;

    long total = 0;
    int tick;
    for (tick = 1; 1; ++tick) {
        do_work();

        struct timeval now;
        gettimeofday(&now, NULL);

        /* How much time has passed since the last tick? */
        long usec_elapsed = (now.tv_sec - last_tick.tv_sec) * 1000000
                          + (now.tv_usec - last_tick.tv_usec);
        last_tick = now;

        /* How time did we spend working this tick? */
        long usec_work = (now.tv_sec - beginning.tv_sec) * 1000000
                          + (now.tv_usec - beginning.tv_usec);

        total += usec_elapsed;
        printf("Worked for %ldus. Average time per tick: %ldus (%ldus since last).\n",
               usec_work, total / tick, usec_elapsed);

        /* Pause if appropriate. */
        long usec_tosleep = slice - usec_work;
        if (usec_tosleep > 0) {
            printf("Sleeping for %ldus.\n", usec_tosleep);
            usleep(usec_tosleep);
        }

        /* Prepare for the next tick. */
        gettimeofday(&beginning, NULL);
    }

    return 0;
}