alarm.c

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

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                       } while (0)

static void
sigalrmHandler(int sig) {
  printf("Signal: %d\n", sig);
}

int main(void) {
  long long freq_nanosecs = 100000000;
  struct itimerval itv = {0};
  struct itimerval oitv = {0};
  //itv.it_interval.tv_sec = freq_nanosecs / 1000000000;
  //itv.it_value.tv_usec = freq_nanosecs % 1000000;
  itv.it_value.tv_sec = 5;
  itv.it_value.tv_usec = 0;
  itv.it_interval.tv_sec = 0;
  itv.it_interval.tv_usec = 500000;

  struct sigaction sa;
  sigemptyset(&sa.sa_mask);
  sa.sa_flags = 0;
  sa.sa_handler = sigalrmHandler;
  if (sigaction(SIGALRM, &sa, NULL) == -1)
  errExit("sigaction");
  setitimer(ITIMER_REAL, &itv, &oitv);

  while(1) {
    struct timespec ts = {0};
    clock_gettime(CLOCK_REALTIME, &ts);
    printf("%ld\n", ts.tv_sec % 2);
    setitimer(ITIMER_REAL, &itv, &oitv);
    printf("%ld\n", oitv.it_value.tv_sec);
    sleep(2);
  }
}