#define _XOPEN_SOURCE 500 /* SUSv2 */

#include <signal.h>       /* sigaction() */
#include <sys/time.h>     /* setitimer() */
#include <assert.h>       /* assert() */
#include <unistd.h>       /* pause() */

static volatile sig_atomic_t flag;

static void
ring(int sig)
{
  flag = !flag;
}

int
main(void)
{
  struct sigaction act;
  int tmp;
  struct itimerval itmv;
  
  act.sa_handler = &ring;
  tmp = sigemptyset(&act.sa_mask); assert(0 == tmp);
  act.sa_flags = 0;
  tmp = sigaction(SIGALRM, &act, 0); assert(0 == tmp);

  itmv.it_value.tv_sec = itmv.it_interval.tv_sec = 0;
  itmv.it_value.tv_usec = itmv.it_interval.tv_usec = 100 * 1000;
  tmp = setitimer(ITIMER_REAL, &itmv, 0); assert(0 == tmp);

  for (;;) {
    while (0 == flag)
      ;
    (void)pause();
  }
}