#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <semaphore.h>
#include <time.h>
#include <assert.h>
#include <errno.h>
#include <signal.h>

sem_t sem;

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

static void
handler(int sig)
{
  write(STDOUT_FILENO, "sem_post() from handler\n", 24);
  if (sem_post(&sem) == -1)
  {
    write(STDERR_FILENO, "sem_post() failed\n", 18);
    _exit(EXIT_FAILURE);
  }
}

int
main(int argc, char *argv[])
{
  struct sigaction sa;
  struct timespec ts;
  int s;

  if (argc != 3)
  {
    fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n",
            argv[0]);
    exit(EXIT_FAILURE);
  }

  if (sem_init(&sem, 0, 0) == -1)
    handle_error("sem_init");

  /* Establish SIGALRM handler; set alarm timer using argv[1] */

  sa.sa_handler = handler;
  sigemptyset(&sa.sa_mask);
  sa.sa_flags = 0;
  if (sigaction(SIGALRM, &sa, NULL) == -1)
    handle_error("sigaction");

  alarm(atoi(argv[1]));

  /* Calculate relative interval as current time plus
     number of seconds given argv[2] */

  if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
    handle_error("clock_gettime");

  ts.tv_sec += atoi(argv[2]);

  printf("main() about to call sem_timedwait()\n");
  while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
    continue;       /* Restart if interrupted by handler */

  /* Check what happened */

  if (s == -1)
  {
    if (errno == ETIMEDOUT)
      printf("sem_timedwait() timed out\n");
    else
      perror("sem_timedwait");
  }
  else
    printf("sem_timedwait() succeeded\n");

  exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
}