#include <stdio.h>
#include <semaphore.h>

sem_t s;

main(int argc, char *argv[])
{
  if( sem_init(&s, 0, 1) < 0 ){
    perror("semaphore init fails");
    exit(1);
  }

  if( (fork()) == 0 ){
    fprintf(stderr, "child waiting...\n");
    sem_wait(&s);
    fprintf(stderr, "child sleeping...\n");
    sleep(3);
    fprintf(stderr, "child stop...\n");
    sem_post(&s);
  } else {
    fprintf(stderr, "parent waiting...\n");
    sem_wait(&s);
    fprintf(stderr, "parent sleeping...\n");
    sleep(3);
    fprintf(stderr, "parent stop...\n");
    sem_post(&s);
  }
}