mutex = ...;


if(fork()) {
  // parent
  file = ...;

  while(true) {
    sem_wait(mutex);
    write_to_file("this is number blah from parent", file);
    sem_post(mutex);
  }
} else {
  file = ...;

  for(int i = 0; i < 5; i++) {
    sem_wait(mutex);
    write_to_file("This is from the child", file);
    sem_post(mutex);
  }

  sem_close(mutex);
}