typedef int semaphore;

int rc, wc;
rc = wc = 0;

semaphore rc_mutex, wc_mutex, readersQueue, db
rc_mutex = wc_mutex = readersQueue = db = 1;

void reader(void) {
  down(&readersQueue);

  down(&rc_mutex);
  rc++;
  if (rc==1) down(&db);
  up(&rc_mutex);

  up(&readersQueue);

  read_db();  /* κρίσιμο τμήμα */

  down(&rc_mutex);
  rc--;
  if (rc==0) up(&db);
  up(&rc_mutex);
}

void writer(void) {
  down(&wc_mutex);
  wc++;
  if (wc==1) down(&readersQueue);
  up(&wc_mutex);

  down(&db);
  write_db();
  up(&db);

  down(&wc_mutex);
  wc--;
  if (wc==0) up(&readersQueue);
  up(&wc_mutex);
}