void clean_thread(void *arg) {
//  printf("clean_thread=%d, alive=%d\n", abs((int) pthread_self()), alive_threads);
    if (pthread_mutex_lock(&readuce_alive_mutex) != 0) {
        err_exit("Błąd: pthread_mutex_lock(readuce_alive_mutex)");
    }
    alive_threads--;
    if (pthread_mutex_unlock(&readuce_alive_mutex) != 0) {
        err_exit("Błąd: pthread_mutex_unlock(readuce_alive_mutex)");
    }
}