问如何使两个线程在一个线程结束后轮流执行各自的临界区

EN

{
    std::mutex mu;
    std::condition_variable cv;
    int turn = 0;

    auto thread_func = [&](int tid, int iters) {
        std::unique_lock<std::mutex> lk(mu);
        lk.unlock();

        for (int i = 0; i < iters; i++) {
            lk.lock();
            cv.wait(lk, [&] {return turn == tid; });

            printf("tid=%d turn=%d i=%d/%d\n", tid, turn, i, iters);
            fflush(stdout);

            turn = !turn;
            lk.unlock();
            cv.notify_all();
        }
    };

    auto th0 = std::thread(thread_func, 0, 20);
    auto th1 = std::thread(thread_func, 1, 25); // Does more iterations

    printf("Made the threads.\n");
    fflush(stdout);

    th0.join();
    th1.join();

    printf("Both joined.\n");
    fflush(stdout);
}