c – 对于简短的共享操作和几个独特的操作,是否有共享互斥的方法

基本理念：

每个读者都在阅读器上复制,而作者可以修改单独的编写者副本.要提交更改,请将writer-object存储在新的reader-object中,该reader-object将从每个新读取器中获取.下一个提交将存储到现在的旧读者对象中.因此,作者需要等待,直到所有旧读者都释放对此对象的锁定.

简单的例子：

struct data{
    int a;
};

struct secured_data {
    data read[2];
    data write;
    std::atomic<int> current = 0;
    std::atomic<int> num_read[2] = 0;

    std::mutex m;

    data* read_lock() {
        // increase reader-counter of current data-object
        int c = current.load();
        num_read[c].fetch_add(1);
        // return current copy
        return &read + c;
    }

    void read_unlock(data* old) {
        // decrease the old data-object (which was returned by read_lock())
        num_read[old - &read].fetch_sub(1);


    }

    data* lock() {
        m.lock();
        // every modification happens on a seperate copy
        write = read[current.load()];
        return &write;
    }

    void unlock() {
       int c;
       // wait until all readers using an old copy have released their lock
       do {
           c = current.load();
       } while (num_read[(c + 1) % 2].load() != 0);
       // copy into new read-object
       read[(c + 1) % 2] = write;
       // swap read-object
       // all readers now use an old version
       current.store((c + 1) % 2);
       m.unlock();
    }
}