c++读写锁的实现

一、使用互斥锁和条件变量实现读写锁：

 

class readwrite_lock
{
public:
    readwrite_lock()
        : stat(0)
    {
    }

    void readLock()
    {
        mtx.lock();
        while (stat < 0)
            cond.wait(mtx);
        ++stat;
        mtx.unlock();
    }

    void readUnlock()
    {
        mtx.lock();
        if (--stat == 0)
            cond.notify_one(); // 叫醒一个等待的写操作
        mtx.unlock();
    }

    void writeLock()
    {
        mtx.lock();
        while (stat != 0)
            cond.wait(mtx);
        stat = -1;
        mtx.unlock();
    }

    void writeUnlock()
    {
        mtx.lock();
        stat = 0;
        cond.notify_all(); // 叫醒所有等待的读和写操作
        mtx.unlock();
    }

private:
    mutex mtx;
    condition_variable cond;
    int stat; // == 0 无锁；> 0 已加读锁个数；< 0 已加写锁
};

二、使用2个互斥锁实现读写锁：

 

class readwrite_lock
{
public:
    readwrite_lock()
        : read_cnt(0)
    {
    }

    void readLock()
    {
        read_mtx.lock();
        if (++read_cnt == 1)
            write_mtx.lock();

        read_mtx.unlock();
    }

    void readUnlock()
    {
        read_mtx.lock();
        if (--read_cnt == 0)
            write_mtx.unlock();

        read_mtx.unlock();
    }

    void writeLock()
    {
        write_mtx.lock();
    }

    void writeUnlock()
    {
        write_mtx.unlock();
    }

private:
    mutex read_mtx;
    mutex write_mtx;
    int read_cnt; // 已加读锁个数
};

用mutex和conditon实现写优先的读写锁

class RWLock {
private:
    pthread_mutex_t mxt;
    pthread_cond_t cond;
    int rd_cnt;//等待读的数量
    int wr_cnt;//等待写的数量

public:
    RWLock() :rd_cnt(0), wr_cnt(0) {
        pthread_mutex_init(&mxt,NULL);
        pthread_cond_init(&cond,NULL);
    }

    void readLock() {
        pthread_mutex_lock(&mxt);

        ++rd_cnt;
        while(wr_cnt > 0)
            pthread_mutex_wait(&cond, &mxt);

        pthread_mutex_unlock(&mxt);
    }

    void readUnlock() {
        pthread_mutex_lock(&mxt);

        --rd_cnt;
        if (rd_cnt == 0 )
            pthread_cond_signal(&cond);

        pthread_mutex_unlock(&mxt);
    }

    void writeLock() {
        pthread_mutex_lock(&mxt);

        ++wr_cnt;
        while (wr_cnt + rd_cnt >=2)
            pthread_cond_wait(&cond, &mxt);

        pthread_mutex_unlock(&mxt);
    }

    void writerUnlock() {
        pthread_mutex_lock(&mxt);

        --wr_cnt;
        if(wr_cnt==0)
            pthread_cond_signal(&cond);

        pthread_mutex_unlock(&mxt);
    }
};