Question

我上课

class Device
{
    enum State {eStopped, eRunning}
    State flag = eStopped;
public:
    void run(){
        if(flag==eRunning){return;}
        /*some codes to start this device*/
        flag = eRunning;
    }
    void stop(){
        if(flag==eStopped){return;}
        /*some codes to stop this device*/
        flag = eStopped;
    }
    void doMaintenance(){
        if(flag==eRunning){return;} // We can't do maintenance when the device is running
        /*Here, the flag may be modified to eRunning in other threads*/
    }
}

在doMaintenance()函数中，flag检查之后，其他线程将更改(flag==eRunning)。我如何优雅地防止这种情况发生？

Answer 1

还有其他问题需要解决。例如，假设设备处于停止状态。然后，在两个线程中，执行run()。然后，两个线程开始执行相同的启动顺序。我们也应该注意这一点。

因此，最简单的解决方案是不允许run()，stop()和doMaintenance()中的任何一个同时运行。这可以通过互斥锁轻松解决：

class Device
{
    enum State {eStopped, eRunning}
    State flag = eStopped;
    std::mutex m_mutex;
public:
    void run(){
        std::scoped_lock lock(m_mutex);
        if(flag==eRunning){return;}
        /*some codes to start this device*/
        flag = eRunning;
    }
    void stop(){
        std::scoped_lock lock(m_mutex);
        if(flag==eStopped){return;}
        /*some codes to stop this device*/
        flag = eStopped;
    }
    void doMaintenance(){
        std::scoped_lock lock(m_mutex);
        if(flag==eRunning){return;} // We can't do maintenance when the device is running
        /*Here, the flag may be modified to eRunning in other threads*/
    }
}

如何防止另一个线程修改状态标志？

1 个答案: