信号量是最早出现的用来解决进程同步与互斥问题的机制，包括一个称为信号量的变量及对它进行的PV原语操作。

 

实际上 Windows 系统提供了 Semaphore 内核对象实现信号量及 PV 操作, 其 P 操作相当于 WaitForSingleObject(hSemaphore, INFINITE),  V 操作相当于 ReleaseSemaphore(hSemaphore, 1, NULL),  以下代码利用 Event 内核对象与计数器模拟实现了 PV 操作:

 

class Signal
{
private:
    LONG m_nCount;
    HANDLE m_hEvent;

public:
    Signal() { m_nCount = 0; m_hEvent = NULL; };
    Signal(int nInit) { m_nCount = nInit; m_hEvent = NULL; };

    ~Signal() { if (m_hEvent) CloseHandle(m_hEvent); };

    BOOL pAction() 
    {
        if (0 > InterlockedDecrement(&m_nCount))
        {
            if (m_hEvent == NULL)
                m_hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
            return (WAIT_OBJECT_0 == WaitForSingleObject(m_hEvent, INFINITE));
        }
        return TRUE;
    };

    BOOL vAction()
    {
        if (0 <= InterlockedIncrement(&m_nCount))
        {
            if (m_hEvent)
                return PulseEvent(m_hEvent);
        }
        return TRUE;
    };
};

 

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