線程的互斥和同步（7）- Qt的條件變量QWaitCondition

QWaitCondition 提供了一個用於同步線程的條件變量。它允許一個線程後告訴其他的線程已經滿足了某種條件；一個或多個線程可以阻塞等待 QWaitCondition 來使用 wakeOne() 或 wakeAll() 設置條件。使用 wakeOne() 喚醒一個隨機選擇的線程或者 wakeAll() 喚醒全部等待的線程。

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下面是 QWaitCondition 類中，常用的函數介紹：

bool wait (QMutex *lockedMutex, unsigned long time = ULONG_MAX)

釋放鎖資源等待等待條件。lockedMutex 初始化必須是上鎖的狀態，根據調用的線程。如果 lockedMutex 沒有上鎖，他的行爲是未知的。如果 lockedMutex 是可遞歸的鎖，則該函數立即返回。 lockedMutex 將被解鎖，調用線程將被阻塞，直到滿足以下任一條件:

• 另一個線程使用 wakeOne() 或 wakeAll() 發出信號。在這種情況下，這個函數將返回true 。
• 時間毫秒已經過去。如果時間是ULONG_MAX(默認值)，那麼等待將永遠不會超時(必須通知事件)。如果等待超時，這個函數將返回 false 。

lockedMutex 將返回到相同 locked 狀態。此函數提供了允許原子性的從鎖定狀態轉換到等待狀態。。

void wakeOne () 和 void wakeAll ()

表示喚醒一個或多個線程。喚醒的順序是根據操作系統的調度決定的。

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上一篇文章我們介紹了使用信號量實現 生產者-消費者 例子
線程的互斥和同步（6）- Qt的信號量QSemaphore
條件變量是一個針對信號量的可替代選擇，本篇文章將使用條件變量替代信號量，實現同樣的效果：

數據定義，頭文件：

#include <QSemaphore>
#include <QWaitCondition>
#include <QMutex>
#include <atomic>

extern "C" int g_data[20];
extern "C" int g_nStartIndex;
extern "C" int g_nEndIndex;
extern "C" int g_nCurrentCreated;
extern "C" QWaitCondition g_notFullCondition;	// 未滿條件變量
extern "C" QWaitCondition g_notEmptyCondition;	// 未空條件變量
extern "C" QMutex g_mutex;

cpp文件

int g_data[20] = {-1};
int g_nStartIndex = 0;
int g_nEndIndex = 0;
int g_nCurrentCreated = 0;
QWaitCondition g_notFullCondition;
QWaitCondition g_notEmptyCondition;
QMutex g_mutex;

生產者實現：

void ProducerThread::run(void)
{
    while (1)
    {
        g_mutex.lock();
        // 當緩存滿了的時候，等待是否有消費者消費
        if (g_nCurrentCreated == 20)
            g_notFullCondition.wait(&g_mutex);
        g_mutex.unlock();

        // 生產數據
        g_data[g_nEndIndex] = number;
        std::cout << "Created Data: " << number++ \
                  << ", Index is " << g_nEndIndex << std::endl;

        g_nEndIndex++;
        if (g_nEndIndex == 20)
            g_nEndIndex = 0;

        ::Sleep(100);

        g_mutex.lock();
        g_nCurrentCreated++;
        // 喚醒消費者
        g_notEmptyCondition.wakeAll();
        g_mutex.unlock();
    }
}

消費者實現：

void ConsumerThread::run(void)
{
    while (1)
    {
        g_mutex.lock();
        // 等待
        if (g_nCurrentCreated == 0)
            g_notEmptyCondition.wait(&g_mutex);
        g_mutex.unlock();

        // 消費數據
        std::cout << "Read Value: " << g_data[g_nStartIndex] \
                  << ", Index is " << g_nStartIndex << std::endl;

        g_nStartIndex++;
        if (g_nStartIndex == 20)
            g_nStartIndex = 0;

        ::Sleep(2000);

        // 釋放資源
        g_mutex.lock();
        g_nCurrentCreated--;
        // 喚醒生產者生產
        g_notFullCondition.wakeAll();
        g_mutex.unlock();
    }
}

程序運行結果：
Created Data: 0, Index is 0
Created Data: 1, Index is 1
Read Value: 0, Index is 0
Created Data: 2, Index is 2
Created Data: 3, Index is 3
Created Data: 4, Index is 4
Created Data: 5, Index is 5
Created Data: 6, Index is 6
Created Data: 7, Index is 7
Created Data: 8, Index is 8
Created Data: 9, Index is 9
Created Data: 10, Index is 10
Created Data: 11, Index is 11
Created Data: 12, Index is 12
Created Data: 13, Index is 13
Created Data: 14, Index is 14
Created Data: 15, Index is 15
Created Data: 16, Index is 16
Created Data: 17, Index is 17
Created Data: 18, Index is 18
Created Data: 19, Index is 19
Read Value: 1, Index is 1
Created Data: 20, Index is 0
Read Value: 2, Index is 2
Created Data: 21, Index is 1
Read Value: 3, Index is 3
Created Data: 22, Index is 2
Read Value: 4, Index is 4
Created Data: 23, Index is 3
…

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作者：douzhq
個人博客主頁：不會飛的紙飛機
文章同步頁(可下載完整代碼)：線程的互斥和同步（6）- Qt的條件變量QWaitCondition