多執行緒的互斥（下）——訊號量

阿新 • • 發佈：2020-08-08

訊號量的概念

-訊號量是特殊的執行緒鎖

-訊號量允許N個執行緒同時訪問臨界資源

-Qt中直接支援訊號量(QSemaphore)

執行緒鎖是用來保護臨界資源的，每個執行緒鎖每次只允許一個執行緒進行訪問臨界資源。

QSemaphore sem(1)定義了一個訊號量，並且N的值為1，意味著一次只允許一個執行緒去訪問臨界資源。

sem.acquire()當前執行緒嘗試著獲取這個特殊的執行緒鎖，首先到該函式中檢視n的值是不是大於0的，如果是的話，就可以獲取這把特殊的執行緒鎖。如果n的值為0，那就必須等待。

再論生產消費者的問題：

在前面的實驗中已經做過一個生產者，一個消費者，一個倉庫。在本課程中實現n個生產者，n個消費者，n個倉庫。如何做到併發高效性呢，此時就需要使用QSemphore訊號量了。

#include <QCoreApplication>
#include <QThread>
#include <QDebug>
#include <QSemaphore>

const int SIZE = 5; //用來定義倉庫的個數
unsigned char g_bufer[SIZE] = {0};
QSemaphore g_sem_free(SIZE);  //表示5個倉庫都是空閒的
QSemaphore g_sem_used(0);   //初始值為0，表示有多少個倉庫已經被使用了

class Producer : public QThread
{
 
protected:
    void run()
    {
        while(true)
        {
            int value = qrand()%100;

            g_sem_free.acquire();
            for(int i=0; i<SIZE; i++)
            {
                if( !g_bufer[i] )
                {
                    g_bufer[i] = value;
                    qDebug()  
<< objectName() << "generate (" << i << "," << value << ")" << endl;
                    break;
                }
            }

            g_sem_used.release();
            sleep(2);
        }
    }
};

class Consumer : public QThread
{
protected:
    void run()
    {
        while(true)
        {
            g_sem_used.acquire();
            for(int i=0; i<SIZE; i++)
            {
                if( g_bufer[i] )
                {
                    int value = g_bufer[i];
                    g_bufer[i] = 0;
                    qDebug() << objectName() << "consume (" << i << "," << value << ")" <<endl;
                    break;
                }
            }
            g_sem_free.release();
            sleep(2);

        }
    }
};

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    Producer p1;
    Producer p2;
    Producer p3;

    Consumer c1;
    Consumer c2;

    p1.setObjectName("p1");
    p2.setObjectName("p2");
    p3.setObjectName("p3");
    c1.setObjectName("c1");
    c2.setObjectName("c2");


    p1.start();
    p2.start();
    p3.start();
    c1.start();
    c2.start();

    return a.exec();
}

列印結果如下：

"p1" generate ( 1 , 41 )

"p3" generate ( 2 , 41 )

"p2" generate ( 0 , 41 )

"c2" consume ( 1 , 41 )

"c1" consume ( 0 , 41 )

"p2" generate ( 0 , 67 )

"p3" generate ( 1 , 67 )

"p1" generate ( 3 , 67 )

"c1" consume ( 1 , 67 )

"c2" consume ( 0 , 67 )

"p2" generate ( 0 , 34 )

"p3" generate ( 1 , 34 )

"p1" generate ( 4 , 34 )

"c1" consume ( 1 , 34 )

"c2" consume ( 0 , 34 )

"p2" generate ( 0 , 0 )

"p3" generate ( 0 , 0 )

"c1" consume ( 2 , 41 )

"c2" consume ( 3 , 67 )

"p1" generate ( 0 , 0 )

"p2" generate ( 0 , 69 )

"c1" consume ( 0 , 69 )

"p3" generate ( 0 , 69 )

"c2" consume ( 0 , 69 )

"p1" generate ( 0 , 69 )

"c1" consume ( 0 , 69 )

"p2" generate ( 0 , 24 )

"c2" consume ( 0 , 24 )

"p3" generate ( 0 , 24 )

"c1" consume ( 0 , 24 )

"p1" generate ( 0 , 24 )

"c2" consume ( 0 , 24 )

"p2" generate ( 0 , 78 )

"c1" consume ( 0 , 78 )

"p3" generate ( 0 , 78 )

"c2" consume ( 0 , 78 )

"p2" generate ( 0 , 58 )

"c1" consume ( 0 , 58 )

"p1" generate ( 0 , 78 )

"c2" consume ( 0 , 78 )

"p3" generate ( 0 , 58 )

"c1" consume ( 0 , 58 )

""p2" generate ( 0 , 62 )

c2" consume ( 4 , 34 )

"p1" generate ( 1 , 58 )

"c1" consume ( 0 , 62 )

"p3" generate ( 0 , 62 )

"c2" consume ( 0 , 62 )

"p1" generate ( 0 , 62 )

"c1" consume ( 0 , 62 )

"p2" generate ( 0 , 64 )

"c2" consume ( 0 , 64 )

"p3" generate ( 0 , 64 )

"c1" consume ( 0 , 64 )

"p1" generate ( 0 , 64 )

"c2" consume ( 0 , 64 )

"p3" generate ( 0 , 5 )

"c1" consume ( 0 , 5 )

"p2" generate ( 0 , 5 )

"c2" consume ( 0 , 5 )

"p1" generate ( 0 , 5 )

"c1" consume ( 0 , 5 )

"p3" generate ( 0 , 45 )

"c2" consume ( 0 , 45 )

"p2" generate ( 0 , 45 )

"c1" consume ( 0 , 45 )

"p1" generate ( 0 , 45 )

"c2" consume ( 0 , 45 )

"p3" generate ( 0 , 81 )