16:50:392017-03-31

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以 生产、消费者的例子说明

#include <QtCore/QCoreApplication>

#include <QWaitCondition>

#include <QThread>

#include <QMutex>

#include <iostream>

const int DataSize = 100;

const int BufferSize = 1;

char buffer[BufferSize];

QWaitCondition bufferIsNotFull;

QWaitCondition bufferIsNotEmpty;

QMutex mutex;

int usedSpace;

 

class Producer : public QThread

{

protected:

    void run()

    {

        for (int i = 0; i < DataSize; ++i)

        {

            mutex.lock();//A

            while (usedSpace == BufferSize)

            {

                bufferIsNotFull.wait(&mutex);//B

            }

            std::cerr<<"P";//C

            ++usedSpace;

            bufferIsNotEmpty.wakeAll();

            mutex.unlock();

        }

    }//…G

};

class Consumer : public QThread

{

protected:

    void run()

    {

        for (int i = 0; i < DataSize; ++i)

        {

            mutex.lock();//D

            while (usedSpace == 0)

            {

                bufferIsNotEmpty.wait(&mutex);//E

            }

            std::cerr<<"C";//F

            --usedSpace;

            bufferIsNotFull.wakeAll();

            mutex.unlock();

        }

        std::cerr<<std::endl;

    }//…K

};

int main(int argc, char *argv[])

{

   Producer producer;

   Consumer consumer;

   producer.start();

   consumer.start();

   producer.wait();

   consumer.wait();

return 0;

}

 

根据QWaitCondition::wait(lockedMutex)的说明:Releases the lockedMutex and waits on the wait condition. The lockedMutex must be initially locked by the calling thread.

在调用wait之前应将lockedMutex锁住 见上面代码的B和A处，以及E、D处的调用关系。

之所以先执行Releases the lockedMutex 是因为，调用wait(lockedMutex)后程序直接跳转到G（执行完B）或K处（执行完D），而不再执行C、F处的代码，所以要release the lockedmutex

否则不能保证mutex的lock、unlock的成对执行。