Make one qthread check the state of the other

[kindlychung]

I am doing an exercise qt console application on threading, here is the code:

// make two thread, one checking on the state of the other
 
 
    //////////////////////////////////////
    // main.cpp
 
    #include "mytimer.h"
    #include "mythread.h"
    #include "checkthread.h"
    #include <QCoreApplication>
    #include <QString>
    #include <QFile>
    #include <QDebug>
 
 
    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
        MyThread mThread1;
        mThread1.name = "thread 1";
        mThread1.start(QThread::HighestPriority);
        CheckThread mCheck(&mThread1);
        mCheck.start();
 
        return a.exec();
    }
 
    ///////////////////////////////////////
    // mythread.h
 
    #ifndef MYTHREAD_H
    #define MYTHREAD_H
 
    #include <QThread>
    #include <QtCore>
 
    class MyThread : public QThread
    {
    public:
        MyThread();
        void run();
        QString name;
        bool stop;
        int sum;
    };
 
    #endif // MYTHREAD_H
 
    //////////////////////////////////////
    // mythread.cpp
 
    #include "mythread.h"
 
    MyThread::MyThread()
    {
        sum = 0;
    }
 
    void MyThread::run()
    {
        qDebug() << this->name << " running...";
        for(int i=0; i<1000; i++) {
            this->sum += i;
            qDebug() << this->name << " counting " << sum;
            this->sleep(1);
            if(this->stop) {
                break;
            }
        }
    }
 
    //////////////////////////////////////
    // checkthread.h
 
 
    #ifndef CHECKTHREAD_H
    #define CHECKTHREAD_H
 
    #include <QThread>
    #include "mythread.h"
 
    class CheckThread : public QThread
    {
        Q_OBJECT
    public:
        explicit CheckThread(QObject *parent = 0);
        explicit CheckThread(MyThread *tocheck);
        void run();
        MyThread *tocheck_;
    };
 
    #endif // CHECKTHREAD_H
 
    //////////////////////////////////////
    // checkthread.cpp
 
 
    #include "checkthread.h"
 
    CheckThread::CheckThread(QObject *parent) :
        QThread(parent)
    {
    }
 
    CheckThread::CheckThread(MyThread *tocheck) :
        tocheck_(tocheck)
    {
    }
 
    void CheckThread::run() {
        while(true) {
            this->sleep(1);
            if(tocheck_->sum > 15) {
                tocheck_->stop = true;
            }
        }
    }

// make two thread, one checking on the state of the other


    //////////////////////////////////////
    // main.cpp

    #include "mytimer.h"
    #include "mythread.h"
    #include "checkthread.h"
    #include <QCoreApplication>
    #include <QString>
    #include <QFile>
    #include <QDebug>


    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
        MyThread mThread1;
        mThread1.name = "thread 1";
        mThread1.start(QThread::HighestPriority);
        CheckThread mCheck(&mThread1);
        mCheck.start();

        return a.exec();
    }

    ///////////////////////////////////////
    // mythread.h

    #ifndef MYTHREAD_H
    #define MYTHREAD_H

    #include <QThread>
    #include <QtCore>

    class MyThread : public QThread
    {
    public:
        MyThread();
        void run();
        QString name;
        bool stop;
        int sum;
    };

    #endif // MYTHREAD_H

    //////////////////////////////////////
    // mythread.cpp

    #include "mythread.h"

    MyThread::MyThread()
    {
        sum = 0;
    }

    void MyThread::run()
    {
        qDebug() << this->name << " running...";
        for(int i=0; i<1000; i++) {
            this->sum += i;
            qDebug() << this->name << " counting " << sum;
            this->sleep(1);
            if(this->stop) {
                break;
            }
        }
    }

    //////////////////////////////////////
    // checkthread.h


    #ifndef CHECKTHREAD_H
    #define CHECKTHREAD_H

    #include <QThread>
    #include "mythread.h"

    class CheckThread : public QThread
    {
        Q_OBJECT
    public:
        explicit CheckThread(QObject *parent = 0);
        explicit CheckThread(MyThread *tocheck);
        void run();
        MyThread *tocheck_;
    };

    #endif // CHECKTHREAD_H

    //////////////////////////////////////
    // checkthread.cpp


    #include "checkthread.h"

    CheckThread::CheckThread(QObject *parent) :
        QThread(parent)
    {
    }

    CheckThread::CheckThread(MyThread *tocheck) :
        tocheck_(tocheck)
    {
    }

    void CheckThread::run() {
        while(true) {
            this->sleep(1);
            if(tocheck_->sum > 15) {
                tocheck_->stop = true;
            }
        }
    }

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The expected behavior is that mThread1 shoud count to 15 and then stop,
but instead it is stuck at 0.

Interestingly (strangely, i mean), if I add the following code into the main.cpp file, then it runs
ok:

void Write(QString Filename)
    {
        QFile fh(Filename);
        if(!fh.open(QFile::WriteOnly | QFile::Text))
        {
            qDebug() << "Could not open file for writing";
            return;
        }
        QTextStream out(&fh);
        out << "hi world";
        fh.flush();
        fh.close();
    }
 
    void Read(QString Filename)
    {
        QFile fh(Filename);
        if(!fh.open(QFile::ReadOnly | QFile::Text))
        {
            qDebug() << "Could not open file for writing";
            return;
        }
        QTextStream in(&fh);
        QString mtext = in.readAll();
        qDebug() << mtext;
    }

void Write(QString Filename)
    {
        QFile fh(Filename);
        if(!fh.open(QFile::WriteOnly | QFile::Text))
        {
            qDebug() << "Could not open file for writing";
            return;
        }
        QTextStream out(&fh);
        out << "hi world";
        fh.flush();
        fh.close();
    }

    void Read(QString Filename)
    {
        QFile fh(Filename);
        if(!fh.open(QFile::ReadOnly | QFile::Text))
        {
            qDebug() << "Could not open file for writing";
            return;
        }
        QTextStream in(&fh);
        QString mtext = in.readAll();
        qDebug() << mtext;
    }

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I am using qt 4.8 on a kubuntu 13.10 machine, and the ide is qt creator 3.0.1

[anda_skoa]

You don't initialize MyThread::stop.
If my chance it is true, the thread will exit in the first iteration of its loop.

Also for a correct program you would need to protect the access of variables that are used by more than one thread. e.g. using QMutex

Cheers,
_

[kindlychung]

Yes, you are right!

But this does not explain why the Read and Write functions in main.cpp solves the problem. Very puzzling.

[anda_skoa]

But this does not explain why the Read and Write functions in main.cpp solves the problem. Very puzzling.

Coincidence.

Cheers,
_