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;
}
}
}

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;
}

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,
_