QMutex and QDataStream

[babu198649]

hi
i need to protect the access of a variable which is currently being filled with the QDataStream(in a multithreading program).

in the following code does the access of the int variable blocksize is locked or the datastream is locked ,or both.

QDataStream datastream(socket);
QMutex mutex;
mutex.lock();
datastream >> blocksize;
mutex.unlock();

[ChristianEhrlicher]

None. You don't lock variables.
The only thing you do here is that you make sure that the code line between lock() and unlock() is not executed by two or more threads at the same time. Nothing more.

[wysota]

Actually even that is not the case here as each thread will have its own mutex instance so they won't be exclusive. The above code is really a no-op (apart from the line operating on the stream).

[babu198649]

thanks for u r replys

here is my header file.

//plot.h Include files are not specified to reduce the post size

class th : public QThread
{
	Q_OBJECT
 
public:
	th();
	void run();
 
	quint32 blocksize;
 
private slots:
	void read_socket();
 
private:
QTcpSocket *socket;
};
 
class PLOT : public QMainWindow
{
	Q_OBJECT
public:
	PLOT();
 
private:
	th t;
};
 
#endif /*PLOT_H_*/

class th : public QThread
{
	Q_OBJECT

public:
	th();
	void run();

	quint32 blocksize;

private slots:
	void read_socket();

private:
QTcpSocket *socket;
};

class PLOT : public QMainWindow
{
	Q_OBJECT
public:
	PLOT();

private:
	th t;
};

#endif /*PLOT_H_*/

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//plot.cpp

th::th() :blocksize(0)
{
 
}
 
void th::run()
{
	socket=new QTcpSocket();
	socket->connectToHost("192.168.1.6", 9999);
	connect(socket, SIGNAL(readyRead()), this, SLOT(read_socket()));
	exec();
}
 
void th::read_socket()
{
	QDataStream in(socket);
	if (blocksize == 0)
	{
		if (socket->bytesAvailable() < (int)sizeof(quint32))
			return;
 
		in >> blocksize;
	}
}
 
PLOT::PLOT()
{
	t.start();
 
       //waiting here till the t.blocksize variable is getting filled but had not worked and never 
      //gets out of the loop,i have tried declaring the blocksize variable in the [B]th[/B] 
      //class as[B] volatile  quint32 blocksize[/B] and even it has not worked.
      //only [B]works[/B] when introduce a 
      //delay here as [B]QTest::qWait(3000);[/B]
 
     while (t.blocksize == (qint32)0)
	{
		qDebug()<<" PLOT inside while "<<t.blocksize;
		coloum=(int)(t.blocksize/360);
	}
}

th::th() :blocksize(0)
{

}

void th::run()
{
	socket=new QTcpSocket();
	socket->connectToHost("192.168.1.6", 9999);
	connect(socket, SIGNAL(readyRead()), this, SLOT(read_socket()));
	exec();
}

void th::read_socket()
{
	QDataStream in(socket);
	if (blocksize == 0)
	{
		if (socket->bytesAvailable() < (int)sizeof(quint32))
			return;
		
		in >> blocksize;
	}
}

PLOT::PLOT()
{
	t.start();

       //waiting here till the t.blocksize variable is getting filled but had not worked and never 
      //gets out of the loop,i have tried declaring the blocksize variable in the [B]th[/B] 
      //class as[B] volatile  quint32 blocksize[/B] and even it has not worked.
      //only [B]works[/B] when introduce a 
      //delay here as [B]QTest::qWait(3000);[/B]

     while (t.blocksize == (qint32)0)
	{
		qDebug()<<" PLOT inside while "<<t.blocksize;
		coloum=(int)(t.blocksize/360);
	}
}

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where should i modify the code to work.

[wysota]

Read about QWaitCondition please.

[babu198649]

If i have to use the QWaitCondition i have to use the mutex,which means i need two classes subclassed from QThread , and also i have to do gui in my program.i tried to use QWaitCondition but i am stuck,i am unable to figure where to wait and where to wakeup the thread(one of the class in the above program does not inherit QThread.)

Do i have to have multiple inheritance(QThread and QWidget ) to solve the above problem in which i have do both gui and threading.


why the above code never gets out of while.
here is a sample code

#include <QtGui>
#include <QtNetwork>
 
class trd : public QThread
{
public:
	void run()
	{
		while (1)
			qDebug()<<"run";
	}
};
 
class sub : public QWidget
{
public:
	sub()
	{
		trd th;
		th.start();
 
		while (1)
		qDebug()<<"sub";
	}
};
 
int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	sub t;
	return a.exec();
}

#include <QtGui>
#include <QtNetwork>

class trd : public QThread
{
public:
	void run()
	{
		while (1)
			qDebug()<<"run";
	}
};

class sub : public QWidget
{
public:
	sub()
	{
		trd th;
		th.start();
	
		while (1)
		qDebug()<<"sub";
	}
};

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	sub t;
	return a.exec();
}

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in this code both the run and sub gets printed alternatively , the same idea i am using to make work in the program(waiting for the while condition to fail) in the above post.where did i made the error.

thanks

[wysota]

I don't really understand what you mean by "where to wait and where to wake"... Don't you know where you want to halt execution of one thread and when to let it run? It is your algoritm - you should know best.

[babu198649]

Consider two classes class GUI(which subclasses QWidget) and class THD(which subclasses QThread) ,now if i want to wait in class G for a condition which is set by class THD ,what should i do.

here is a simple code what i have explained above.

#include <QtGui>
 
class THD : public QThread
{
public:
	int i;
 
	THD():i(0)
	{
 
	}
 
	void run()
	{
		i=1;
	}
};
 
class GUI : public QWidget
{
public:
	GUI()
	{
		THD th;
		th.start();
 
		//wait until i is changed and then continue(how to wait)
	}
};
 
int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	GUI t;
	return a.exec();
}

#include <QtGui>

class THD : public QThread
{
public:
	int i;
	
	THD():i(0)
	{
		
	}
	
	void run()
	{
		i=1;
	}
};

class GUI : public QWidget
{
public:
	GUI()
	{
		THD th;
		th.start();
		
		//wait until i is changed and then continue(how to wait)
	}
};

int main(int argc, char *argv[])
{
	QApplication a(argc, argv);
	GUI t;
	return a.exec();
}

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[wysota]

Call wait() where you want to wait and wakeAll() where you want to release the other thread. What is the problem?

QMutex m;
QWaitCondition wc;
GUI(){THD th; th.start(); wc.wait(&m); }
...
void run(){ i=0; wc.wakeAll(); }

QMutex m;
QWaitCondition wc;
GUI(){THD th; th.start(); wc.wait(&m); }
...
void run(){ i=0; wc.wakeAll(); }

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By the way, in this case you could easily use signals and slots instead of a mutex...

[babu198649]

thanks wysota
But the widget is not getting displayed after adding the wc.wait(&m) (i have called show() in main). it seems that it gets stopped until the wc.wakeAll() is called .But i want it to wake according to the buttons pressed in the GUI widget.
should i have to start the thread in a slot.

in this case you could easily use signals and slots instead of a mutex

in this case you could easily use signals and slots instead of a mutex

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the above code is just to explain my problem . i am using the ideas in different program which is large to post here.

[wysota]

it seems that it gets stopped until the wc.wakeAll() is called

Isn't that what you wanted? You wanted to stop execution of a thread until a variable is changed in another thread. That's exactly what happens.

in this case you could easily use signals and slots instead of a mutex

the above code is just to explain my problem . i am using the ideas in different program which is large to post here.

Then maybe you should try expressing your problems more clearly. If you describe your problem incorrectly, you can only blame yourself for getting a wrong answer. Without seeing your code I can only say "redesign code to use signals and slots and rely on the event queue for synchronization".