Reading from TCP Socket crashes program

[OnionRingOfDoom]

Ok, so I have a program which acts as a server, and can write data to a socket. Now when I use the client program to connect to the server, the client connects just fine, but it crashes whenever it goes to read the data. I'm not entirely sure how the whole QIODevice thing plays into it, but what I wrote makes sense, at least to me :P. Anyway, here's the code:

// Server/Client-related connections
     connect(&tcpServer, SIGNAL(newConnection()), this, SLOT(acceptConnection()));
     connect(&tcpServer, SIGNAL(connectionAccepted()), this, SLOT(sendSpeed()));
     connect(&tcpClient, SIGNAL(connected()), this, SLOT(readSpeed()));
 
     connect(&tcpClient, SIGNAL(error(QAbstractSocket::SocketError)), this, SLOT(displayError(QAbstractSocket::SocketError)));
     connect(&tcpClient, SIGNAL(state(QAbstractSocket::SocketState)), this, SLOT(displayStatus(QAbstractSocket::SocketState)));
 
     // Set up layout
     layout->addWidget(connectButton, 0, 0, 1, 2);
     layout->addWidget(slider, 1, 0, 1, 2);
     layout->addWidget(label, 3, 0); 
     //layout->addWidget(label2, 3, 0);  
     layout->addWidget(ipLabel, 5, 0);
     win->setLayout(layout);
 
} // MainWindow()
 
////////////////////////////////////////////////////////////////////////
// SERVER FUNCTIONS                                                   //
 
// startServer(QString ip, quint16 port) - Starts the server
////////////////////////////////////////////////////////////////////////
void MainWindow::startServer(QString ip, quint16 port)
{
     statusBar->showMessage("Server created", 0);
     if (!tcpServer.listen(QHostAddress(ip), port))
     {
          statusBar->showMessage(tr("Unable to start the server: %1.", 0).arg(tcpServer.errorString()));
          return;
     }
     else
     {
          statusBar->showMessage(tr("Listening on port %1...", 0).arg(tcpServer.serverPort()));
     }
}
 
// acceptConnection() - Completes the connection to the client
////////////////////////////////////////////////////////////////////////
void MainWindow::acceptConnection()
{
     slider->setAmpValue(10);
     slider->setFreqValue(10);
     clientConnection = tcpServer.nextPendingConnection();
     statusBar->showMessage("Connection established.", 0);
     emit connectionAccepted();
}
 
// sendSpeed() - Sends the slider speed to the client
////////////////////////////////////////////////////////////////////////
void MainWindow::sendSpeed()
{
     QByteArray block;
     QDataStream out(&block, QIODevice::WriteOnly);
     value = slider->ampValue();
 
     // Write the slider value to the output buffer
     out.setVersion(QDataStream::Qt_4_0);
     out << value;
 
     // Send the linear slider value to the client
     clientConnection->write(block);
}
 
// SERVER FUNCTIONS
 
////////////////////////////////////////////////////////////////////////
// CLIENT FUNCTIONS                                                   //
 
// getServer() - Establishes a connection to the server
////////////////////////////////////////////////////////////////////////
void MainWindow::getServer(QString ip, quint16 port)
{
     blockSize = 0;
     tcpClient.connectToHost(QHostAddress(ip), port);
}
 
// readSpeed() - Reads the slider speed value into a label
////////////////////////////////////////////////////////////////////////
void MainWindow::readSpeed()
{
     QDataStream in(&tcpClient);
     in.setVersion(QDataStream::Qt_4_0);
 
     quint8 sliderValue;
     in >> sliderValue;
     label->setNum(sliderValue);
}

// Server/Client-related connections
     connect(&tcpServer, SIGNAL(newConnection()), this, SLOT(acceptConnection()));
     connect(&tcpServer, SIGNAL(connectionAccepted()), this, SLOT(sendSpeed()));
     connect(&tcpClient, SIGNAL(connected()), this, SLOT(readSpeed()));
     
     connect(&tcpClient, SIGNAL(error(QAbstractSocket::SocketError)), this, SLOT(displayError(QAbstractSocket::SocketError)));
     connect(&tcpClient, SIGNAL(state(QAbstractSocket::SocketState)), this, SLOT(displayStatus(QAbstractSocket::SocketState)));
     
     // Set up layout
     layout->addWidget(connectButton, 0, 0, 1, 2);
     layout->addWidget(slider, 1, 0, 1, 2);
     layout->addWidget(label, 3, 0); 
     //layout->addWidget(label2, 3, 0);  
     layout->addWidget(ipLabel, 5, 0);
     win->setLayout(layout);
     
} // MainWindow()

////////////////////////////////////////////////////////////////////////
// SERVER FUNCTIONS                                                   //

// startServer(QString ip, quint16 port) - Starts the server
////////////////////////////////////////////////////////////////////////
void MainWindow::startServer(QString ip, quint16 port)
{
     statusBar->showMessage("Server created", 0);
     if (!tcpServer.listen(QHostAddress(ip), port))
     {
          statusBar->showMessage(tr("Unable to start the server: %1.", 0).arg(tcpServer.errorString()));
          return;
     }
     else
     {
          statusBar->showMessage(tr("Listening on port %1...", 0).arg(tcpServer.serverPort()));
     }
}

// acceptConnection() - Completes the connection to the client
////////////////////////////////////////////////////////////////////////
void MainWindow::acceptConnection()
{
     slider->setAmpValue(10);
     slider->setFreqValue(10);
     clientConnection = tcpServer.nextPendingConnection();
     statusBar->showMessage("Connection established.", 0);
     emit connectionAccepted();
}

// sendSpeed() - Sends the slider speed to the client
////////////////////////////////////////////////////////////////////////
void MainWindow::sendSpeed()
{
     QByteArray block;
     QDataStream out(&block, QIODevice::WriteOnly);
     value = slider->ampValue();
     
     // Write the slider value to the output buffer
     out.setVersion(QDataStream::Qt_4_0);
     out << value;

     // Send the linear slider value to the client
     clientConnection->write(block);
}

// SERVER FUNCTIONS

////////////////////////////////////////////////////////////////////////
// CLIENT FUNCTIONS                                                   //

// getServer() - Establishes a connection to the server
////////////////////////////////////////////////////////////////////////
void MainWindow::getServer(QString ip, quint16 port)
{
     blockSize = 0;
     tcpClient.connectToHost(QHostAddress(ip), port);
}

// readSpeed() - Reads the slider speed value into a label
////////////////////////////////////////////////////////////////////////
void MainWindow::readSpeed()
{
     QDataStream in(&tcpClient);
     in.setVersion(QDataStream::Qt_4_0);
     
     quint8 sliderValue;
     in >> sliderValue;
     label->setNum(sliderValue);
}

To copy to clipboard, switch view to plain text mode 

[jacek]

Could you post the backtrace?

[OnionRingOfDoom]

Sure! Lemme just figure out how to make MinGW do that

[jacek]

Sure! Lemme just figure out how to make MinGW do that

You need gdb for this.

C:\...\gdb app.exe
(gdb) run
<program crashes>
(gdb) bt
<backtrace>
(gdb) quit

[OnionRingOfDoom]

Ok, lemme install this doohicky

[OnionRingOfDoom]

#0 0x10047934 in _size_of_stack_reserve__ ()
Cannot access memory at address 0x200000

that's what it gave me.

[jacek]

#0 0x10047934 in _size_of_stack_reserve__ ()
Cannot access memory at address 0x200000

that's what it gave me.

Only this? Maybe there was something more?

If there is no main() in the backtrace, it might mean that there is some problem with libraries. Make sure you have "QT += network" in your .pro file.

[OnionRingOfDoom]

If there is no main() in the backtrace, it might mean that there is some problem with libraries. Make sure you have "QT += network" in your .pro file.

Yep, QT += network" is in the .pro file.

If you want, I can upload the entire source somewhere, and you can see what it does on your end?

[jacek]

If you want, I can upload the entire source somewhere, and you can see what it does on your end?

According to Murphy's law it will work on my system

Does your program crash before it starts?

[OnionRingOfDoom]

According to Murphy's law it will work on my system

Does your program crash before it starts?

Nope, I have to start up the program in server mode, then start another instance of the program in client mode. As soon as I click OK on the connect dialog box after typing in the propper port and IP, the server indicates that the client has sucessfully connected, and then the client crashes.

[jacek]

and then the client crashes.

And there was only one entry in the backtrace? Did you compile your application in debug mode?

Anyway, you try to read data from the socket before they arrive --- you must wait for readyRead() signal.

[OnionRingOfDoom]

...How do I compile it in debug mode?

[jacek]

...How do I compile it in debug mode?

Add "CONFIG += debug" to your .pro file (and remove "release" if it's there).

[OnionRingOfDoom]

Ok so I changed the connection to do the readSpeed() function whenever readyRead() was emitted. However, now nothing happens when the client connects.

I also added a new connection at the end of the acceptConnection() function, which is as follows: connect(slider, SIGNAL(valueChanged(int)), this, SLOT(sendSpeed()));
so now, whenever I move the slider (and it outputs valueChanged signal), it crashes.

[OnionRingOfDoom]

After further fiddling, it seems that it's not the server's writing to the IO device that's the problem, it's the client's reading from the IO device that's causing the crash. Moving the slider thus causes the server to write data, and the client tries to read it or something, but crashes.

[jacek]

Does it crash if you change MainWindow::readSpeed() to:

void MainWindow::readSpeed()
{
     label->setNum(0);
}

void MainWindow::readSpeed()
{
     label->setNum(0);
}

To copy to clipboard, switch view to plain text mode 

?

Did you try to generate that backtrace again?

[OnionRingOfDoom]

yes, it still crashes, oddly enough...
and I'm trying to compile in debug mode, but I have to fix all these errors switching to debug mode seems to have created.

[OnionRingOfDoom]

Oh awesome! It's acting like it can't find the QtNetwork include file anymore...
Any idea why adding that CONFIG thing to the .pro file would make it think it couldn't find QtNetwork, but could still find every other Qt file I included?

[jacek]

yes, it still crashes, oddly enough...

Is that label variable initialized?

It's acting like it can't find the QtNetwork include file anymore...

Can it find "QTcpSocket? Did you compile Qt in debug mode or maybe you have only release version?

[OnionRingOfDoom]

First question: yes, it is initialised
And I fixed the other problem, when I remade the .pro file, it didn't put in the
QT += network thing.