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);
}

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[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?