question about "fortune server"

[Dumbledore]

Hi, I'm stumped as to of why this code causes my application to crash when I telnet to it. It's almost directly copied from the fortune example here: http://doc.trolltech.com/4.1/network-fortuneserver.html

void send() {
  QByteArray block;
  QDataStream out(&block, QIODevice::WriteOnly);
  out.setVersion(QDataStream::Qt_4_0);
  out << (quint16)0;
  out << "here's some text i'm sending...";
     out.device()->seek(0);
     out << (quint16)(block.size() - sizeof(quint16));
 
     QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
     connect(clientConnection, SIGNAL(disconnected()),
             clientConnection, SLOT(deleteLater()));
 
     clientConnection->write(block);
     clientConnection->disconnectFromHost();
}

void send() {
  QByteArray block;
  QDataStream out(&block, QIODevice::WriteOnly);
  out.setVersion(QDataStream::Qt_4_0);
  out << (quint16)0;
  out << "here's some text i'm sending...";
     out.device()->seek(0);
     out << (quint16)(block.size() - sizeof(quint16));

     QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
     connect(clientConnection, SIGNAL(disconnected()),
             clientConnection, SLOT(deleteLater()));

     clientConnection->write(block);
     clientConnection->disconnectFromHost();
}

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[marcel]

Probably because clientConnection is NULL. You don't test for that, yet it is possible for it to be.

And about the byte array: you're writing something to ot, next you're positioning the file pointer to the start of the stream and write something again. The first data gets lost.

[Dumbledore]

Yeah, like I said it's from an example so I'm trying to learn network programming with QT.

I'm just trying to not crash at this point. This still crashes though:

QByteArray block;
  QDataStream out(&block, QIODevice::WriteOnly);
  out.setVersion(QDataStream::Qt_4_0);
  out << "here's some text i'm sending...";
 
 
     QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
     if ( NULL != clientConnection ) {
       connect(clientConnection, SIGNAL(disconnected()),
	       clientConnection, SLOT(deleteLater()));
 
       clientConnection->write(block);
       clientConnection->disconnectFromHost();
     }

QByteArray block;
  QDataStream out(&block, QIODevice::WriteOnly);
  out.setVersion(QDataStream::Qt_4_0);
  out << "here's some text i'm sending...";


     QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
     if ( NULL != clientConnection ) {
       connect(clientConnection, SIGNAL(disconnected()),
	       clientConnection, SLOT(deleteLater()));

       clientConnection->write(block);
       clientConnection->disconnectFromHost();
     }

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[jpn]

Could we see the backtrace from debugger?

[marcel]

Can't you debug it, to see exactly at what line it crashes?

[Dumbledore]

I've never used a debugger before. In other words, the code looks fine and something else is going on?

EDIT: Alright I discovered it's because the instance of tcpServer it was using had never been allocated yet. It was a wild pointer. Thanks for the help, normally I don't ask such stupid questions but I'm completely new to this stuff.

[marcel]

I've never used a debugger before. In other words, the code looks fine and something else is going on?

Code looks good all the time...
You should start using a debugger. These days you can't do any serious programming without one.