You can only debug what you know is wrong. In this case, with my cursory knowledge of Qt, I know this code should work. (Well I know it doesn't but I don't know why.)

Problem description: I can start multiple apps and they can connect to one another, but as soon as a connection is terminated all of the apps segfault.

The problem is most assuredly within here:
Qt Code: 
  1. void MSocketThread::run() {
  2.   MSocket mSocket;
  3.   if ( 0 != socketDescriptor ) {
  4.     if ( !mSocket.setSocketDescriptor(socketDescriptor) ) {
  5.       std::cerr << "Failed to set SocketDescriptor." << endl;
  6.     }
  7.     /* Output who joined the server */
  8.     QHostAddress ipaddress = mSocket.peerAddress(); 
  9.     QString ipString(tr("<font color=blue>%1 has joined.</font>").arg(ipaddress.toString()));
  10.     emit relayIncomingText(ipString);
  11.   }
  12.   connect(&mSocket, SIGNAL(connected()), 
  13. 	  this, SIGNAL(connectionEstablished()));
  14.   connect(&mSocket, SIGNAL(inboundText(QString)), 
  15. 	  this, SIGNAL(relayIncomingText(QString)));
  16.   connect(this, SIGNAL(sigSend(QString)), 
  17. 	  &mSocket, SLOT(send(QString)));
  18.   connect(&mSocket, SIGNAL(error()),	  
  19. 	  this, SIGNAL(quit()));  
  20.   connect(&mSocket, SIGNAL(disconnected()), this, SLOT(quit()));
  21.  
  22.   if ( 0 == socketDescriptor ) {
  23.       mSocket.connectToHost(ip, port);
  24.   }
  25.  
  26.  
  27.   exec(); //Begin event loop.  
  28. }
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