A Simple Example of a Qt Socket Application

int main(int argc, char *argv[]) {
 QCoreApplication app(argc, argv);
 // code ...
 return 0;
}

The Qt C++ class library is probably best know for being a library that is used to build graphic user interfaces (GUIs). The Qt library also includes a set of core classes with strings, hash tables and the link. It also includes a networking library. Qt is one of the best open source libraries available for C++ and if you're building an application with the othe rQt classes, its nature to use the networking classes if you need network communication. Unfortunately the documentation on the networking classes is not very good. When I was struggling with the Qt networking classes I wrote this model application for a simple client/server. I'm posting it here for my own future reference and in the hope that it will help others avoid the hours of trying to figure out why my application was not working.

/**
   This is a model for threaded client/server communication using the Qt QtNetwork class library.

   A threaded server is created that waits for a client connection.  When the client
   connects, the client sends a string (terminated by a \r) to the server.  The server
   reads the string and echos it back to the client.  The client then prints out the string.
*/

#include "windows.h"
#include "QMutex"
#include "QThread"
#include "QTcpServer"
#include "QTcpSocket"
#include "QVector"
#include "QString"

class SocketThreadBase
{
public:
  SocketThreadBase(const QString &ipAddr, const ushort port ) : mIP( ipAddr ), mPort( port )
  {
    mRunThread = false;
  }
  enum Identity {
    BAD_IDENTITY,
    SERVER,
        CLIENT
  };
  virtual void startThread() = 0;
  virtual void stopThread() = 0;
  virtual Identity getIdentity() = 0;
protected:
  QMutex mMutex;
  const QString mIP;
  const ushort mPort;

  void setRunThread( bool newVal );
  bool getRunThread();
  void run();
  QString readLine(QTcpSocket *socket );
  int waitForInput( QTcpSocket *socket );
  void writeLine( QTcpSocket *client, const QString &line );
private:
  bool mRunThread;
};


class EchoServer : public SocketThreadBase, QThread
{
public:
  EchoServer(const QString &ipAddr, const ushort port ) : SocketThreadBase( ipAddr, port ) {}
  void startThread();
  void stopThread();
  Identity getIdentity() { return SERVER; }
protected:
  void run();
  void echoServer( QTcpSocket *client );
};

class EchoClient : public SocketThreadBase, QThread
{
public:
  EchoClient(const QString &ipAddr, const ushort port ) : SocketThreadBase( ipAddr, port ) { mDone = false; }
  void startThread();
  void stopThread();
  bool isDone();
  Identity getIdentity() { return CLIENT; }
protected:
  void run();
  void setDone( const bool newVal );

private:
  static const unsigned int CONNECT_TIME_OUT = 5 * 1000; // 5 seconds
  static const char *mStrings[];
  static const uint mNumStrings;
  bool mDone;
};



void SocketThreadBase::setRunThread(bool newVal )
{
  QMutexLocker lock( &mMutex );
  mRunThread = newVal;
}

bool SocketThreadBase::getRunThread()
{
  QMutexLocker lock( &mMutex );
  return mRunThread;
}


int SocketThreadBase::waitForInput( QTcpSocket *socket )
{
  int bytesAvail = -1;
  while (socket->state() == QAbstractSocket::ConnectedState && getRunThread() && bytesAvail < 0) {
    if (socket->waitForReadyRead( 100 )) {
      bytesAvail = socket->bytesAvailable();
    }
    else {
      Sleep( 50 );
    }
  }
  return bytesAvail;
}

QString SocketThreadBase::readLine(QTcpSocket *socket )
{
  QString line = "";
  int bytesAvail = waitForInput( socket );   
  if (bytesAvail > 0) {
    int cnt = 0;
    bool endOfLine = false;
    bool endOfStream = false;
    while (cnt < bytesAvail && (!endOfLine) && (!endOfStream)) {
      char ch;
      int bytesRead = socket->read(&ch, sizeof(ch));
      if (bytesRead == sizeof(ch)) {
	cnt++;
	line.append( ch );
	if (ch == '\r') {
	  endOfLine = true;
	}
      }
      else {
	endOfStream = true;
      }
    }
  }
  return line;
}


void SocketThreadBase::writeLine(QTcpSocket *socket, const QString &line)
{
  if (line.length() > 0) {
    socket->write( line.toAscii() );
    if (! socket->waitForBytesWritten()) {
      if (getIdentity() == SERVER) {
	printf("Server - ");
      }
      else if (getIdentity() == CLIENT) {
	printf("Client - ");
      }
      else {
	printf("Unknown identity - ");
      }
      printf("writeLine: the write to the socket failed\n");
    }
  }
}

void EchoServer::echoServer(QTcpSocket *client )
{
  QString line;
  do {   
    line = readLine( client );
    if (line.length() > 0) {
      writeLine( client, line );
    }
  } while (line.length() > 0);
}


/**
  Note that Qt requires that the QTcpServer be in the same thread it is used in.
  So it cannot be declared as a class variable.  It must be in the run thread.
*/
void EchoServer::run()
{
  QHostAddress serverAddr( mIP );
  QTcpServer server;
  if (server.listen(serverAddr, mPort)) {
    printf("EchoServer::run: listen() succeeded\n");
    while (server.isListening() && getRunThread()) {
      if (server.waitForNewConnection(100)) {
	printf("EchoServer::run: got a TCP connection\n");
	QTcpSocket *client = server.nextPendingConnection();
	echoServer( client );
      }
      else {
	Sleep( 100 );
      }
    } // while
  }
  else {
    printf("EchoServer::run: listen operation failed\n");
  }
}

void EchoServer::startThread()
{
  setRunThread( true );
  start();
}

void EchoServer::stopThread()
{
  setRunThread( false );
}


void EchoClient::setDone(const bool newVal)
{
  QMutexLocker lock( &mMutex );
  mDone = newVal;
}

bool EchoClient::isDone()
{
  QMutexLocker lock( &mMutex );
  return mDone;
}

/**
    Note that the QTcpSocket object must be in the run thread.  It cannot
    be a class variable.
*/
void EchoClient::run()
{
  QTcpSocket socket;
  QHostAddress hostAddr( mIP );
  socket.connectToHost( hostAddr, mPort );
  if (socket.waitForConnected( CONNECT_TIME_OUT )) {
    QHostAddress hostAddr = socket.localAddress();
    QString addr = "";
    if (hostAddr != QHostAddress::Null) {
      addr = hostAddr.toString();
    }
    printf("Client connected");
    if (addr.length() > 0) {
      printf(" on address %s:%04d", addr.toAscii().data(), socket.localPort() );
    }
    printf("\n");
    int ix = 0;
    while (socket.state() == QAbstractSocket::ConnectedState && ix < mNumStrings) {
      QString line( mStrings[ix] );
      writeLine(&socket, line);
      QString echoedLine = readLine( &socket );
      if (echoedLine.length() > 0) {
	if (line != echoedLine) {
	  printf("line and echoed line doesn't match\n");
	}
	else {
	  printf("%s\n", line.toAscii().data() );
	}
      }
      ix++;
    } // while
  }
  else {
    printf("Client socket failed to connect\n");
  }
  setDone( true );
}


void EchoClient::startThread()
{
  setRunThread( true );
  start();
}

void EchoClient::stopThread()
{
}


const char *EchoClient::mStrings[] = {
  "    'Twas brillig, and the slithy toves\r",
  "        Did gyre and gimble in the wabe:\r",
  "    All mimsy were the borogoves,\r",
  "        And the mome raths outgrabe.\r",
  " \r",
  "    \"Beware the Jabberwock, my son!\r",
  "        The jaws that bite, the claws that catch!\r",
  "    Beware the Jubjub bird, and shun\r",
  "        The frumious Bandersnatch!\"\r",
  " \r",
  "    He took his vorpal sword in hand: \r",
  "        Long time the manxome foe he sought--\r",
  "    So rested he by the Tumtum tree, \r",
  "        And stood awhile in thought. \r",
  " \r",
  "    And, as in uffish thought he stood,\r",
  "        The Jabberwock, with eyes of flame,\r",
  "    Came whiffling through the tulgey wood,\r",
  "        And burbled as it came!\r",
  " \r",
  "    One, two!  One, two!  And through and through\r",
  "        The vorpal blade went snicker-snack!\r",
  "    He left it dead, and with its head\r",
  "        He went galumphing back. \r",
  " \r",
  "    \"And hast thou slain the Jabberwock?\r",
  "        Come to my arm, my beamish boy!\r",
  "    O frabjous day! Callooh!  Callay!\"\r",
  "        He chortled in his joy.\r",
  " \r",
  "    'Twas brillig, and the slithy toves\r",
  "        Did gyre and gimble in the wabe:\r",
  "    All mimsy were the borogoves, \r",
    "        And the mome raths outgrabe\r"
};

const uint EchoClient::mNumStrings = sizeof EchoClient::mStrings / sizeof( *EchoClient::mStrings );



int main(int argc, char *argv[])
{
  const QString IP =     "127.0.0.3";
  const ushort port = 9999;
  EchoServer server(IP, port );
  EchoClient client(IP, port );
  server.startThread();
  client.startThread();
  while (! client.isDone()) {
    Sleep( 100 );
  }
  return 0;
}