Question

I am writing a server as a Qt console application. I have the server set up to wait for a socket connection, but I also need to allow a user to input commands into the server for managing it. Both are working independently. However, the problem I ran into is that when I’m in a while loop accepting and processing input commands, the server doesn’t accept connections.

I have a Socket class, and in its constructor, I have:

connect(server,SIGNAL(newConnection()),this, SLOT(newConnection()));

Right under that in the constructor, I call a function that has a more in-depth version of this for getting commands from the user:

QTextStream qin(stdin, QIODevice::ReadOnly);
QString usrCmd;

while(usrCmd != "exit" && usrCmd != "EXIT") {   
    //Get command input and process here
}

Inside newConnection(), I just accept the next connection and then use the socket.

QTcpSocket *serverSocket = server->nextPendingConnection();

How can I make it so the socket can wait for connections and wait for user-inputed commands at the same time?

Answer 1

Problem with your code is because you are blocking event loop with your while loop. So, the solution to your problem is to read from stdin asynchronously. On Linux (and on Mac, I guess), you can use QSocketNotifier to notify when the data is arrived on stdin, and to read it manually), as per various internet sources.

As I am using Windows, I would suggest you to do it in this way (which should work on all platforms):

1. Open the thread for reading data from stdin
2. Once you get some data (perhaps line?) you can use Qt signal-slot mechanism to pass the data to main thread for processing without blocking the event loop.

So, this is the pseudocode. MainAppClass should your existing server class, just edit the constructor to create new thread, and add new slot for processing the data.

class Reader: public QThread
{
    Q_OBJECT
 public:
    Reader(QObject * parent = 0 ): QThread(parent){}

    void run(void)
    {
         forever{
             std::string data;
             std::getline (std::cin, data);

             if(data == "exit")
             {
                 emit exitServer();
                 return;
             }


             emit dataReady(QString::fromStdString(data));
         }
     }

signals:
    void dataReady(QString data);
    void exitServer();

};



class MainAppClass: public QObject
{
    Q_OBJECT
public:
    MainAppClass()
    {
        Reader * tr = new Reader(this);
        connect(tr, SIGNAL(dataReady(QString)), this, SLOT(processData(QString)));
        connect(tr, SIGNAL(exitServer()), this, SLOT(exitServer()));
        tr->start();
    }

public slots:
    void processData(QString data)
    {

        std::cout << "Command: " << data.toStdString() << std::endl;
    }

    void exitServer()
    {
       std::cout << "Exiting..." << std::endl;
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainAppClass myapp; //your server

    app.exec();
    return 0;
}

Since I wrote simple guidelines how to use QTcpSocket, here is the brief

When you get client QTcpSocket, connect readyRead() signal to some slot, and read data from sender() object. You don’t need to read anything in the constructor.

For reading you can use standard QIODevice functions.

Note: this is pseudo code, and you may need to change few things (check the state of the stream on reading, save pointer to sockets in some list, subscribe to disconnected() signal, call listen() in constructor, check if QTcpServer is listening, etc).

So, you need to have slot onReadyRead() in your class which will have the following code:

void Server::readyReadSlot()
{
    QTcpSocket *client = (QTcpSocket*)sender(); // get socket which emited the signal
    while(client->canReadLine()) // read all lines! 
                                 // If there is not any lines received (you may not always receive 
                                 // whole line as TCP is stream based protocol),
                                 // you will not leave data in the buffer for later processing.
    {
        QString line = client->readLine(); 
        processLine(line); // or emit new signal if you like
    }
}

Inside newConnection() you need to connect readyRead() signal with your slot.

void Server::newConnection()
{
    QTcpSocket *clientSocket = server->nextPendingConnection(); 
    connect(clientSocket, SIGNAL(readyRead()), this, SLOT(readyReadSlot()));
}