QTcpSocket never emits readyRead when connected

  • I have been connecting to a network through an ssh command on a Linux machine:

    ssh user@host.colo.edu
    and then I am prompted in the terminal for a password:

    at which point, I am connected to the network. I then run netcat (a simple Unix utility that reads and writes data across network connections, using the TCP or UDP protocol) with the command:

    nc -l -p 1313
    where 1313 is the port number and JSON messages begin to appear in my terminal window. I would like to use Qt to have a similar result. I am open to suggestions, but I believe I need to use QTcpSocket with a QNetwork Proxy. Here is my mainwindow.cpp (I have changed the host, user, and password to something generic):

    #include "mainwindow.h"
    #include "ui_mainwindow.h"
    MainWindow::MainWindow(QWidget *parent) :
        ui(new Ui::MainWindow)
        serverSocket = new QTcpSocket(this);
        connect(serverSocket, SIGNAL(hostFound()),this,SLOT(hostFoundSlot()));
        QNetworkProxy proxy;
        delete ui;
    void MainWindow::readyReadSlot()
        qDebug() << "reading...";
        while (!serverSocket->atEnd())
         QByteArray data = serverSocket->read(100);
         int istop = 0;
    void MainWindow::connectedSlot()
        qDebug() << "connected...";
    void MainWindow::hostFoundSlot()
        qDebug() << "found host...";
    void MainWindow::errorSlot(QAbstractSocket::SocketError)
        qDebug() << "an error...";
    void MainWindow::proxyAuthenticationRequiredSlot(QNetworkProxy, QAuthenticator *)
        qDebug() << "authentication...";
    void MainWindow::stateChangedSlot(QAbstractSocket::SocketState socketState)
        qDebug() << "state changed...";

    And here is my mainwindow.h:

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H
    #include <QMainWindow>
    #include <QTcpSocket>
    #include <QNetworkProxy>
    namespace Ui {
    class MainWindow;
    class MainWindow : public QMainWindow
        explicit MainWindow(QWidget *parent = 0);
        QTcpSocket *serverSocket;
    public slots:
        void readyReadSlot();
        void connectedSlot();
        void hostFoundSlot();
        void errorSlot(QAbstractSocket::SocketError);
        void proxyAuthenticationRequiredSlot(QNetworkProxy,QAuthenticator*);
        void stateChangedSlot(QAbstractSocket::SocketState socketState);
        Ui::MainWindow *ui;
    #endif // MAINWINDOW_H

    And here is the output from debug:

    Debugging starts
    state changed...
    state changed...
    found host...
    state changed...
    Debugging has finished

    At no point do I have any errors or authentication requests, but readyRead signal never emits and readyReadSlot never gets called? Am I missing something?

    [Code tags added ~kshegunov]

  • Lifetime Qt Champion

    Hi and welcome to devnet,

    Something strange here, you seem to log through SSH and then, in your code, you are using a proxy which is not the same thing AFAIK.

  • Thanks. I want to read messages across a network connection. I was explaining how I can see the messages on Linux. I first ssh into the network and then I can run netcat , a simple Unix utility that reads and writes data across network connections, using the TCP or UDP protocol.
    Based on that description, I assumed I would need a QTcpSocket. I want to use my Windows computer and my Qt application to achieve the same end. I don't think I need to ssh, but that is the method I use in Linux. What do you suggest instead?

  • Lifetime Qt Champion

    First you need to determine whether that machine is accessible from the outside on that port and how.

  • Thanks for explaining! Reading data across network connections is a new endeavor for me. I am confused as to what you mean by accessible to the outside on that port. I thought since I had successfully connected using that port number that would mean that it is accessible. My qdebug messages imply I have connected.

  • Lifetime Qt Champion

    There's one other thing that is strange, the documentation of netcat explicitly state that you should not use -p and -l together. So what exactly are you connecting to ?

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