QTcpSocket: Cannot Connect in a LAN

Sajjad Ali

Hello,
I have 2 PCs.
PC1 has IP Address 169.254.211.27
So, I am trying to send data from PC2 to PC1 through QTcpSocket
But it has some problem in establishing connection.
I have also tried waitForConnection. but the still cannot see hello message I sent.
The code in PC1:
main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "receiver.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    Receiver a;
    QQmlApplicationEngine engine;
    const QUrl url(u"qrc:/6-TCPLearn/main.qml"_qs);
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreated,
        &app,
        [url](QObject *obj, const QUrl &objUrl) {
            if (!obj && url == objUrl)
                QCoreApplication::exit(-1);
        },
        Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

receiver.h

#ifndef RECEIVER_H
#define RECEIVER_H

#include <QObject>
#include <QTcpSocket>
#include <QHostAddress>

class Receiver : public QObject
{
    Q_OBJECT
public:
    explicit Receiver(QObject *parent = nullptr);
    void readData();
public slots:
signals:
private:
    QTcpSocket *socket;
};

#endif // RECEIVER_H

receiver.cpp

#include "receiver.h"

Receiver::Receiver(QObject *parent)
    : QObject{parent}
{
    socket = new QTcpSocket(this);
    qInfo() << socket->state();
    socket->connectToHost("127.0.0.1", 1234);
    qInfo() << socket->state();
    connect(socket, &QTcpSocket::readyRead, this, &Receiver::readData);
}

void Receiver::readData()
{
    QByteArray data = socket->readAll();
    qInfo() << "Receiver: " << data;
}

The Code in PC2:
main.cpp


#include <QCoreApplication>
#include "sendtcp.h"
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    SendTCP socket;
    socket.sendData();
    return a.exec();
}

sendtcp.h

#ifndef SENDTCP_H
#define SENDTCP_H

#include <QObject>
#include <QtNetwork/QTcpSocket>
class SendTCP : public QObject
{
    Q_OBJECT
public:
    explicit SendTCP(QObject *parent = nullptr);
    void sendData();
signals:
private:
    QTcpSocket *socket;
};

#endif // SENDTCP_H

sendtcp.cpp

#include "sendtcp.h"

SendTCP::SendTCP(QObject *parent)
    : QObject{parent}
{
    socket = new QTcpSocket(this);
    socket->connectToHost("169.254.211.27",1234);
}

void SendTCP::sendData()
{
    QByteArray data;
    data = "Hello";
    socket->write(data);
}

JonB

@Sajjad-Ali
Your code does a connectToHost() immediately followed by a socket->write(data). Either put the write in a slot on connected() signal or call waitForConnected() --- your code does neither.

I don't think you will get connected() signal given that you call this before starting the Qt event loop (a.exec()). I don't know about waitForConnected() in that situation. Also docs claim taht method may not work well under Windows. You might want delay sendData() until that loop has started.

If it does not work connect a slot to void QAbstractSocket::errorOccurred(QAbstractSocket::SocketError socketError) and show result. You should always do this when writing code for production, and even more so while developing.

ChrisW67

@Sajjad-Ali Quite apart from @JonB's observations:

Your receiver is not listening. You need to look at QTcpServer
You need to ensure that the receiving end does not have a system firewall blocking whatever port you are listening on.