QTcpSocket always connectingState

[Shen sida]

Hi all

I try to connect a tcp socket to send data. But after connectToHost(), the state of socket is connectingState not the connectedState.

I also set the option as follow:

socket->setSocketOption(QAbstractSocket::KeepAliveOption,1);
socket->setSocketOption(QAbstractSocket::LowDelayOption,1);

I'm working on Qt 6.4.2 on Windows 11.
What should I do?
Let me know if you need more information.

Best regards

[J.Hilk]

@Shen-sida more information please,

my 0815 guess is, you're immediately checking the state, and do not listen to the state changed signal and therefore never progress. The connectedState will only come when the event loop is running and processing events.

[Shen sida]

@J-Hilk Thanks for reply first.

I have listen the state changed signal before connectToHost(), see the code as follow.

void CenterManageSystem::init()
{    
    socket = new QTcpSocket(this);

    socket->setSocketOption(QAbstractSocket::KeepAliveOption,1);
    socket->setSocketOption(QAbstractSocket::LowDelayOption,1);

    connect(socket, &QTcpSocket::connected, this, &CenterManageSystem::socketConnected);
    connect(socket, &QTcpSocket::disconnected, this, &CenterManageSystem::socketDisconnected);
    connect(socket, &QTcpSocket::stateChanged, this, &CenterManageSystem::socketStateChanged);
    connect(socket, &QTcpSocket::readyRead, this, &CenterManageSystem::socketReceivedData);

    socket->connectToHost("192.168.5.181",24680);
}

void CenterManageSystem::socketConnected()
{
    qDebug()<<"socket connected";
}

void CenterManageSystem::socketDisconnected()
{
    qDebug()<<"CMS socket disconnected";
}

void CenterManageSystem::socketStateChanged(QAbstractSocket::SocketState state)
{
    qDebug()<<"Socket state changed:"<<state;
}

void CenterManageSystem::socketReceivedData()
{
    receiveData.append(socket->readAll());

    // TODO process data
}

void CenterManageSystem::socketError(QAbstractSocket::SocketError error)
{
    qDebug()<<error;
}

Then I only get the output:

Socket state changed: QAbstractSocket::HostLookupState
Socket state changed: QAbstractSocket::ConnectingState

[Christian Ehrlicher]

And where do you actually connect to an endpoint?

[Shen sida]

@Christian-Ehrlicher
For now I connect it on the same device, that means the server and this program are running on the same devices.
I also tried connect with the loop back IP (127.0.0.1) but it was still in connectingState .

[Christian Ehrlicher]

Ok, now there is a connect. But do you listen and accept on the server side?

[Shen sida]

@Christian-Ehrlicher Sure, the server has been finished and used with other programs.

[Christian Ehrlicher]

I doubt the server accepts the connection or even get a connection. Make sure there is no firewall and the server really gets a connection request.

[Shen sida]

@Christian-Ehrlicher I turn off all the firewalls and try again, but it'is still in the connecting state.

[JonB]

@Shen-sida
Test your server outside of a Qt program, e.g. telnet 192.168.5.181 24680. Does that actually connect?

[SGaist]

Hi,

To add to my fellow, you did not connect the errorOccurred signal.

[JoeCFD]

@Shen-sida You can also ping your server to see if it is visible.

[Shen sida]

@JoeCFD @SGaist

I did more tests and there are some questions about that:

1. when I want to reconnect the socket, there is an error id 19, what is it.

// In my tick function
 if(QAbstractSocket::ConnectedState != socket->state() && socketTimer.elapsed() >= 5000)
    {
        qDebug()<<"Try to reconnect socket";
        socket->disconnectFromHost();
        socket->connectToHost("192.168.5.233",24680);
        socketTimer.restart();
    }

// output as follow
Try to reconnect socket
QAbstractSocket::connectToHost() called when already looking up or connecting/connected to "192.168.5.233"
QAbstractSocket::SocketError(19)

2. I try to setup the socket in main theard and it work well! But it doesn't work if the socket setup in other thread.

3. What's the different between normal object pointer and QPointer<>? I'm still using QPointer<> to declarate this task with tcp socket

class CenterManageSystem : public QObject
{
    ...
private:
    QTcpSocket *socket;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
    QPointer<CenterManageSystem> pCMS = nullptr;
};

// move the object to thread
        pCMS = new CenterManageSystem();
        CenterManageSystemThread = new QThread();
        QThread::connect(CenterManageSystemThread, &QThread::started, pCMS , &CenterManageSystem::main, Qt::DirectConnection);
        pCMS ->moveToThread(CenterManageSystemThread);
        CenterManageSystemThread->start();

[jsulm]

@Shen-sida said in QTcpSocket always connectingState:

QAbstractSocket::connectToHost() called when already looking up or connecting/connected to "192.168.5.233"

This seems to be your problem.

"But it doesn't work if the socket setup in other thread" - why do you want to have the socket in another thread? Qt is assynchronous. If you really want to do that then you have to make sure the QTcpSocket instance is created in another thread (when CenterManageSystem::main is called).

[Shen sida]

@jsulm said in QTcpSocket always connectingState:

If you really want to do that then you have to make sure the QTcpSocket instance is created in another thread (when CenterManageSystem::main is called).

Sure, the instance will be created in &CenterManageSystem::main.
Because this socket will process much data so I move it to another thread then the data can be sent by signal to update UI.