QTcpSocket connecting results in UnknownSocketError with errorString "UnknownError"

Question

Problem

I am unable to get any further information regarding this error:

QAbstractSocket::UnknownSocketError

The QT QAbstractSocket::SocketError provides only a basic explanation that some error has occurred

An unidentified error occurred.

enum value = -1

Calling QTcpSocket::errorString() provides this:

"Unknown error"

There is one question regarding this here on SO but provides no real solution to solving the issue (and what was suggested I have done)

I have absoltely no idea how to further progress with this error, since each time my client attempts to connect (after calling connectToHost()) I get this error.

Code:

//Server

//...
if (tcpServer.listen(QHostAddress().AnyIPv4, 5000)) {
    qDebug() << "tcpserver started on port : 5000";
}
else{
    qDebug() << "tcpserver failed to start";
}
//...

I also went on to explicitly set the server ip to localhost and port 5000, but without success.

//Client

//...
tcp_con = new QTcpSocket(new QObject());
tcp_con->connectToHost("127.0.0.1", 5000);

switch (tcp_con->error()) {
    //...
    case QAbstractSocket::UnknownSocketError:
    qDebug() << "tcp error UnknownSocketError closed : " << tcp_con->errorString();
    return;
    //...
}

Client debug output:

tcp error UnknownSocketError closed :  "Unknown error"

Any advice?

p.s. I looked for some stacktrace/backtrace option, didn't find anything - if there is, please leave a comment

Answer 1

It is wrong to check an error immediately after the connectToHost(), because this is not a complete action, and the errorString() will always return "Unknown error". You have to call the QAbstractSocket::waitForConnected() method like this:

tcp_con->connectToHost("127.0.0.1", 5000);
if (tcp_con->waitForConnected(1000))
    qDebug("Connected!");

Or you can don't call the waitForConnected() and asynchronously wait while the signal connected() will be emitted:

connect(tcp_con, SIGNAL(error(QAbstractSocket::SocketError)),
        this, SLOT(onError(QAbstractSocket::SocketError)));
    connect(tcp_con, SIGNAL(connected()),
        this, SLOT(onConnect()));
//...
void MyClass::onConnect()
{
    qDebug() << "Connected!";
}

void MyClass::onError(QAbstractSocket::SocketError)
{  
    QTcpSocket* sock = (QTcpSocket*)sender();
    qDebug() << "Socket error:" << sock->errorString();
}