CODEWITHSUNDEEP
portqtcpsocket
wasp256
wasp256

Reputation: 6242

Bind a QTcpSocket to a specific port

I am connecting through a QTcpSocket to a QTcpServer. I can specify the listening port on the Server side, but the client chooses a random port for its connection. I have tried to use the method QAbstractSocket::bind but that made no difference.

Here is my code: 

void ConnectionHandler::connectToServer() {
     this->socket->bind(QHostAddress::LocalHost, 2001);
     this->socket->connectToHost(this->ip, this->port);

     if (!this->socket->waitForConnected()) {
           this->socket->close();
           this->errorMsg = this->socket->errorString();
      }

     qDebug() << this->socket->localPort();
}

Does anyone know what I'm missing?

Upvotes: 2

Views: 3701

Answers (1)

Toby Speight
Toby Speight

Reputation: 30968

I reformulated your code into a MCVE: 

#include <QDebug>
#include <QHostAddress>
#include <QTcpSocket>

#include <memory>

int main()
{
    std::unique_ptr<QTcpSocket> socket(new QTcpSocket);

    socket->bind(QHostAddress::LocalHost, 2001);
    qDebug() << socket->localPort(); // prints 2001

    socket->connectToHost(QHostAddress::LocalHost, 25);
    qDebug() << socket->localPort(); // prints 0
}

Why does connectToHost reset the local port to 0?

This appears to be a bug in Qt. In version 5.2.1, QAbstractSocket::connectToHost contains

d->state = UnconnectedState;
/* ... */
d->localPort = 0;
d->peerPort = 0;

In version 5.5, this has changed to

if (d->state != BoundState) {
    d->state = UnconnectedState;
    d->localPort = 0;
    d->localAddress.clear();
}

So it's likely that upgrading your Qt will fix the issue.

Upvotes: 4

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