UDP Network. receiving pakets

[muhuhn]

I am trying to write a programm that communicates with a server over the network. the server implementation is given, i can not change it.

the servers IP address is the 141.47.69.35 and the server aplication listens to port 11111. the server answers to the source port of the request. as a protocoll udp is used. the example code works fine for transmitting data to the server. with a network sniffer i can see that the server responds correcly. but the function readPendingDatagrams() is never called. i gues there is anything wrong with the ports, but i really have no more idea...

#include "cmyudp.h"
#include "iostream"
 
using namespace std;
 
MyUDP::MyUDP(QObject *parent) : QObject(parent)
{
    // initialize port
    port = 11111;
 
    // create a QUDP socket
    udpReceiveSocket = new QUdpSocket(this);
 
    // creat host ip
    hostIp = new QHostAddress("141.47.69.35");
 
    // bind socket
    udpReceiveSocket->connectToHost(*hostIp, port, QIODevice::ReadWrite);
 
    // setup receiver calllback function
    connect(udpReceiveSocket, SIGNAL(readyRead()), this, SLOT(readPendingDatagrams()))
 
}
 
void MyUDP::HelloUDP()
{
    // create send data
    QByteArray Data;
    Data.append("Hallo Hilfe!");
    udpReceiveSocket->write(Data);
}
 
void MyUDP::readPendingDatagrams()
{
    cout << "received!" << endl;
    while (udpReceiveSocket->hasPendingDatagrams()) {
        cout << "received!" << endl;
        }
}

#include "cmyudp.h"
#include "iostream"

using namespace std;

MyUDP::MyUDP(QObject *parent) : QObject(parent)
{
    // initialize port
    port = 11111;

    // create a QUDP socket
    udpReceiveSocket = new QUdpSocket(this);

    // creat host ip
    hostIp = new QHostAddress("141.47.69.35");

    // bind socket
    udpReceiveSocket->connectToHost(*hostIp, port, QIODevice::ReadWrite);

    // setup receiver calllback function
    connect(udpReceiveSocket, SIGNAL(readyRead()), this, SLOT(readPendingDatagrams()))

}

void MyUDP::HelloUDP()
{
    // create send data
    QByteArray Data;
    Data.append("Hallo Hilfe!");
    udpReceiveSocket->write(Data);
}

void MyUDP::readPendingDatagrams()
{
    cout << "received!" << endl;
    while (udpReceiveSocket->hasPendingDatagrams()) {
        cout << "received!" << endl;
        }
}

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[anda_skoa]

You are missing the bind() call.

Cheers,
_

[muhuhn]

Thanks for your response.
Is it possible to mix bind() and connectToHost()? As i understood either the one or the other should be used. Where is the bind() missing? i also tryed solutions only with bind() and the writeDatagram() functions. the result was the same: receiving a paket didn't work.

[anda_skoa]

bind() is needed if you want your UDP socket to listen for data.

connectToHost() is just a convenience for not having to specifiy the receiver when sending packets. I doubt it would conflict with bind().

"Connect" doesn't have any inherent meaning for UDP, the function is in QUdpSocket because it is in its base class and the developers decided to make it do something useful (allowing to you use write without specifying the receipient) instead of it just doing nothing (or generating an error),

Cheers,
_

[muhuhn]

OK, i think i got this. How can i bind a socket to the port that is used as a source port for transmitting? The transmitter picks its source port randomly, thats fine so far. But how do i tell the socket that it has to listen to that randomly choosen port? It would not be a Problem if the Server response would go to a defined port at client side. But i have to listen to a port that i actually don't know when calling bind().

Thanks for your help.

[anda_skoa]

Just to be sure: have you checked if binding to a port makes this also the port used for sending?

If not, you could try connectToHost() and then accessing localPort() for the value to bind()

Cheers,
_