UDP packets received only the first time.

[Eli_a_n]

Hi All, i'm having this weird problem, i only receive the udp packets the first time i send them, all after that aren't shown anymore.

Here is what i have, can you please help me figure out what the problem is ?:

receiver.h

#ifndef RECEIVER_HPP
#define RECEIVER_HPP

#include <QtCore/QObject>

class QUdpSocket;

/**
     * The Receiver class handles the processing of the incoming UDP datagrams.
     * It uses a QUdpSocket to retrieve the datagrams and makes the content of
     * the datagram available to the UI via the 'status' property.
     */
class Receiver : public QObject
{
    Q_OBJECT
    
    // The property that makes the content of a received datagram available to the UI
    Q_PROPERTY(QString status READ status NOTIFY statusChanged)
    
public:
    void initSocket();
    Receiver(QObject *parent = 0);
    
Q_SIGNALS:
    // The change notification signal of the status property
    void statusChanged();
    
private slots:
    void processPendingDatagrams();
    
private Q_SLOTS:
    // This method is called whenever a new datagram arrived on the socket
    //    void processPendingDatagrams();
    
private:
    // The accessor method of the status property
    QString status() const;
    
    // The UDP socket object
    QUdpSocket *m_udpSocket;
    
    // The content of the received datagram
    QString m_status;
};

#endif

receiver.cpp

#include "receiver.h"
#include <QThread>
#include <QtNetwork/QUdpSocket>

Receiver::Receiver(QObject *parent)
    : QObject(parent)
{
    /**
         * Initialize the 'status' property with no data, it won't
         * be shown in the UI until the first datagram arrives.


*/
    m_status = "--";

    m_udpSocket = new QUdpSocket(this);
        m_udpSocket->bind(QHostAddress::AnyIPv4, 4210);

        processPendingDatagrams();
        connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(processPendingDatagrams()), Qt::QueuedConnection);

    /*
    // Create a new UDP socket and bind it against port 45454
    m_udpSocket = new QUdpSocket(this);
    if(m_udpSocket->bind(QHostAddress::LocalHost, 4210))
        qDebug() << "Bind true on port" << 4210;
    else
        qDebug() << "Bind super failed";



         * Create signal/slot connection to invoke processPendingDatagrams() whenever
         * a new datagram is received by the socket.

  //  connect(m_udpSocket, SIGNAL(readyRead()), this,
   //         SLOT(processPendingDatagrams()));*/
}

void Receiver::initSocket(){
    QByteArray Data;
        Data.append("Hello from UDP");

        // Sends the datagram datagram
        // to the host address and at port.
        // qint64 QUdpSocket::writeDatagram(const QByteArray & datagram,
        //                      const QHostAddress & host, quint16 port)

//             m_udpSocket->writeDatagram(Data, QHostAddress::LocalHost, 4210);

}

void Receiver::processPendingDatagrams()
{

    // Now read all available datagrams from the socket
    while (m_udpSocket->hasPendingDatagrams()) {


        // Create a temporary buffer ...
        QByteArray datagram;

        QHostAddress sender;
        quint16 senderPort;
    qDebug() << "2 ";
        // ... with the size of the received datagram ...
        datagram.resize(m_udpSocket->pendingDatagramSize());

        // ... and copy over the received datagram into that buffer.
        m_udpSocket->readDatagram(datagram.data(), datagram.size(), &sender, &senderPort);

        // Update the 'status' property with the content of the received datagram
  //      m_status = tr("%1").arg(datagram.data());



         qDebug() << "Message from: " << sender.toString();
         qDebug() << "Message port: " << senderPort;
         qDebug() << "Message: " << datagram;



    //    emit statusChanged();
    }
}

QString Receiver::status() const
{
    return m_status;
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include<QDebug>
#include <QQmlEngine>
#include <QQmlComponent>
#include <QQmlProperty>
#include "receiver.h"
#include <QQmlContext>
#include "touchitem.h"
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(int argc, char *argv[])
{



    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

 //   QCoreApplication::setAttribute(Qt:QT_QPA_EGLFS_HIDECURSOR, true);
    QCoreApplication::setAttribute(Qt::AA_SynthesizeTouchForUnhandledMouseEvents, true);



    QGuiApplication app(argc, argv);
    app.setAttribute(Qt::AA_SynthesizeTouchForUnhandledMouseEvents, true);
    QQmlApplicationEngine engine;
        QObject::connect(&app, &QGuiApplication::aboutToQuit, &thread, &Thread::stop);

    engine.rootContext()->setContextProperty("applicationDirPath", QGuiApplication::applicationDirPath());
    const QUrl url(QStringLiteral("qrc:/main.qml"));


    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);

    }, Qt::QueuedConnection);

    engine.load(url);
    QString test = "";


    Receiver receiver ;
   receiver.initSocket();

    return app.exec();
}

[6thC]

I don't have time to help your specific example (sorry). However, I'd suggest looking at the functional example projects in Qt Creator. Look for something like "fortune server" and "fortune client" there's a couple examples that should get you right on track.

[jsulm]

@Eli_a_n said in UDP packets received only the first time.:

i only receive the udp packets the first time i send them, all after that aren't shown anymore

What does this mean exactly? Do you stop and start your app and then it does not work?

[Eli_a_n]

@jsulm Hi,

So, i start the app, i send an udp packet from another terminal in the local network, i receive and display it in qt, everything is fine so far. With the app still running, and without changing anything, i send the second packet from the same terminal in the local network and i don't receive it (or it just doesn't display it) in qt.

It's absolutely weird.

What are your thoughts?

[JonB]

@Eli_a_n

        processPendingDatagrams();
        connect(m_udpSocket, SIGNAL(readyRead()), this, SLOT(processPendingDatagrams()), Qt::QueuedConnection);

Probably irrelevant, but I'd get rid of the first processPendingDatagrams();, outside of the connect(SIGNAL(readyRead())), just in case it influences what you see. There is no need for it in your test problem, is there?

m_udpSocket->bind(QHostAddress::AnyIPv4, 4210);

bind() returns a bool. If you don't c heck it, maybe the whole binding isn't working....

void QAbstractSocket::errorOccurred(QAbstractSocket::SocketError socketError): if you don't slot this signal, how would you know if there is some error?

These are the sort of things I would do if developing and I had a problem.

[Eli_a_n]

@JonB thanks for the heads up

Here is what i did:

Receiver::Receiver(QObject *parent)
    : QObject(parent)
{
    m_status = "--";

    m_udpSocket = new QUdpSocket(this);
        if(m_udpSocket->bind(QHostAddress::AnyIPv4, 4210))
            qDebug() << "Bind succesfull on port 4210" ;
        else
            qDebug() << "Bind failed on port 4210";

    
        connect(m_udpSocket, SIGNAL(readyRead()), this,
                SLOT(processPendingDatagrams()), Qt::QueuedConnection);
        connect(m_udpSocket, SIGNAL(disconnected()), this,
                SLOT(Disconnected()));
        connect(m_udpSocket, static_cast<void(QAbstractSocket::*)(QAbstractSocket::SocketError)>(&QAbstractSocket::error),
            [=](QAbstractSocket::SocketError socketError){ qDebug() << "socket error " << socketError; });
}

void Receiver::processPendingDatagrams()
{
    while (m_udpSocket->hasPendingDatagrams()) {

        QByteArray datagram;

        QHostAddress sender;
        quint16 senderPort;
    qDebug() << "2 ";
        datagram.resize(m_udpSocket->pendingDatagramSize());
        m_udpSocket->readDatagram(datagram.data(), datagram.size(), &sender, &senderPort);

         qDebug() << "Message from: " << sender.toString();
         qDebug() << "Message port: " << senderPort;
         qDebug() << "Message: " << datagram;

    }
}

void Receiver::ReceiveData ()
  {
     qDebug() <<"Socket received data...";
  }

  void Receiver::Disconnected ()
  {
    qDebug() <<"Socket got disconnected...";
  }

  void Receiver::Error (QAbstractSocket::SocketError error)
  {
     qDebug() <<"Socket error " << error;
  }

but i get o disconnect message nor an error message:

pi@raspberrypi:~/RTCs/bin $ ./RTCs
QML debugging is enabled. Only use this in a safe environment.

Bind succesfull on port 4210
Message from:  "192.168.68.111"
Message port:  52848
Message:  "test\r\n"

[SGaist]

Hi,

Did you check the examples linked in the details of the QUdpSocket class ?

Compare them to your client and server code to spot the differences.

On a side note, you should move all your connection to the new style so you won't have any runtime surprises.

[JonB]

@Eli_a_n said in UDP packets received only the first time.:

i only receive the udp packets the first time i send them, all after that aren't shown anymore.

One thing. I've seen a fair amount of user questions in this forum. If I come over to your place and find that actually you are only sending one packet, will you give me $5? :) Please verify multiple packets really are being sent...!

[Eli_a_n]

@JonB :)) that would be funny but i'm actually sending the packets using software called UDP - Sender/Reciever from Microsoft Store. To make sure that i have no problem with firewall or anything like that i made a simple app in python and i'm getting all the packets just fine there. So there is a problem with the QT app.

I will try to investigate some more.

By the way, not sure if it makes any difference but i'm running the QT app on raspberry pi.

Regards

[Eli_a_n]

Not sure i'm loading the class correctly.

in my main.cpp i do :

Receiver *rcv = new Receiver();
rcv->initSocket();

where initSocket is:

void Receiver::initSocket(){
    QByteArray Data;
        Data.append("Hello from UDP");

        m_udpSocket = new QUdpSocket(this);
            if(m_udpSocket->bind(QHostAddress::AnyIPv4, 4210))
                qDebug() << "Bind succesfull on port 4210" ;
            else
                qDebug() << "Bind failed on port 4210";


            connect(m_udpSocket, SIGNAL(readyRead()), this,
                    SLOT(processPendingDatagrams()), Qt::QueuedConnection);
            connect(m_udpSocket, SIGNAL(disconnected()), this,
                    SLOT(Disconnected()));
            connect(m_udpSocket, static_cast<void(QAbstractSocket::*)(QAbstractSocket::SocketError)>(&QAbstractSocket::error),
                [=](QAbstractSocket::SocketError socketError){ qDebug() << "socket error " << socketError; });

        // Sends the datagram datagram
        // to the host address and at port.
        // qint64 QUdpSocket::writeDatagram(const QByteArray & datagram,
        //                      const QHostAddress & host, quint16 port)

//             m_udpSocket->writeDatagram(Data, QHostAddress::LocalHost, 4210);

}

shoud i start the Receiver class in a thread or something?

using the debuger, after i receive the first packet, using the "step over" i end up back in the main.cpp at the line :
return app.exec();

Is this normal ?

[Christian Ehrlicher]

Please simply use the examples from Qt to see if it works for you as @SGaist already told you. There we know the code exactly and we know they're working fine for us. We don't know your program and you don't provide a minimal, compilable example so we can't see what else you're doing.

See e.g. https://doc.qt.io/qt-5/qtnetwork-broadcastreceiver-example.html and https://doc.qt.io/qt-5/qtnetwork-broadcastsender-example.html

[Eli_a_n]

@Christian-Ehrlicher Hi, what do you mean? in my initial post i provided the main.cpp and both full receiver files, what else would you need? Anyway,

I loaded the broadcast receiver project you suggested and currently i have the following cases:

1. Launching from one terminal the broadcast receiver app you suggested and from another terminal using netcat -u 127.0.0.1 45454 and netcat -u 192.168.68.107 45454 (which is the IP of my raspberry pi on the local network) i get all the messages that i'm sending

2. without changing anything in the case 1 above, i'm sending the packets from:

• Ubuntu terminal - netcat -u 192.168.68.107 45454
• Windows UDP - Sender/receiver application
• Packet sender application on windows

From none of the apps listed above manage to receive the packets

3. Closing the broadcast receiver app and opening a simple python udp receiver i can receive the packets from all mentioned at case 2

4. Closing the python receiver app and opening my initial application i can receive only the first packet sent.

Any thoughts ?

Regards

[Eli_a_n]

OK, so i don't know exactly why but I got it working.

I believe that the following were the problems:

m_udpSocket->bind(QHostAddress::AnyIPv4, 4210) 
**instead of** 
udpSocket->bind(45454, QUdpSocket::ShareAddress); as mentioned in the broadcast receiver example

connect(m_udpSocket, &QUdpSocket::readyRead,  this, &Receiver::processPendingDatagrams);
**instead of** 
connect(m_udpSocket, SIGNAL(readyRead()), this,  SLOT(ReceiveData()), Qt::QueuedConnection); what i used in my initial application when posting for help

    while (m_udpSocket->hasPendingDatagrams()) {
        datagram.resize(int(m_udpSocket->pendingDatagramSize()));
        m_udpSocket->readDatagram(datagram.data(), datagram.size());

        qDebug() << "Message: " << datagram.constData();
    }

**instead of** 

while (m_udpSocket->hasPendingDatagrams()) {
        QByteArray datagram;

        QHostAddress sender;
        quint16 senderPort;
    qDebug() << "2 ";
        datagram.resize(m_udpSocket->pendingDatagramSize());
        m_udpSocket->readDatagram(datagram.data(), datagram.size(), &sender, &senderPort);


         qDebug() << "Message from: " << sender.toString();
         qDebug() << "Message port: " << senderPort;
         qDebug() << "Message: " << **datagram;**

//as per my initial program
    }

Thank you all for all your involvement and support
Regards
Elian N.