How to receive proper UDP packet in QT? - c++

Question

I am trying write a QT program to receive UDP packet.I am trying to receive from Packet Sender software
This is my code
socket = new QUdpSocket(this);
bool result = socket->bind(QHostAddress("150.100.50.88"),45454);
qDebug() << result;
if(result)
{
qDebug << "PASS";
}
else
{
qDebug << "FAIL";
}
processPendingDatagrams();
connect(socket, SIGNAL(readyRead()), this, SLOT(processPendingDatagrams()),Qt::QueuedConnection);
void UDP::processPendingDatagrams()
{
QHostAddress sender;
u_int16_t port;
while (socket->hasPendingDatagrams())
{
QByteArray datagram;
datagram.resize(socket->pendingDatagramSize());
socket->readDatagram(datagram.data(),datagram.size(),&sender,&port);
qDebug() <<"Message From :: " << sender.toString();
qDebug() <<"Port From :: "<< port;
qDebug() <<"Message :: " << datagram;
} //! [2]
}
UDP.h:
class UDP : public QObject
{
Q_OBJECT public:
explicit UDP(QObject *parent = 0);
signals:
public slots:
void SendDatagram(u_int8_t,u_int8_t,u_int8_t);
private slots:
void processPendingDatagrams();
private :
QUdpSocket *socket;
};
The readReady signal and corresponding slot are not working . I can see the packets in Wireshark.
If a I try receive the packets in continuously in a loop I am able see the datagrams.What can be the reason for signals and Slots for not working.Sending operation is working well.

Answer 1

This code work for me. Try it please.
.pro:
#-------------------------------------------------
#
# Project created by QtCreator 2017-03-10T11:44:19
#
#-------------------------------------------------
QT += core gui network
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = test
TEMPLATE = app
SOURCES += main.cpp\
mainwindow.cpp
HEADERS += mainwindow.h
FORMS += mainwindow.ui
mainwindow.cpp:
#include "mainwindow.h"
#include <QDebug>
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent)
{
socket = new QUdpSocket(this);
bool result = socket->bind(QHostAddress::AnyIPv4, 45454);
qDebug() << result;
if(result)
{
qDebug() << "PASS";
}
else
{
qDebug() << "FAIL";
}
processPendingDatagrams();
connect(socket, SIGNAL(readyRead()), this, SLOT(processPendingDatagrams()),Qt::QueuedConnection);
}
MainWindow::~MainWindow()
{
}
void MainWindow::processPendingDatagrams()
{
qDebug() << "in !";
QHostAddress sender;
u_int16_t port;
while (socket->hasPendingDatagrams())
{
QByteArray datagram;
datagram.resize(socket->pendingDatagramSize());
socket->readDatagram(datagram.data(),datagram.size(),&sender,&port);
qDebug() <<"Message From :: " << sender.toString();
qDebug() <<"Port From :: "<< port;
qDebug() <<"Message :: " << datagram;
}
}
mainwindow.h:
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QUdpSocket>
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private slots:
void processPendingDatagrams();
private:
QUdpSocket *socket = nullptr;
};
#endif // MAINWINDOW_H
main.cpp:
#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
w.show();
return a.exec();
}
tried with netcat with the command:
netcat -u 127.0.0.1 45454
once you ran the command, just type anything and press return.

Answer 2

In my case this did not work, because there was always a "dummy" datagram left somewhere in memory, even if hasPendingDatagrams() returned false, I had to call readDatagram one more time to let my program receive newer datagrams. My answer is here anyways https://stackoverflow.com/a/74056997/11414500
Good luck!

Answer 3

#Gabriel answer did't work for me because it's using the old connect syntax. I updated it to the following (Qt 6):
// udpclient.h
#ifndef UDPCLIENT_H
#define UDPCLIENT_H
#include <QObject>
#include <QUdpSocket>
class UdpClient: public QObject
{
Q_OBJECT
public:
explicit UdpClient(QObject *parent);
void initSocket();
private:
void processPendingDatagrams();
QUdpSocket *socket=nullptr;
};
#endif // UDPCLIENT_H
// udpclient.cpp
#include "udpclient.h"
UdpClient::UdpClient(QObject *parent) : QObject(parent) {
}
void UdpClient::initSocket()
{
socket = new QUdpSocket(this);
bool result = socket->bind(QHostAddress::AnyIPv4, 4999);
if(result)
{
qDebug() << "Socket PASS";
}
else
{
qDebug() << "Socket FAIL";
}
processPendingDatagrams();
connect(socket, &QUdpSocket::readyRead, this, &UdpClient::processPendingDatagrams);
}
void UdpClient::processPendingDatagrams()
{
qDebug() << "in !";
QHostAddress sender;
quint16 port;
while (socket->hasPendingDatagrams())
{
QByteArray datagram;
datagram.resize(socket->pendingDatagramSize());
socket->readDatagram(datagram.data(),datagram.size(),&sender,&port);
qDebug() <<"Message From :: " << sender.toString();
qDebug() <<"Port From :: "<< port;
qDebug() <<"Message :: " << datagram;
}
}
It can be used in a function or main() as:
auto udpClient = new UdpClient(0);
udpClient->initSocket();