Can I use QUdpSockets wo polling or custom classes in Qt? - c++

Question

Here is a short UDP server example in Qt below which does work but what I don't like is that I'm polling to see if new data is available. I've come across some examples of a readyRead() but they all seem to introduce a qt class. Do I need to use a qt class in order to take advantage of the readyRead() signal?
Here is the working but simple UDP server implemented entirely in main:
#include <QDebug>
#include <QUdpSocket>
#include <QThread>
int main(int argc, char *argv[])
{
QUdpSocket *socket = new QUdpSocket();
u_int16_t port = 7777;
bool bindSuccess = socket->bind(QHostAddress::AnyIPv4, port);
if (!bindSuccess) {
qDebug() << "Error binding to port " << port << " on local IPs";
return a.exec();
}
qDebug() << "Started UDP Server on " << port << endl;
QHostAddress sender;
while (true) {
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.data();
}
QThread::msleep(20);
}
return 0;
}
Here is an example of the readyRead() signal:
https://www.bogotobogo.com/Qt/Qt5_QUdpSocket.php
I haven't really figured out how to get this to work yet. I must be doing something wrong. Here is the UDP connection code i'm trying:
#include "myudp.h"
MyUDP::MyUDP(QObject *parent) : QObject(parent) {
}
void MyUDP::initSocket(u_int16_t p) {
port = p;
udpSocket = new QUdpSocket(this);
bool bindSuccess = udpSocket->bind(QHostAddress::LocalHost, port);
if (!bindSuccess) {
qDebug() << "Error binding to port " << port << " on local IPs";
return;
}
connect(udpSocket, SIGNAL(readyRead()), this, SLOT(readPendingDatagrams()));
}
void MyUDP::readPendingDatagrams() {
QHostAddress sender;
while (udpSocket->hasPendingDatagrams()) {
QByteArray datagram;
datagram.resize(udpSocket->pendingDatagramSize());
udpSocket->readDatagram(datagram.data(), datagram.size(), &sender, &port);
qDebug() << "Message From :: " << sender.toString();
qDebug() << "Port From :: " << port;
qDebug() << "Message :: " << datagram.data();
}
}
myudp.h
#include <QObject>
#include <QUdpSocket>
class MyUDP : public QObject
{
Q_OBJECT
public:
explicit MyUDP(QObject *parent);
void initSocket(u_int16_t p);
u_int16_t port;
QUdpSocket *udpSocket;
signals:
public slots:
void readPendingDatagrams();
};
new main.cpp
int main(int argc, char *argv[])
{
MyUDP *myUDP = new MyUDP(0);
myUDP->initSocket(port);
while (true) {
usleep(1000);
}
return 0;
}
I am testing with:
netcat 127.0.0.1 -u 7777
{"cid"="0x1234123412341", "fill_level"=3245 }<cr>

Answer 1

What you're doing wrong is that you're not letting Qt's event loop run. i.e. this is incorrect:
int main(int argc, char *argv[])
{
MyUDP *myUDP = new MyUDP(0);
myUDP->initSocket(port);
while (true) {
usleep(1000);
}
return 0;
}
... instead, you should have something like this:
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
// connect needs to occur after QCoreApplication declaration
MyUDP *myUDP = new MyUDP(0);
myUDP->initSocket(port);
return app.exec();
}
... it is inside the app.exec() call where a Qt application spends most of its time (app.exec() won't return until Qt wants to quit), and there is where Qt will handle your UDP socket's I/O and signaling needs.

Answer 2

Please modify your processPendingDatagrams like this, to let newer incoming data be processed:
void MyUDP::readPendingDatagrams() {
QHostAddress sender;
uint16_t port;
QByteArray datagram; // moved here
while (udpSocket->hasPendingDatagrams()) {
//QByteArray datagram; // you don't need this here
datagram.resize(udpSocket->pendingDatagramSize());
udpSocket->readDatagram(datagram.data(), datagram.size(), &sender, &port);
qDebug() << "Message From :: " << sender.toString();
qDebug() << "Port From :: " << port;
qDebug() << "Message :: " << datagram.data();
}
// God knows why, there is always one more "dummy" readDatagram call to make,
// otherwise no new readyRead() will be emitted, and this function would never be called again
datagram.resize(udpSocket->pendingDatagramSize());
socket->readDatagram(datagram.data(),datagram.size(),&sender,&port);
}