Qt UDP read error - c++

Question

I try to write a little client-server program. A client should detect all servers running in the same LAN. I tried to implement a UDP-Broadcast with qt but my read function returns -1 all the time. If I call the dataAvailable()-function on the socket, it says that 4 bytes are available to read, but for some reason it fails to read them.
For now I'm trying to receive the broadcast on the same machine and in the same program
Here's my main:
#include <iostream>
#include <QString>
#include <QThread>
#include "../include/network.h"
using namespace std;
using namespace network;
int main () {
Network *n = new Network ();
n->broadcast("test");
if (n->dataAvailable()) {
cout << "Data available: ";
cout << n->getData().toStdString() << std::endl;
} else {
cout << "No data" << endl;
}
delete n;
}
and my Network-class:
#ifndef NETWORK
#define NETWORK
#include <QObject>
#include <QUdpSocket>
#include <iostream>
namespace network {
class Network : public QObject {
Q_OBJECT
public:
const static int BROADCAST_PORT = 52433;
explicit Network(QObject *parent = 0) {
socket = new QUdpSocket ();
socket->bind (QHostAddress::Any, BROADCAST_PORT);
}
~Network () {
delete socket;
}
bool dataAvailable () {
return socket->hasPendingDatagrams();
}
QString getData () {
if (!dataAvailable()) {
return "";
}
char *data = 0;
std::cout << socket->pendingDatagramSize() << std::endl;
std::cout << QString("recv: %1")
.arg(socket->readDatagram(data,
socket->pendingDatagramSize())).toStdString();
return QString(data);
}
void broadcast(QString data) {
QUdpSocket *broadcast = new QUdpSocket ();
broadcast->connectToHost(QHostAddress::Broadcast, BROADCAST_PORT);
std::cout << broadcast->write(data.toStdString().c_str())
<< std::endl << std::endl;
delete broadcast;
}
private:
QUdpSocket *socket;
};
}
#endif // NETWORK
the Output is the following:
4
Data available: 4
recv -1
Which means
4 Bytes are sent from the broadcast
the socket detects that 4 bytes are available
the attempt to read the 4 bytes fails
the QString returned by getData() is empty
Unfortunately I could not find any possibility to get more information on the error

Answer 1

You definitely need to allocate memory for reading datagram. Now you try to write it to null pointer.
// char *data = 0;
std::cout << socket->pendingDatagramSize() << std::endl;
QVector<char> buffer(socket->pendingDatagramSize()); // create buffer
std::cout << QString("recv: %1")
.arg(socket->readDatagram(buffer.data(),
socket->pendingDatagramSize())).toStdString();