Qt Write to UDP Socket on Linux Machine and Windows Read from Socket with Python

Question

=========== UPDATE: ===========

I am now using the broadcast ip 10.6.0.3 and a higher port, 57890. I can see the traffic in Windows Network Monitor but still cannot get Python to read it.

==============================

I have a linux machine writing a 'hello' message to a UDP socket on ip address 10.6.0.2 using Qt and QUDPSocket. I also have a Windows machine that I want to read that 'hello' message using Python. From my Qt Code, I can see that the data is being written, however, the Python program on Windows sits forever on "Waiting for data". The windows machine can indeed ping 10.6.0.2. What am I doing wrong?

Test sequence:

UdpBroadcaster client; // Binds socket
sleep(10); // During this time , startup Python program
client.WriteData(); // Write hello message

Qt Output:

Initializing UDP Socket 
Determined ip address to be:  "10.6.0.2" 
Successfully bound to:   QHostAddress( "10.6.0.2" )   Port:  5678 
Sending UDP Datagram now! 
Wrote this many bytes:  8 
Message From:  "10.6.0.2" 
With Port Number  5678 
Message:  "Hello!!!"

Qt Code:

// Constructor
UdpBroadcaster::UdpBroadcaster(QObject *parent) :QObject(parent)

{

    // Init socket
    m_UdpSocket = new QUdpSocket(this);

    // Figure out our ip address to broadcast on
    QString ipAddress;
    foreach (const QHostAddress &address, QNetworkInterface::allAddresses()) {
        if (address.protocol() == QAbstractSocket::IPv4Protocol && address != QHostAddress(QHostAddress::LocalHost))
            if ( address.toString().contains("10.") ) {
                ipAddress = address.toString();
                qDebug() << "Determined ip address to be: " << ipAddress;
                break;
            }
    }

    // Bind to localhost
    m_Port = 5678;
    m_AddressToUse = QHostAddress(ipAddress);
    bool didBind = m_UdpSocket->bind(m_AddressToUse,m_Port);
    if ( !didBind ) {
        qDebug() << "Error! Cannot bind to: " << m_AddressToUse << " Port: " << m_Port;
    }
    else {
        qDebug() << "Successfully bound to: " << m_AddressToUse << " Port: " << m_Port;
    }

    // Connect the ready read socket
    connect(m_UdpSocket,SIGNAL(readyRead()),this,SLOT(readReady()));

}

// Write data to socket
void UdpBroadcaster::WriteData() {

    // Debug
    qDebug() << "Sending UDP Datagram now!";

    // Construct a message to send
    QByteArray msg;
    msg.append("Hello!!!");
    qint64 numberOfBytesWritten = m_UdpSocket->writeDatagram(msg,m_AddressToUse,m_Port);
    if ( numberOfBytesWritten == -1 ) {
        qDebug() << "Error! Could not write data to socket...";
    }
    else {
        qDebug() << "Wrote this many bytes: " << numberOfBytesWritten;
    }

}

// Do this when a datagram is available
void UdpBroadcaster::readReady() {

    // Buffer to receive data
    QByteArray buffer;
    buffer.resize(m_UdpSocket->pendingDatagramSize());

    QHostAddress sender;
    quint16 port;
    m_UdpSocket->readDatagram(buffer.data(),buffer.size(),&sender,&port);

    qDebug()<<"Message From: " << sender.toString();
    qDebug()<<"With Port Number " << port;
    qDebug()<<"Message: " << buffer;

}

Python Program:

import socket
import time

def main():

    UDP_IP = "10.6.0.2"
    UDP_PORT = 5678

    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    sock.connect((UDP_IP, UDP_PORT))
    cnt = 0
    while cnt < 10:
        print "Waiting for data"
        data, addr = sock.recvfrom(1024) # buffer size is 1024 bytes
        print "received message:", data
        time.sleep(1)
        cnt = cnt + 1

    ## Wait so we don't lose our terminal
    filename = raw_input()            

if __name__ == '__main__':
    main()

Have a try at using "netcat", also known as "nc", to act as either the sender or receiver to debug your code. It is dead simple to use and available on Windows, Mac & Linux. — Mark Setchell

PhilBot PhilBot · Accepted Answer · 2014-04-29T15:20:43

I had to allow my program through the Windows 7 Firewall . Now it works fine.

Qt Write to UDP Socket on Linux Machine and Windows Read from Socket with Python

1 Answers