QT 5.0 Qudpsocket wont trigger waitforbyteswritten

Question

I have bound a udp socket and i am trying to write a udp datagram for a specific adress so here is my code:

UDP::UDP(QObject *parent) :
    QObject(parent)
{
    socket = new QUdpSocket(this);
    QObject::connect(socket,SIGNAL(bytesWritten(qint64)),this,SLOT(bytesWritten(qint64)));
    QObject::connect(socket,SIGNAL(readyRead()),this,SLOT(readyRead()));
}
void UDP::connect(int port){

if(socket->state()==QAbstractSocket::BoundState){
    qDebug()<<"already bound try to disconnect";
    socket->close();
}
     if(socket->bind(QHostAddress::Any,port,QAbstractSocket::ShareAddress|QAbstractSocket::Reuse    AddressHint)){
        qDebug()<< "listening";
        emit changeStatus("UDP: escutando("+QString::number(port)+")");
    }else{
        qDebug()<< "error no bind";
        emit changeStatus("UDP: erro no bind("+QString::number(port)+")");
    }
}

void UDP::bytesWritten(qint64 bytes){
    qDebug()<<"bytes written:"<<bytes;
    emit changeStatus("UDP: enviado");
}

void UDP::readyRead(){
    qDebug()<<"ready to read";
    QByteArray datagram;
    QHostAddress sender;
    quint16 senderPort;

    datagram.resize(socket->pendingDatagramSize());
    socket->readDatagram(datagram.data(),datagram.size(),&sender,&senderPort);
    qDebug()<< "datagram received from " << sender.toString();

    emit receivedUDPdata(datagram);

}
bool UDP::write(QString dest,int port, QString buffer, int size){
    char *finalOutput;
    finalOutput = new char[size];
    unsigned int j=0;

    for(unsigned int i=0;i<size;i++){
        if(buffer[i]== '\\'){
            switch (buffer.at(++i).toLatin1()){
            case 'r':
                finalOutput[j++] = '\r';
                break;
            case 'n':
                finalOutput[j++]= '\n';
                break;
            case 't':
                finalOutput[j++]='\t';
                break;
            case '\\':
                finalOutput[j++]='\\';
                break;
            }
        }else{
            finalOutput[j++]= buffer[i].toLatin1();
        }
    }
    socket->writeDatagram(finalOutput,j,QHostAddress(dest),port);
    if(!socket->waitForBytesWritten(2000)){
        qDebug() << "UDP Couldnt write.";
        emit changeStatus("UDP: Não foi possível escrever");
        return false;
    }
    return true;
}

The variable dest is QString, but after i call socket->writeDatagram it enters the if below as if it had fail, but actually the bytes are written (i receive then) and the byteswritten signal is emitted. So why it is entering the if clause?

ps(i tested the ip dest as 127.0.0.1) and the socket->errorString() prints "Unknown error"

Can you share more of your code? what's before `socket->writeDatagram()` ? — dschulz, Mar 15 '13 at 00:00
Don't you get the following when you run your code: QAbstractSocket::waitForBytesWritten() is not allowed in UnconnectedState ? — Alen Stojanov, Nov 26 '13 at 23:32

score 0 · Answer 1 · answered Jan 12 '16 at 18:13

QAbstractSocket::waitForBytesWritten() is for TCP sockets, after calling write().

With a QUdpSocket the API is writeDatagram(), which sends immediately (at least at the Qt level, the OS has a buffer I suppose). For proof of that, see the implementation of writeDatagram() in src/network/socket/qudpsocket.cpp, which emit bytesWritten(sent);

So there is no point in calling waitForBytesWritten() after writeDatagram().

QT 5.0 Qudpsocket wont trigger waitforbyteswritten

1 Answers1