CODEWITHSUNDEEP
c++multithreadingqtsocketstcp
Karol Drach
Karol Drach

Reputation: 25

toLocal8bit send over TCP

Im creating TCP Server/Client application in QT Creator framework. I want to get some data from UI input field and send it over TCP.

Im doing something like this in client application:

 void MainWindow::on_btn_login_clicked()
{
    QByteArray text = (ui->login_input->text()).toLocal8Bit();
    char* out = text.data();
    connection->ConnectAndSendData(out);
}

and in the ConnectAndSendData function:

void TcpConnect::ConnectAndSendData(const char* data)
{
    socket = new QTcpSocket(this);
    int port = 1234;
    socket->connectToHost("localhost", port);

    if(socket->waitForConnected(3000))
    {
        qDebug() << "connected to s. localhost at port " << port;
        socket->flush();
        socket->write(data, sizeof(data));
        qDebug() << data << "\n";

        socket->waitForReadyRead();
        char* serverresponse;
        socket->read(serverresponse, 128);

        if(serverresponse == MESSAGE_LOGINRQ)
            socket->write(data);
        socket->flush();
        socket->close();
    }
    else
    {
        /**/
    }
}

and the data in line socket->write(data, sizeof(data)); is properly send to server, but when server echoes it, it looks like "something/x00/x00/x00/x00" or somethinglike that. Also when i to do something like this:

#define MESSAGE_WANTLOGIN "wanlogin"
socket->write(MESSAGE_WANTLOGIN, sizeof(MESSAGE_WANTLOGIN));

message is messed up with those null signs.

on the server side receiving data look as simple as:

void Thread::readyRead()
{
    socket->flush();
    QByteArray data = socket->readAll();
    qDebug() << "data received: " << data;
    if(data == MESSAGE_WANTLOGIN)
    {
        socket->write(MESSAGE_LOGINRQ);
    } else
    {
        qDebug() << "error not messageloginrq";
    }
}

and like u can assume, though i send "wanlogin" message, server receiving something like "wanlogin/x00/x00" and this if obviously returns false.

this trash is applied on the end of data, and this impossible to check what message was send. The other thing is that maximum size of send data is 8 chars, but also to data of this length trash is applied so it looks like "wanlogin/x00/x00"; however, when i type more chars, for example 10, the send data is just cut to 8 signs, with no /x00s.

So my question is how to clear data from those /x00s and how to send more than 1 byte of information(i need it e.g. to send login and password of user). Sorry if there's some stupid mistake, its my first client/server application which also using multithreading for each client.

Upvotes: 0

Views: 334

Answers (1)

Benjamin T
Benjamin T

Reputation: 8311

sizeof(data) is 4 or 8 depending if you are on a 32-bit or 64-bit machine. It is not the size of your data, but the size (in byte) of a pointer.

So what happens is that your actual wanlogin is in fact a 6 character string, and you end up sending 2 more bytes. In this case you are lucky: the char array returned by data() is null-terminated, so you have one extra 0 that you can access, but accessing the second 0 is undefined behavior i.e anything can happen.

The solution is to use strlen() instead of sizeof. Or, better, to directly call write() with a QByteArray by changing ConnectAndSendData(const char* data) to ConnectAndSendData(const QByteArray &data).

void MainWindow::on_btn_login_clicked()
{
    const QByteArray text = (ui->login_input->text()).toLocal8Bit();
    connection->ConnectAndSendData(text);
}

void TcpConnect::ConnectAndSendData(const QByteArray & data)
{
    socket = new QTcpSocket(this);
    quint16 port = 1234;
    socket->connectToHost("localhost", port);

    if(socket->waitForConnected(3000))
    {
        qDebug() << "connected to s. localhost at port " << port;
        socket->write(data);
        ...
    }
    ...
}

Upvotes: 2

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