Transferring large files over TCP in Qt

Question

I am newbie on TCP file transferring over sockets. So just in terms of self-lerning, I want to modify the example "Loopback" to make it send a large file (e.g 100Mb to 2Gb) from server to client once the connections is established. My problem is that I have no idea how to split the file in order to now when the transmission has to finish. Let me insert a piece of code to make this easier to understand:

server.cpp

void Dialog::acceptConnection()
{
    tcpServerConnection = tcpServer.nextPendingConnection();
    connect(tcpServerConnection,SIGNAL(connected()), this, SLOT(startTransfer()));
    connect(tcpServerConnection, SIGNAL(bytesWritten(qint64)), this, SLOT(updateServerProgress(qint64)));
    connect(tcpServerConnection, SIGNAL(error(QAbstractSocket::SocketError)), this, SLOT(displayError(QAbstractSocket::SocketError)));

    serverStatusLabel->setText(tr("Accepted connection"));
    startTransfer();
}
void Dialog::startTransfer()
{
    file = new QFile("file_path");
    if (!file->open(QIODevice::ReadOnly))
    {
        serverStatusLabel->setText("Couldn't open the file");
        return;
    }
    int TotalBytes = file->size();

    bytesToWrite = TotalBytes - (int)tcpServerConnection->write(<-WHAT HERE!!!->));
    serverStatusLabel->setText(tr("Connected"));
}
void Dialog::updateServerProgress(qint64 numBytes)
{
    bytesWritten += (int)numBytes;

    // only write more if not finished and when the Qt write buffer is below a certain size.
    if (bytesToWrite > 0 && tcpServerConnection->bytesToWrite() <= 4*PayloadSize)
        bytesToWrite -= (int)tcpServerConnection->write(<-WHAT HERE!!!->));

    serverProgressBar->setMaximum(TotalBytes);
    serverProgressBar->setValue(bytesWritten);
    serverStatusLabel->setText(tr("Sent %1MB").arg(bytesWritten / (1024 * 1024)));
}

I've seen some solution that use readAll() but I don't think qt can handle a buffer with 2Gb of data inside... So, as mentioned, my question is how to slip up the file to by write over tcpServerConnection? I would like to know if it's recommended to use QDataStream for that purpose (QDataStream out(&file, QIODevice::WriteOnly);), as well.

Thanks!

PD: Note the mark <-WHAT HERE!!!-> on the code.

Answer 1

Ok, thanks to Basile Starynkevitch I find out the solution. It's as easy as set:

buffer = file->read(PayloadSize);

And then send it through Tcp. In local network I achieve to transfer 397Mb in 40.11 sec. Thanks,