Qt: QSslSocket::bytesWritten() signal is emitted too often

Question

I use this code to transfer a large file through a socket without spikes in memory usage :

    connect(socket, SIGNAL(bytesWritten(qint64)), this, SLOT(refillSocketBuffer(qint64)));
    refillSocketBuffer(128*1024);
}

void FtpRetrCommand::refillSocketBuffer(qint64 bytes)
{
    if (!file->atEnd()) {
        socket->write(file->read(bytes));
    } else {
        socket->disconnectFromHost();
    }
}

This works fine with QTcpSocket , but with an encrypted QSslSocket, the bytesWritten() signal is emitted constantly, which causes my function to write to the socket all the time, way quicker than it can send the data though the socket, so eventually its memory usage goes to 400 MB and the OS kills it.

Answer 1

I just found the answer after some more digging, it was in the documentation actually. It seems that I should use encryptedBytesWritten() for SSL sockets instead:

Note: Be aware of the difference between the bytesWritten() signal and the encryptedBytesWritten() signal. For a QTcpSocket, bytesWritten() will get emitted as soon as data has been written to the TCP socket. For a QSslSocket, bytesWritten() will get emitted when the data is being encrypted and encryptedBytesWritten() will get emitted as soon as data has been written to the TCP socket.

So I needed to change this code:

connect(socket, SIGNAL(bytesWritten(qint64)), this, SLOT(refillSocketBuffer(qint64)));

to this:

if (socket->isEncrypted()) {
    connect(socket, SIGNAL(encryptedBytesWritten(qint64)), this, SLOT(refillSocketBuffer(qint64)));
} else {
    connect(socket, SIGNAL(bytesWritten(qint64)), this, SLOT(refillSocketBuffer(qint64)));
}