Data / file transfer via TCP

[Jeff_T69]

Hey there,

I want to realize a data/file transfer between 2 classes, one client and one server. I allready get an connection using the QT Classes "QTcpServer" "QTcpSocket". And I can send messages using this:

//code here
void Server::sendString()  {
	// send via QByteArray 
	QByteArray block;
	QDataStream out(&block, QIODevice::WriteOnly);
	out.setVersion(QDataStream::Qt_5_5);
	out << (quint16)0;
	// create data for the client 
	QString data;
	data.append(tr("Just some bla "));
	out << data;
	out.device()->seek(0);
	out << (quint16)(block.size() - sizeof(quint16));
	// get the next client-connection
	QTcpSocket *clientConnection =
		tcpServer->nextPendingConnection();
	// signal  
	connect(clientConnection, SIGNAL(disconnected()),
		clientConnection, SLOT(deleteLater()));
	// write the string into the socket
	clientConnection->write(block);
	// disconnect
	clientConnection->disconnectFromHost();
}

now I want to send files. I tried it using class "QFile", but I failed.
Is my approach right?? How would you do a data/file transfer via TCP?

thank you for your help.

greetings
Thomas

[mrjj]

hi
It seems fine. If you can send the QString Data, you can send anything you like.
If you want to send a binary file, then you should never ever convert to string or char *
as you will only get up till first 0 (zero) it finds.
so to send say an image: (pseudo code-ish)

QFile file("c:/mypic.jpg");
file.open(QIODevice::ReadOnly);
QByteArray mydata=file.readAll();
then then clientConnection->write(mydata).

On the receiver end, also read it as QByteArray and
save to file again.

QByteArray data;
// read ...
QFile file("C:/path/myimage.jpg"); // change path
file.open(QIODevice::WriteOnly);
file.write(data);
file.close();

[Jeff_T69]

Hi,
thank you for your help!!
I tried to implement it, with your advice, but my program just create an empty file.
There could be a problem with my server class, because I did't understand how I get the bytes in a DataStream.
I thouht the command file.readAll() reads the whole file and QDataStream puts this into my Socket.
But now I think it isn't that easy!??

In my Server Class

void Server::sendData()  {
	QFile file("C:/Uploads/test.zip");
	if (!file.open(QFile::ReadOnly))
	{
	ui.label_Server_Status_Header ->setText(
      tr("Could not open the file for reading"));
		return;
	}
	QByteArray mydata = file.readAll();
	QDataStream out(&mydata, QIODevice::WriteOnly);

    out.setVersion(QDataStream::Qt_5_5);
	out.device()->seek(0); // sets device
	//out << (quint64) mydata.size();
	//out << (quint16)0;
	out << (quint64) mydata.size();  // filesize // mydata or file.size() ???
	out << mydata;

    // get the next client-connection
    QTcpSocket *clientConnection =
        tcpServer->nextPendingConnection();
    // signal  
    connect(clientConnection, SIGNAL(disconnected()),
        clientConnection, SLOT(deleteLater()));
    // write the string into the socket
    clientConnection->write(mydata);
    // disconnect
    clientConnection->disconnectFromHost();
}

In my Client Class

// read Servers datas
void Client::readData() {
QDataStream in(tcpSocket);
	in.setVersion(QDataStream::Qt_5_5);
  
	if (blockSize == 0) {     
		// data to read available   
		if ( tcpSocket->bytesAvailable() <         
			(int)sizeof(quint16) )        
			return;     
		in >> blockSize; 
	} 
	if (tcpSocket->bytesAvailable() < blockSize)    
		return;

	QByteArray nextByte;
	// read ...
	in >> nextByte;
	
	/*if (nextByte == currentByte) {
     QTimer::singleShot(0, this, SLOT(requestNewConnection()));
     return; 
	}*/

	currentByte = nextByte;
	QFile file("C:/Downloads/test.zip"); // download path
	file.open(QIODevice::WriteOnly);
	file.write(currentByte);
	file.close();
 
  ui.getTimeButton->setEnabled(true);
}

[mrjj]

Hi
you must use the debugger to find out what went wrong.
you write the size of the zip file as a header.

Do you get the size over correctly ?
Meaning that blockSize is actually mydata.size();
also after
in >> nextByte;
if you
qDebug() << "bytes read: " << nextByte.size();

what you get as output?

[mcosta]

Hi,

what do you do in server is

socket->write(data);
socket->disconnectFromHost();

IMO is not correct because you should wait that the bytes are written before closing the connection for instance

socket->write(data);
// Wait until data are written to the native socket buffer
socket->waitForBytesWritten();
socket->disconnectFromHost();

[SGaist]

Hi,

One other thing:
You are creating your QDataStream on your file buffer, then you write the block size in place of the first bytes of your file.

Taking the fortune server examples code, it should be:

QByteArray block; // Data that will be sent
QDataStream out(&block, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_5);
out << (quint64)0; // Space for size of data
out << file.readAll(); // Actual data
out.device()->seek(0);
out << (quint64)(block.size() - sizeof(quint64));

[kshegunov]

@Jeff_T69
Hello,

This part:

    QByteArray mydata = file.readAll();
    QDataStream out(&mydata, QIODevice::WriteOnly);

    out.setVersion(QDataStream::Qt_5_5);
    out.device()->seek(0); // sets device
    //out << (quint64) mydata.size();
    //out << (quint16)0;
    out << (quint64) mydata.size();  // filesize // mydata or file.size() ???
    out << mydata;

looks very suspicious. I don't get it. You read the file, then create a data stream attached to the buffer containing the file and then write it over with the << operator? What exactly is it that this part of the code is trying to achieve?
I believe simply doing this is sufficient:

QTcpSocket * socket = tcpServer->nextPendingConnection();
socket->write(mydata);
if (!socket->waitForBytesWritten())
     ; // Some error occured
socket->disconnectFromHost();

Similarly on the client side:

if (tcpSocket->bytesAvailable() <= 0)
    return;

while (tcpSocket->state() == QAbstractSocket::ConnectedState)  {
    tcpSocket->waitForReadyRead();
    file.write(tcpSocket->read(tcpSocket->bytesAvailable()));
}

Additionally, don't open the file at every read, just open it once and then write. The way you're doing it, the file contents will always be overwritten on each subsequent call.

Kind regards.

EDIT:
It seems @SGaist beat me to it and posted while I was writing. I see now what the idea of the first part of the code is.

[Jeff_T69]

Thank you very much for all your replies. I see what went wrong.
Now it works very well.

Server Class

void Server::sendData()  {
	QFile file("C:/Uploads/Test.zip");
	
	if (!file.open(QFile::ReadOnly))
	{
	ui.label_Server_Status_Header ->setText(
      tr("Could not open the file for reading"));
		return;
	}

	QByteArray block; // Data that will be sent
	QDataStream out(&block, QIODevice::WriteOnly);
	out.setVersion(QDataStream::Qt_5_5);
	out << (quint64)0; // Space for size of data
	out << file.readAll(); // Actual data
	out.device()->seek(0);
	out << (quint64)(block.size() - sizeof(quint64));

    // get the next client-connection
    QTcpSocket *clientConnection =
        tcpServer->nextPendingConnection();
    // signal  
    connect(clientConnection, SIGNAL(disconnected()),
        clientConnection, SLOT(deleteLater()));
    // write the string into the socket
    clientConnection->write(block);
	// Wait until data are written to the native socket buffer
	clientConnection->waitForBytesWritten();
    // disconnect
    clientConnection->disconnectFromHost();
}

Client Class

// read Servers datas
void Client::readData() {
QDataStream in(tcpSocket);
	in.setVersion(QDataStream::Qt_5_5);
  
	if (blockSize == 0) {     
		// data to read available   
		if ( tcpSocket->bytesAvailable() <         
			(int)sizeof(quint16) )        
			return;     
		in >> blockSize; 
	} 
	if (tcpSocket->bytesAvailable() < blockSize)    
		return;

	QByteArray nextByte;
	// read
	in >> nextByte;

	currentByte = nextByte;
	QFile file("C:/Downloads/Test.zip"); // download path
	
	file.open(QIODevice::WriteOnly);
	file.write(currentByte);
	file.close();
 
  ui.getTimeButton->setEnabled(true);
}

[karti gesar]

hi all,
If file size is 1000 kb it is possible to send all the data

[VRonin]

@karti-gesar yes, of course

[karti gesar]

@VRonin

QFile file("D:/serial_Data.txt");
    file.open(QIODevice::ReadOnly);
    QByteArray mydata=file.readAll();
     socket->write(mydata);
//Serial_Data file size is 1072kb but only 1 kb is writting

[VRonin]

add socket->waitForBytesWritten(-1);

P.S.
Since your file is a text file probably what you are reading is not what you want and/or it's not solid to change. you should use:

QFile file("D:/serial_Data.txt");
if(file.open(QIODevice::ReadOnly | QIODevice::Text)){
const auto mydata=QTextStream(&file).readAll();
QDataStream socketStream(socket);
socketStream << mydata;
socket->waitForBytesWritten(-1);
}

[karti gesar]

k i will try

[karti gesar]

QFile file("D:/serial_Data.txt");
 if(file.open(QIODevice::ReadOnly| QIODevice::Text))
   {
        const auto mydata=QTextStream(&file).readAll();
       QDataStream socketStream(socket);
       socketStream << mydata;
       socket->waitForBytesWritten(-1);
     qDebug()<<mydata.constData();
   }

this what i used....for file size smaller it's working ..for huge file size it not writing

[VRonin]

@karti-gesar said in data/ file transfer via TCP:

huge

You should define huge but in this case you can use:

QFile file("D:/serial_Data.txt");
if(file.open(QIODevice::ReadOnly | QIODevice::Text)){
QDataStream socketStream(socket);
QTextStream fileStream(&file);
for(QString mydata=fileStream.readLine();!mydata.isEmpty();mydata=fileStream.readLine())
socketStream << mydata;
socket->waitForBytesWritten(-1);
}

[Talha Celik]

@Jeff_T69 what is block size ?