If everything is working as expected, don't forget to update the thread's title to solved so other forum users may know that a solution has been found :)

if (blockSize == 0) {
if (clientConnection->bytesAvailable() < (int)sizeof(quint16))
return;
in >> blockSize;
}
if (clientConnection->bytesAvailable() < blockSize)
return;@

Doesn't that mean that all data is arrived?

Have a look at the Fortune Server and Client example. It explains one way of doing the transfer from one side to the other.

Hope it helps

thank you, your answer was helpful!
So now this works:
@QByteArray ba;
QBuffer buffer(&ba);
image.save(&buffer, "PNG");

QByteArray ba2 = buffer.data();
const char *imageData = ba2.data();
int imageSize = ba2.size();

QByteArray ba3 = QByteArray(imageData, imageSize);
QImage test;
test.loadFromData(ba3);@

and I send it like this:
@QByteArray block;
QDataStream out(&block, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_0);
out << (quint16)0;
out << scaleFactor;
out << imageData;
out << imageSize;
out.device()->seek(0);
out << (quint16)(block.size() - sizeof(quint16));
tcpSocket->write(block);@

and receive it so:
@QImage img;
double scaleFactor;
char *imageData;
const char *cImageData;
int imageSize;

in >> scaleFactor;
in >> imageData;
in >> imageSize;
cImageData = imageData;
qDebug() << "scaleFactor:" << scaleFactor << "imageSize:" << imageSize; //right numbers
//this still doesn't work:
QByteArray ba = QByteArray(cImageData, imageSize); //doesn't work with imageData, too
img.loadFromData(ba);@

Do you have any idea why the last two lines still don't work? "scaleFactor" and "imageSize" are transmitted correctly.

when I write

@in >> img; //my QImage@

I get this in the Application Output: "QIODevice::seek: Cannot call seek on a sequential device"

You need to send the data size of the image you are sending before the data so you can rebuild it on the other side.

Using the QByteArray(const char *) constructor, your are telling it it's a string so it will stop at the first \0 char. You need to use the other constructor giving the size of the char array.

Hope it helps

why you don't try with

@
out << image; // your QImage object

in >> image;
@

QImage implements
@
QDataStream& operator << (QDataStream & str, const QImage& img);
QDataStream& operator >> (QDataStream & str, QImage& img);
@

so you can write directly code like mine