QPixmap.loadFromData() does not load image from QByteArray - qt

Question

I'm creating a socket-based program to send a screenshot from one user to another user. I need to convert a screenshot to a byte array before sending. After I convert my screenshot to a QByteArray I insert 4 bytes to the beginning of the array to mark that it is a picture (it is the number 20 to tell me it is a picture and not text or something else).
After I send the byte array via a socket to other user, when it is received I read the first 4 bytes to know what it is. Since it was a picture I then convert it from a QByteArray to QPixmap to show it on a label. I use secondPixmap.loadFromData(byteArray,"JPEG") to load it but it not load any picture.
This is a sample of my code:
void MainWindow::shootScreen()
{
originalPixmap = QPixmap(); // clear image for low memory situations
// on embedded devices.
originalPixmap = QGuiApplication::primaryScreen()->grabWindow(0);
scaledPixmap = originalPixmap.scaled(500, 500);
QByteArray bArray;
QBuffer buffer(&bArray);
buffer.open(QIODevice::WriteOnly);
originalPixmap.save(&buffer,"JPEG",5);
qDebug() << bArray.size() << "diz0";
byteArray= QByteArray();
QDataStream ds(&byteArray,QIODevice::ReadWrite);
int32_t c = 20;
ds << c;
ds<<bArray;
}
void MainWindow::updateScreenshotLabel()
{
this->ui->label->setPixmap(secondPixmap.scaled(this->ui->label->size(), Qt::KeepAspectRatio, Qt::SmoothTransformation));
}
void MainWindow::on_pushButton_clicked()
{
shootScreen();
}
void MainWindow::on_pushButton_2_clicked()
{
secondPixmap = QPixmap();
QDataStream ds(&byteArray,QIODevice::ReadOnly);
qint32 code;
ds>>code;
secondPixmap.loadFromData(byteArray,"JPEG");
updateScreenshotLabel();
}

Answer 1

Your MainWindow::on_pushButton_2_clicked implementation looks odd. You have...
QDataStream ds(&byteArray,QIODevice::ReadOnly);
which creates a read-only QDataStream that will read it's input data from byteArray. But later you have...
secondPixmap.loadFromData(byteArray,"JPEG");
which attempts to read the QPixmap directly from the same QByteArray -- bypassing the QDataStream completely.
You can also make use of the QPixmap static members that read from/write to a QDataStream. So I think you're looking for something like...
QDataStream ds(&byteArray,QIODevice::ReadOnly);
qint32 code;
ds >> code;
if (code == 20)
ds >> secondPixmap;
And likewise for your MainWindow::shootScreen implementation. You could reduce your code a fair bit by making use of QDataStream & operator<<(QDataStream &stream, const QPixmap &pixmap).