Sending pictures via qt TCP/IP

Question

I'm trying to send pictures via a QTCPsocket, with localhost (127.0.0.1) it works fine. If I'm sending it to an other pc via ethernet, sometimes Qt receveices it in 2 times, see debug output.

How can i fix it?

Server side:

void server::writePic(QString fileName)
{
   name = QString("%1.png").arg(counter);
   counter++;

   qDebug() << name;

   pic.load(fileName, "PNG");

   pic.setText("name",name);
   pic.setText("datum","20-3-2018");

   QByteArray ba;              // Construct a QByteArray object
   QBuffer buffer(&ba);        // Construct a QBuffer object using the QbyteArray
   pic.save(&buffer, "PNG"); // Save the QImage data into the QBuffer
   qDebug() << ba;
   socket->flush();
   socket->write(ba);          // Send the QBuffer (QbyteArray) over a socket
   socket->waitForBytesWritten();
   socket->flush();
}

Client side:

void Client:: readyRead()
{
   ImageBuffer->open(QIODevice::ReadWrite);
   socket->waitForReadyRead(1);
   QByteArray Temp;
   Temp = socket->readAll();
   ImageBuffer->write(Temp);
   pic.loadFromData(ImageBuffer->buffer());
   std::stringstream fileName;
   fileName <<"C:/pics/" << pic.text("name").toStdString();

   if(!pic.isNull())
   {
      qDebug() << "Image file was received ";
      qDebug() << pic.text("name");
      qDebug() << pic.text("datum");
      qDebug() << "size = " << Temp.size();
      pic.save(fileName.str().c_str(),"PNG");
   }else{
      qDebug() << "Pic is NULL";
      qDebug() << "size = " << Temp.size();
   }
}

Debug output

Server side:

"C:/.../Analysis/test_images/Foto01.png"
"1.png"
We wrote:  25156
File has been removed
"C:/.../Analysis/test_images/Foto02.png"
"2.png"
We wrote:  26755
File has been removed

---

Client side:

Not succeeded:

Pic is NULL 
size =  18980 
Pic is NULL 
size =  6176 

Succeeded:

Image file was received 
"2.png"
"20-3-2018"
size =  26755

Answer 1

Nothing ensures you to receive your picture in a single read. That's true. one solution is to encapsulate your picture in a frame you will define, containing info about that picture. One simple (and not really bulletproof) way would be to add the size of your picture as the first 4 bytes of your frame. On the receive side, your could read it, and wait for additional data as long as you did nor receive at lease the expected side. For sure you should improve structure and processing to handle packet loss and spurious disconnections.