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Incomplete data in QDataStream when reading from QTcpSocket

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  • S Offline
    S Offline
    SGaist
    Lifetime Qt Champion
    wrote on 10 Jan 2020, 21:22 last edited by
    #7

    Hi,

    Did you consider using read transactions ?

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    C 1 Reply Last reply 10 Jan 2020, 21:30
    1
    • S SGaist
      10 Jan 2020, 21:22

      Hi,

      Did you consider using read transactions ?

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      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on 10 Jan 2020, 21:30 last edited by
      #8

      @SGaist said in Incomplete data in QDataStream when reading from QTcpSocket:

      Did you consider using read transactions ?

      But then you have to make sure that your QDataStream stays alive all the time since this will not work with sequential devices otherwise:

      void foo::onDataAvailable()
      {
        QDataStream ds(m_socket);
        ds.startTransaction()
        if (readFromDataStream(ds))
          ds.commitTransaction();
        else
          ds.rollbackTransaction();
      }
      

      I had this error in my codebase once and it took some time until I figured out what went wrong.

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      • R Offline
        R Offline
        RBLL
        wrote on 10 Jan 2020, 21:37 last edited by RBLL 1 Oct 2020, 21:37
        #9

        I just validated that calling mServerTask->getBufferX(i) twice does not modify the data. The function is a single line that returns a float. I'm not familiar with read transactions but I'll look into those to try and work around my issue. I'm still puzzled as to what happens with the QDataStream that makes my data "corrupted"...

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        • R RBLL
          10 Jan 2020, 21:17

          Ya that is again a mistake on my part. The line is supposed to go before the
          write(block). I actually found the problem, but I'm still clueless as to what happens:

          In my server, I get these bufferX and bufferY from another object like so:

          for (size_t i = 0; i < BUFFERX_SIZE; i++)
          {
          	out << mServerTask->getBufferX(i);
          	wBufferX[i] = mServerTask->getBufferX(i);
          }
          

          Now, if I execute this, I have the same behavior as I did previously: after index 315-350, values are unknown. However, executing this instead (note the commented line):

          for (size_t i = 0; i < BUFFERX_SIZE; i++)
          {
          	//out << mServerTask->getBufferX(i);
          	wBufferX[i] = mServerTask->getBufferX(i);
          }
          

          Will result in wBufferX containing all the data. It seems the QDataStream is interfering / corrupting the data?

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          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on 10 Jan 2020, 21:42 last edited by
          #10

          @RBLL said in Incomplete data in QDataStream when reading from QTcpSocket:

          out << mServerTask->getBufferX(i);

          So when getBuffer() is const this line can not do anything harmful to wBufferX

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          • R Offline
            R Offline
            RBLL
            wrote on 10 Jan 2020, 21:54 last edited by RBLL 1 Oct 2020, 21:56
            #11

            The function was not declared const, but even when making it const, I have the same behavior. In any case, that is the get function : it does not modify the data:

            const float ServerTask::getBufferX(size_t index)
            {
            	return mBufferX[index];
            }
            

            Now, for debugging purpose, I added an extra loop in front to see if I would have different results like so:

            for (size_t i = 0; i < BUFFERX_SIZE; i++)
            {
            	wBufferX2[i] = ServerTask->getBufferX(i);
            }
            
            for (size_t i = 0; i < BUFFERX_SIZE; i++)
            {
            	out << ServerTask->getBufferX(i);
            	wBufferX[i] = ServerTask->getBufferX(i);
            }
            

            Now, this is the what I have inside wBufferX2:

            8f6e2b6d-5a02-4d39-a010-dfee0786e5b8-image.png

            And this is what I have inside wBufferX:

            446653e1-fa1f-41b2-8a01-474501e6c63f-image.png

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            • C Online
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              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on 10 Jan 2020, 22:03 last edited by
              #12

              @RBLL said in Incomplete data in QDataStream when reading from QTcpSocket:

              wBufferX

              Is this memory big enough? Are you on Linux? I would take a look with valgrind since this really looks like a memory allocation problem

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              • R Offline
                R Offline
                RBLL
                wrote on 10 Jan 2020, 22:05 last edited by
                #13

                No, this is on Windows. But both buffers are declared the same like so:

                float wBufferX[BUFFERX_SIZE];
                float wBufferX2[BUFFERX_SIZE];
                

                It would be surprising if it had anything to do with memory allocation but at this point I don't see many other options...

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                • H Online
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                  hskoglund
                  wrote on 10 Jan 2020, 22:08 last edited by
                  #14

                  Hi, it could be a transmission timing problem, about 350 floats is pretty close to MTU size (the normal Ethernet packet size). So it could be that while the first packet is xmitted, out<<... wreaks havoc with the rest of the data waiting to be xmitted (just guessing :-)

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                  • R Offline
                    R Offline
                    RBLL
                    wrote on 10 Jan 2020, 22:16 last edited by
                    #15

                    When debugging I had a breakpoint before the write command. Nothing had been sent to the client yet when those 2 buffers had different values.

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                    • H Online
                      H Online
                      hskoglund
                      wrote on 10 Jan 2020, 22:25 last edited by
                      #16

                      Maybe that QByteArray is the culprit, you could try rewrite into more vanilla standard:

                      ...
                      {
                          QByteArray block;
                          QBuffer buffer(&block);
                          buffer.open(QIODevice::WriteOnly);
                          QDataStream out(&buffer);
                      ...
                      

                      at least you would expose more stuff to the debugger :-)

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                      16/16

                      10 Jan 2020, 22:25

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