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Reading from COM port yields strange results

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  • mrjjM mrjj

    @mzimmers
    If the MSVC is using native serial API , it might be buffered more.

    mzimmersM Offline
    mzimmersM Offline
    mzimmers
    wrote on last edited by
    #7

    @mrjj said in Reading from COM port yields strange results:

    @mzimmers
    If the MSVC is using native serial API , it might be buffered more.

    So...are there any settings I can change, or do I need to handle the input stream in the application?

    I'm still unclear why I'm getting readyRead signals with 0 bytes available for reading, too.

    hskoglundH 1 Reply Last reply
    0
    • mrjjM Offline
      mrjjM Offline
      mrjj
      Lifetime Qt Champion
      wrote on last edited by
      #8

      So...are there any settings I can change, or do I need to handle the input stream in the application?

      As far as I know, you should handle the case when comes in bulks.
      buffer = buffer + port->readAll();

      I'm still unclear why I'm getting readyRead signals with 0 bytes available for reading, too.

      That also puzzle me as not seen that. I wonder if you use flow control or tx etc?

      1 Reply Last reply
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      • K Offline
        K Offline
        kuzulis
        Qt Champions 2020
        wrote on last edited by kuzulis
        #9

        I'm still unclear why I'm getting readyRead signals with 0 bytes available for reading, too.

        It is impossible in principle, because the readyRead() emits only when QSP internally reads bytes more than 0.

        PS: You can parse like this (pseudocode):

        int startIndex = -1;
        int endIndex = -1;
        
        void CdSerial::read()
        {
            for (;;) {
                const auto av  = serial->bytesAvailable();
                if (av < 11) // <XML></XML> - minimum size
                    return;
        
                const auto peeked = serial->peek(av); 
        
                startIndex = peeked.indexOf("<XML>");
        
                if (startIndex == -1) {
                    serial->read(av); // clear all as we did not found start tag
                } else if (startIndex > 0) {
                    serial->read(startIndex); // synchronize up to first start tag
                    startIndex = -1;
                } else {
                    // we have found the start tag, so, now we need to found the end tag
                    endIndex = peeked.indexOf("</XML>");  
                    if (endIndex == -1) {
                         // we did not found end tag yet
                         return;
                    } else {
                         // we found the end tag
                         const auto content = serial->read(endIndex + 6); // +6 == length of </XML> tag
                         // now, content contains whole <XML>blablabla</XML> 
                         // parse it 
        
                        // prepare to next synching
                        startIndex = -1;
                        endIndex = -1;
                    }
                }
            }
        }
        
        1 Reply Last reply
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        • mzimmersM mzimmers

          @mrjj said in Reading from COM port yields strange results:

          @mzimmers
          If the MSVC is using native serial API , it might be buffered more.

          So...are there any settings I can change, or do I need to handle the input stream in the application?

          I'm still unclear why I'm getting readyRead signals with 0 bytes available for reading, too.

          hskoglundH Online
          hskoglundH Online
          hskoglund
          wrote on last edited by
          #10

          @mzimmers This might happen if there was an error reading the data, like buffer overflow, try checking what port->error(); says.

          1 Reply Last reply
          0
          • K Offline
            K Offline
            kuzulis
            Qt Champions 2020
            wrote on last edited by
            #11

            This might happen if there was an error reading the data, like buffer overflow

            not!

            1 Reply Last reply
            0
            • mzimmersM Offline
              mzimmersM Offline
              mzimmers
              wrote on last edited by
              #12

              The error code isn't revealing anything (other than a design flaw I've posted in another thread).

              Every 2nd time that readyRead() signals, the read returns 0 bytes. According to the docs:

              readyRead() is not emitted recursively; if you reenter the event loop or call waitForReadyRead() inside a slot connected to the readyRead() signal, the signal will not be reemitted (although waitForReadyRead() may still return true).
              

              It doesn't seem to me like I need a waitForReadyRead() call...anyone disagree with this?

              1 Reply Last reply
              0
              • mrjjM Offline
                mrjjM Offline
                mrjj
                Lifetime Qt Champion
                wrote on last edited by mrjj
                #13

                @mzimmers said in Reading from COM port yields strange results:

                waitForReadyRead

                I agree. its for blocking use and you are using the asynchronous api so no need for that.

                1 Reply Last reply
                1
                • K Offline
                  K Offline
                  kuzulis
                  Qt Champions 2020
                  wrote on last edited by
                  #14

                  @mzimmers said in Reading from COM port yields strange results:

                  Every 2nd time that readyRead() signals, the read returns 0 bytes

                  Did you hear me? It is impossible!

                  1 Reply Last reply
                  0
                  • mzimmersM Offline
                    mzimmersM Offline
                    mzimmers
                    wrote on last edited by
                    #15

                    I found the cause of the readyRead() firing with 0 bytes available -- it had to do with my QComboBox somehow emitting 2 signals when the value changed. That problem is solved.

                    It's still a mystery to me why the response to my discovery request comes in pieces, but I think I've got it fixed. This routine also trims off any spurious traffic (the com port is used for much more than responding to discovery requests).

                    void CdSerial::read()
                    {
                        buffer += port->readAll();
                        qDebug() << "buffer contains " << buffer.size() << " characters.";
                    
                        if (buffer.contains(XML_START_TAG))
                        {
                            if (buffer.contains(XML_STOP_TAG))
                            {
                                // trim off any "fluff" and convert to QString.
                                trim(buffer, qBuffer);
                                emit newText(qBuffer);
                                buffer.clear();
                            }
                        }
                        else // currently not interested in any non-XML traffic.
                        {
                            buffer.clear();
                        }
                    }
                    

                    It's a lot of work being done in a slot, but given that this is responding to a comparatively slow device (the serial port), I'm assuming it's OK. I welcome any feedback on my routine above.

                    Thanks for all the assistance.

                    kshegunovK 1 Reply Last reply
                    0
                    • mzimmersM mzimmers

                      I found the cause of the readyRead() firing with 0 bytes available -- it had to do with my QComboBox somehow emitting 2 signals when the value changed. That problem is solved.

                      It's still a mystery to me why the response to my discovery request comes in pieces, but I think I've got it fixed. This routine also trims off any spurious traffic (the com port is used for much more than responding to discovery requests).

                      void CdSerial::read()
                      {
                          buffer += port->readAll();
                          qDebug() << "buffer contains " << buffer.size() << " characters.";
                      
                          if (buffer.contains(XML_START_TAG))
                          {
                              if (buffer.contains(XML_STOP_TAG))
                              {
                                  // trim off any "fluff" and convert to QString.
                                  trim(buffer, qBuffer);
                                  emit newText(qBuffer);
                                  buffer.clear();
                              }
                          }
                          else // currently not interested in any non-XML traffic.
                          {
                              buffer.clear();
                          }
                      }
                      

                      It's a lot of work being done in a slot, but given that this is responding to a comparatively slow device (the serial port), I'm assuming it's OK. I welcome any feedback on my routine above.

                      Thanks for all the assistance.

                      kshegunovK Offline
                      kshegunovK Offline
                      kshegunov
                      Moderators
                      wrote on last edited by
                      #16

                      I'd consider implementing it with QXmlStreamReader instead of reinventing the wheel. It supports incremental parsing too, which is quite convenient for the thing you seem to be trying to accomplish.

                      Read and abide by the Qt Code of Conduct

                      mzimmersM 1 Reply Last reply
                      0
                      • kshegunovK kshegunov

                        I'd consider implementing it with QXmlStreamReader instead of reinventing the wheel. It supports incremental parsing too, which is quite convenient for the thing you seem to be trying to accomplish.

                        mzimmersM Offline
                        mzimmersM Offline
                        mzimmers
                        wrote on last edited by
                        #17

                        @kshegunov I actually did experiment with QXmlStreamReader. Given the nature of the input stream, though, I found it safer to form the string and then parse it. At this time, I'm not really doing anything with the response other than displaying it, so parsing isn't necessary. I'm sure that will soon change, and I'll probably take another look at it. Thanks for the suggestion.

                        1 Reply Last reply
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