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    Solved Reading from COM port yields strange results

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    • mzimmers
      mzimmers last edited by mzimmers

      I'm writing a program that communicates with an embedded device via USB-UART interface. In simplest terms, I'm writing out a request for the embedded device to identify itself, and I read the response. Here's a snippet from the read logic:

          QSerialPort *port;
      ...
          connect(port, &QSerialPort::readyRead, this, &CdSerial::read);
      ...
      void CdSerial::read()
      {
          QByteArray buffer;
          int nBytes = port->bytesAvailable();
          if (nBytes > 0)
          {
              buffer = port->readAll();
          }
      ...
      

      The communication is working, but two things are happening that are giving me problems:

      • I'm expecting a response of about 600 characters. This response should come in on a single read, but it's being broken up into pieces. This is not being caused by newlines.
      • between each "valid" read, I get a signal but nBytes comes back as 0. I'm not sure whether this is contributing to my problem.

      I encountered this problem in Windows 10, and verified it on Windows 7. We don't get this behavior when using a MSVS program to do the same thing.

      Is there some additional set-up of the port I need to do? I've already set the buffer size to 1024. I've read through the pages on QSerialPort and QIODevice, and haven't found anything.

      Any help is appreciated. Thanks...

      1 Reply Last reply Reply Quote 0
      • mrjj
        mrjj Lifetime Qt Champion last edited by mrjj

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

        This response should come in on a single read,

        But it wont always. Its not how it works. Depending on internal cache etc, it will/might be broken up.

        But its easy to fix

        buffer = port->readAll();
        should be
        buffer += port->readAll();
        and when buffer.size() == expected_size you process the input.

        and QByteArray buffer; should move to a class member and not be defined locally.

        mzimmers 1 Reply Last reply Reply Quote 3
        • mzimmers
          mzimmers last edited by

          The message coming back is of variable size, so I can't use the size as a criterion for termination. I can use the string "</XML>" though.

          I didn't post the entire read() routine for brevity, but I guess I should:

          void CdSerial::read()
          {
              QByteArray buffer;
              static QString qBuffer;
              int nBytes = port->bytesAvailable();
              if (nBytes > 0)
              {
                  buffer = port->readAll();
              }
              // only show XML formatted input.
              if (strstr(buffer.toStdString().c_str(), XmlStartTag) != NULL)
              {
                  // may need to trim off some text ahead of the <XML>.
                  buffer.right(buffer.indexOf(XmlStartTag));
                  XmlState = STARTED;
              }
              if (strstr(buffer.toStdString().c_str(), XMLStopTag) != NULL)
              {
                  // may need to trim off some text behind the </XML>.
                  buffer.truncate(buffer.indexOf(XMLStopTag) + XMLStopTag.length());
                  XmlState = ENDING;
              }
          
              if ((XmlState == STARTED) || (XmlState == ENDING))
              {
                  qDebug() << nBytes << " bytes read from serial port.";
                  qDebug() << buffer.toHex() << endl;
          
                  qBuffer = buffer;
                  emit newText(qBuffer);
                  if (XmlState == ENDING)
                  {
                      XmlState = ENDED;
                  }
              }
          }
          

          I welcome any critique of this method.

          Thanks...

          1 Reply Last reply Reply Quote 0
          • mzimmers
            mzimmers @mrjj last edited by

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

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

            This response should come in on a single read,

            But it wont always. Its not how it works. Depending on internal cache etc, it will/might be broken up.

            But it seems to be when another program built in MSVC does the same thing. That's why I'm wondering whether I've failed to properly configure the port or something.

            mrjj 1 Reply Last reply Reply Quote 0
            • mrjj
              mrjj Lifetime Qt Champion last edited by

              Hi
              When input length is dynamic a data maker is often used.
              So if /XML can be used for that its a valid design.
              But you still need buffer += port->readAll(); :)

              1 Reply Last reply Reply Quote 0
              • mrjj
                mrjj Lifetime Qt Champion @mzimmers last edited by

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

                mzimmers 1 Reply Last reply Reply Quote 0
                • mzimmers
                  mzimmers @mrjj last edited by

                  @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.

                  hskoglund 1 Reply Last reply Reply Quote 0
                  • mrjj
                    mrjj Lifetime Qt Champion last edited by

                    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 Reply Quote 0
                    • K
                      kuzulis Qt Champions 2020 last edited by kuzulis

                      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 Reply Quote 2
                      • hskoglund
                        hskoglund @mzimmers last edited by

                        @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 Reply Quote 0
                        • K
                          kuzulis Qt Champions 2020 last edited by

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

                          not!

                          1 Reply Last reply Reply Quote 0
                          • mzimmers
                            mzimmers last edited by

                            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 Reply Quote 0
                            • mrjj
                              mrjj Lifetime Qt Champion last edited by mrjj

                              @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 Reply Quote 1
                              • K
                                kuzulis Qt Champions 2020 last edited by

                                @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 Reply Quote 0
                                • mzimmers
                                  mzimmers last edited by

                                  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.

                                  kshegunov 1 Reply Last reply Reply Quote 0
                                  • kshegunov
                                    kshegunov Moderators @mzimmers last edited by

                                    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

                                    mzimmers 1 Reply Last reply Reply Quote 0
                                    • mzimmers
                                      mzimmers @kshegunov last edited by

                                      @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 Reply Quote 0
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