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How to calculate 8-bit Checksum (2’s complement)?

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  • JonBJ JonB

    @ahsan737
    You'll have to be more specific about just what you mean. "Hex format" is not relevant, as per your code you'll convert any strings into the actual numbers and do the math on that. 8-bit checksum is often just sum() % 255 or (uint8_t)sum(), perhaps you want a ~ in front of it for your "2’s complement". See maybe https://stackoverflow.com/questions/31151032/writing-an-8-bit-checksum-in-c or https://stackoverflow.com/questions/27505240/computing-a-8-bit-checksum-in-c.

    Your current

                char hex[hexString.size()];
                strcpy(hex, hexString); 
                QByteArray data=QByteArray::fromHex(hex);
    

    is simply a waste of time & space, you can just QByteArray::fromHex(hexString);. Worse that that, you are actually copying one more character (the terminating \0) from hexString than the size you have allocated for stack variable hex, so you're lucky if this does not corrupt your stack....

    As a side note your, code is assuming that socket->readAll() inside socket->bytesAvailable() will read all bytes, sufficient for all your expected variables (and no more than that). This is dangerous, readAll() only returns however many happen to be there at the instant you call bytesAvailable(), which may not yet be all sent (or could even be more than for one structure, depending on your protocol). You should buffer your input till you have enough for your structure, you could look at QDataStream::startTransaction().

    ahsan737A Offline
    ahsan737A Offline
    ahsan737
    wrote on last edited by
    #3

    @JonB
    Thank you so much for pointing out mistakes.
    for checksum: I have to calculate checksum for given data and make comparison with the checksum reference value.

    checksum.JPG

    JonBJ 1 Reply Last reply
    0
    • ahsan737A ahsan737

      @JonB
      Thank you so much for pointing out mistakes.
      for checksum: I have to calculate checksum for given data and make comparison with the checksum reference value.

      checksum.JPG

      JonBJ Online
      JonBJ Online
      JonB
      wrote on last edited by JonB
      #4

      @ahsan737
      You haven't said how that 36 is calculated from the input bytes. I already suggested to you

      is often just sum() % 255 or (uint8_t)sum(), perhaps you want a ~ in front of it for your "2’s complement".

      You can do it from here, I'm not going to figure how you come up with your checksum number or do the C++ for you.

      ahsan737A 1 Reply Last reply
      0
      • hskoglundH Offline
        hskoglundH Offline
        hskoglund
        wrote on last edited by
        #5

        Hi, should be something like this:

        QByteArray receivedBytes = socket->readAll();
        
        char c = 0;
        for (auto b : receivedBytes)
            c += b;
        
        c = 256 - c;
        qDebug() << QString::number(c,16);
        

        (I got 36 when i tested with

         QByteArray receivedBytes = QByteArray::fromHex("D4 0A 1B 73 0F 3A 0F 41 00 A7 0A 14")
        
        JonBJ ahsan737A 2 Replies Last reply
        3
        • JonBJ JonB

          @ahsan737
          You haven't said how that 36 is calculated from the input bytes. I already suggested to you

          is often just sum() % 255 or (uint8_t)sum(), perhaps you want a ~ in front of it for your "2’s complement".

          You can do it from here, I'm not going to figure how you come up with your checksum number or do the C++ for you.

          ahsan737A Offline
          ahsan737A Offline
          ahsan737
          wrote on last edited by
          #6

          @JonB Thanks, got your point. And I have already mentioned checksum type twice (question title, and description).

          1 Reply Last reply
          0
          • hskoglundH hskoglund

            Hi, should be something like this:

            QByteArray receivedBytes = socket->readAll();
            
            char c = 0;
            for (auto b : receivedBytes)
                c += b;
            
            c = 256 - c;
            qDebug() << QString::number(c,16);
            

            (I got 36 when i tested with

             QByteArray receivedBytes = QByteArray::fromHex("D4 0A 1B 73 0F 3A 0F 41 00 A7 0A 14")
            
            JonBJ Online
            JonBJ Online
            JonB
            wrote on last edited by JonB
            #7

            @hskoglund said in How to calculate 8-bit Checksum (2’s complement)?:

            c = 256 - c;
            qDebug() << QString::number(c,16);

            Slight correction: This generates a number in range 1--256. Isn't the checksum supposed to be a byte 0--255 (uint8_t cs)? Yet you say you did get 36 on the data, so perhaps not, but surprises me....

            hskoglundH 1 Reply Last reply
            0
            • hskoglundH hskoglund

              Hi, should be something like this:

              QByteArray receivedBytes = socket->readAll();
              
              char c = 0;
              for (auto b : receivedBytes)
                  c += b;
              
              c = 256 - c;
              qDebug() << QString::number(c,16);
              

              (I got 36 when i tested with

               QByteArray receivedBytes = QByteArray::fromHex("D4 0A 1B 73 0F 3A 0F 41 00 A7 0A 14")
              
              ahsan737A Offline
              ahsan737A Offline
              ahsan737
              wrote on last edited by
              #8

              @hskoglund
              thank you for such a detailed response. It holds true for other data strings that I have tested.

              1 Reply Last reply
              1
              • JonBJ JonB

                @hskoglund said in How to calculate 8-bit Checksum (2’s complement)?:

                c = 256 - c;
                qDebug() << QString::number(c,16);

                Slight correction: This generates a number in range 1--256. Isn't the checksum supposed to be a byte 0--255 (uint8_t cs)? Yet you say you did get 36 on the data, so perhaps not, but surprises me....

                hskoglundH Offline
                hskoglundH Offline
                hskoglund
                wrote on last edited by
                #9

                @JonB No worries, a char has only 8 bits so the value 256 will be truncated to 0, i.e. the range will still be 0..255.
                Basically you're translating from signed arithmetic to unsigned d:o using 2's-complement.

                JonBJ 1 Reply Last reply
                0
                • hskoglundH hskoglund

                  @JonB No worries, a char has only 8 bits so the value 256 will be truncated to 0, i.e. the range will still be 0..255.
                  Basically you're translating from signed arithmetic to unsigned d:o using 2's-complement.

                  JonBJ Online
                  JonBJ Online
                  JonB
                  wrote on last edited by JonB
                  #10

                  @hskoglund
                  Yes I know that (bits, signed, overflow wrapping), I thought the result would be wrong. E.g. if there is, say, just 1 byte with value 0, then 256 - c will generate 256 == char(0) == 0x00 for the checksum. I expected that to need to be 255 - c => 255 == char(255) == 0xFF for the 2's complement? So I don't get it. But since your answer matches what the OP expects, there you are....

                  EDIT OK, I looked up https://en.wikipedia.org/wiki/Two's_complement

                  The two's complement is calculated by inverting the digits and adding one

                  I didn't know about the "adding one"! :) I thought it was just ~number, got it now!

                  M 1 Reply Last reply
                  1
                  • JonBJ JonB

                    @hskoglund
                    Yes I know that (bits, signed, overflow wrapping), I thought the result would be wrong. E.g. if there is, say, just 1 byte with value 0, then 256 - c will generate 256 == char(0) == 0x00 for the checksum. I expected that to need to be 255 - c => 255 == char(255) == 0xFF for the 2's complement? So I don't get it. But since your answer matches what the OP expects, there you are....

                    EDIT OK, I looked up https://en.wikipedia.org/wiki/Two's_complement

                    The two's complement is calculated by inverting the digits and adding one

                    I didn't know about the "adding one"! :) I thought it was just ~number, got it now!

                    M Offline
                    M Offline
                    mpergand
                    wrote on last edited by mpergand
                    #11

                    @JonB response is correct.

                    From the code I posted earlier, you can add:

                    // checksum = 36h 54d
                    uint8_t c=0;
                    for(uint8_t v : data) c+=v;
                    c-=cs;  // remove the checksum value
                    c=~c;   // 1's complement
                    c++;    // 2's complement
                    
                    if(c != cs)
                        {
                        qDebug()<<"checksum error";
                        }
                    
                    ahsan737A 1 Reply Last reply
                    2
                    • M mpergand

                      @JonB response is correct.

                      From the code I posted earlier, you can add:

                      // checksum = 36h 54d
                      uint8_t c=0;
                      for(uint8_t v : data) c+=v;
                      c-=cs;  // remove the checksum value
                      c=~c;   // 1's complement
                      c++;    // 2's complement
                      
                      if(c != cs)
                          {
                          qDebug()<<"checksum error";
                          }
                      
                      ahsan737A Offline
                      ahsan737A Offline
                      ahsan737
                      wrote on last edited by
                      #12

                      @mpergand
                      thank you very much.

                      1 Reply Last reply
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