Making of checksum using XOR

This is a discussion on Making of checksum using XOR within the C Programming forums, part of the General Programming Boards category; Im trying to make a checksum of the content in a datapacket struct and want to make sure I get ...

  1. #1
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    Making of checksum using XOR

    Im trying to make a checksum of the content in a datapacket struct and want to make sure I get it right. Lets say that the struct have three variables where on is an array like this:

    Code:
    typedef struct {
       char a;
       char b;
       char c[10];
    } DataPacket;
    Should I then make the checksum like this?

    Code:
    char checksum;
    
    checksum = a ^ b;
    
    for(i = 0; i < sizeof(c); i++){
       checksum ^= c[i];
    }

  2. #2
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    Yes, you can. And it will be a slightly "better" checksum than using addition, because all bits have an equal chance of being detected as erroneous. But you can only detect an odd-number of failures for each bit.

    An addition checksum fails to detect a double failure of the upper-most bit [assuming the checksum is equal size to the transmission unit, e.g. 1 byte checksum].

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  3. #3
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    Perfect, Thank you.

  4. #4
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    That's called an LRC.
    In practice, both checksum and LRC are inferrior to a good CRC.
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  5. #5
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    Aah, I see. Im trying to comunicate with a device that uses checksum so Im stuck with that.
    The docs I have says "checksum (exclusive OR of above 124 bytes)" I just wanted to make sure that my approach would work or I would be stuck re-sending the first packet. :-)

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