Thread: Making of checksum using XOR

  1. #1
    Registered User
    Join Date
    Jan 2009

    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:

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

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

  2. #2
    Kernel hacker
    Join Date
    Jul 2007
    Farncombe, Surrey, England
    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].

    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
    Registered User
    Join Date
    Jan 2009
    Perfect, Thank you.

  4. #4
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    That's called an LRC.
    In practice, both checksum and LRC are inferrior to a good CRC.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

  5. #5
    Registered User
    Join Date
    Jan 2009
    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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