checksum calculation

Question

To calculate CRC I found a piece of code but I am not understanding the concept. Here is the code:

count =128 and ptr=some value;


calcrc(unsigned char *ptr, int count)
{
    unsigned short  crc;
    unsigned char i;

    crc = 0;

    while (--count >= 0)
    {
        crc = crc ^ (unsigned short)*ptr++ << 8;
        i = 8;
        do
        {
            if (crc & 0x8000)
                crc = crc << 1 ^ 0x1021;
            else
                crc = crc << 1;
        } while(--i);
    }
    return (crc);
}

Please any body explain and tell me the logic.

I suggest you be more specific about what you don't understand. Is it the syntax or semantics of the algorithm, something else??? — andreas buykx, May 11 '09 at 11:08

score 4 · Answer 1 · answered May 11 '09 at 11:09

This looks like a CRC (specifically it looks like CRC-16-CCITT, used by things like 802.15.4, X.25, V.41, CDMA, Bluetooth, XMODEM, HDLC, PPP and IrDA). You might want to read up on the CRC theory on the linked-to Wikipedia page, to gain some more insight. Or you can view this as a "black box" that just solves the problem of computing a checksum.

score 3 · Answer 2 · answered May 11 '09 at 11:38

You will probably need to know that in C, the ^ operator is a bitwise XOR operator and the << operator is the left shift operator (equivalent to multiplication by 2 to the power of the number on the right of the operator). Also the crc & 0x8000 expression is testing for the most significant bit set of the variable crc. This will help you to work out a low level explanation of what is occurring when this runs, for a high level explanation of what a CRC is and why you might need it, read the Wikipedia page or How Stuff Works.

score 3 · Answer 3 · answered May 11 '09 at 11:53

3

One famous text on CRCs is "A Painless Guide to CRC Error Detection Algorithms" by Ross Williams. It takes some time to absorb but it's pretty thorough.

answered May 11 '09 at 11:53

Craig McQueen

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score 1 · Answer 4 · edited May 23 '17 at 12:13

1

Take a look at my answer to How could I guess a checksum algorithm?

edited May 23 '17 at 12:13

Community

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answered Nov 12 '09 at 13:47

selwyn

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checksum calculation

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