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CRC TCP/IP Lean

Hi All,

Can someone explain what the following code is doing? I'm assuming it has something to do with CRC and overflow.

The full function and some hints can be found here but don't have the book as of yet.

http://books.google.com/books?id=NOLVo6KjmVIC&pg=PA310&lpg=PA310&dq=void+check_byte(BYTE+b)&source=bl&ots=jMj-Xp3B0R&sig=loy2s4WwaqiJl3WT7i350g3xqUc&hl=en&ei=tAm6S5C_KYv88wSJutDqAw&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBEQ6AEwAw#v=onepage&q=void%20check_byte(BYTE%20b)&f=false

void check_byte(BYTE b)
{
    if (checkflag)
    {
        if ((checklo = b+checklo) < b)
        {
            if (++checkhi == 0)
                checklo++;
        }
    }
    else
    {
        if ((checkhi = b+checkhi) < b)
        {
            if (++checklo == 0)
                checkhi++;
        }
    }
    checkflag = !checkflag;
}

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Infinity08

It calculates a 16bit checksum.

This function is called for every byte of the input data. The resulting checksum will be in the chackhi and checklo BYTEs. The checkflag is just used to alternate between the checkhi and checklo BYTEs.

Kyle Abrahams, PMP

ASKER

Still not seeing how this is working:

The code passes in IP's (LWORD) for the b parameter.

I get the checkflag is alternating between low and high but assuming checklo is 0.

checklo = b+ checklo < b will always evaluate to false.

It's been a while since I've been this low (mainly .Net now). . . what's an LWORD and how is it cast to a byte again?

ASKER CERTIFIED SOLUTION

Infinity08

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Kyle Abrahams, PMP

ASKER

Thanks.