[net.dcom] Problem with CRC calculation...need help

mason@ryesone.UUCP (Dave Mason) (04/03/86)

I thought I had a working CRC routine, but it doesn't seem to net to
a zero.  For background a CRC-CCITT or CRC-16 calculated on a message
(plus 2 zero bytes) gives 2 CRC characters.  If you calculate the CRC
on the message with the CRC characters replacing the zero bytes should
give zeros for the CRC.  My CRC routine gives a number that is zero mod 256,
but not necessarily 0.  Any help would be appreciated.  The code follows:

int crc;
main() {
	check("asdfks");
	check("abc");
	check("1231455");
}
check(omsg) char *omsg; {
    int i,j;
    int ch;
    char *msg;
	printf(">>>>>>>>\n");
	msg=omsg;
	crc=0;
	while (ch = *msg++)
		docrc(ch);
	docrc(0);
	docrc(0);
	i=crc/256 % 256;
	j=crc %256;
	printf("------\n");
	msg=omsg;
	crc=0;
	while (ch = *msg++)
		docrc(ch);
	docrc(i);
	docrc(j);
	printf("crc=%d\n",crc);
}
docrc(ch) int ch; {
    register int n;
    register int i,t;
	n=ch;
	n &= 255;
	printf("ch=%d  ",n);
	i=8;
	t=crc;
	while (--i>=0) {
		n<<=1;
		t<<=1;
		if (n & 256)
			++t;
		if (crc<0)
			t ^= 0x1021;
		crc=t;
	  }
	printf("crc=%d\n",crc);
}
Thanks...
-- 
usenet:	..!utzoo!utcsri!mason	Dave Mason, U. Toronto CSRI
	..!utzoo!ryesone!mason	Dave Mason, Ryerson Polytechnical Institute
CSNET:	mason@Toronto
ARPA:	mason%Toronto@CSNet-Relay
BITNET:	FCTY7053@RYERSON.BITNET

mason@ryesone.UUCP (Dave Mason) (04/06/86)

> I thought I had a working CRC routine, but it doesn't seem to net to
> a zero.
Turns out it *was* right, but I was mangling the checksum when I put it
on the line.  The error was in my routine to validate the crc.  Basically
I divided a negative number by 256 as an attempt to shift right by 8.
> check(omsg) char *omsg; {
>     int i,j;
>     int ch;
>     char *msg;
> 	printf(">>>>>>>>\n");
> 	msg=omsg;
> 	crc=0;
> 	while (ch = *msg++)
> 		docrc(ch);
> 	docrc(0);
> 	docrc(0);
> 	i=crc/256 % 256;
<<<<<< 	i=crc>>8;
> 	j=crc %256;
> 	printf("------\n");
> 	msg=omsg;
> 	crc=0;
> 	while (ch = *msg++)
> 		docrc(ch);
> 	docrc(i);
> 	docrc(j);
> 	printf("crc=%d\n",crc);
> }
------
I wish computers were as predictable as dice!
-- 
usenet:	..!utzoo!utcsri!mason	Dave Mason, U. Toronto CSRI
	..!utzoo!ryesone!mason	Dave Mason, Ryerson Polytechnical Institute
CSNET:	mason@Toronto
ARPA:	mason%Toronto@CSNet-Relay
BITNET:	FCTY7053@RYERSON.BITNET