Re: Lockless file reading

[Valdis . Kletnieks]

On Thu, 28 Aug 2003 06:17:46 +0300, Timo Sirainen said:

> How about checksum[n] = data[n-1] ^ data[n]? That looks like it would 
> work.

Unfortunately, if you swap the contents of 'n' and 'n-1', the ^ remains the same,
as it's a commutative operation.

If you want to go down this route, you *really* want to use at least a CRC
here, and understand *why* the incremental computation of the IP header
checksum works at all (hint - the fact it's so simple puts a significant upper
bound on the error-detection strength of the checksum...)

1071 Computing the Internet checksum. R.T. Braden, D.A. Borman, C.
     Partridge. Sep-01-1988. (Format: TXT=54941 bytes) (Updated by
     RFC1141) (Status: UNKNOWN)

1141 Incremental updating of the Internet checksum. T. Mallory, A.
     Kullberg. Jan-01-1990. (Format: TXT=3587 bytes) (Updates RFC1071)
     (Updated by RFC1624) (Status: INFORMATIONAL)

1624 Computation of the Internet Checksum via Incremental Update. A.
     Rijsinghani, Ed.. May 1994. (Format: TXT=9836 bytes) (Updates
     RFC1141) (Status: INFORMATIONAL)

http://www.ietf.org/rfc/rfc1071.txt
http://www.ietf.org/rfc/rfc1141.txt
http://www.ietf.org/rfc/rfc1624.txt

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