gno/usr.bin/cksum/cksum.1

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.\"	@(#)cksum.1	8.2 (Berkeley) 4/28/95
.\"	$Id: cksum.1,v 1.2 1998/02/15 00:25:44 gdr-ftp Exp $
.\"
.TH CKSUM 1 "December 1997" "GNO" "Commands and Applications"
.SH NAME
.BR cksum ,
.BR sum
\- display file checksums and block counts
.SH SYNOPSIS
.BR cksum
.RB [ "-o 1" " | " 2 " | " 3
.RI "]  [" file\0 ...]
.PP
.BR sum
.RI [ file\0 ...]
.SH DESCRIPTION
The
.BR cksum
utility writes to the standard output three whitespace separated
fields for each input file.
These fields are a checksum
.BR CRC ,
the total number of octets in the file and the file name.
If no file name is specified, the standard input is used and no file name
is written.
.PP
The
.BR sum
utility is identical to the
.BR cksum
utility, except that it defaults to using historic algorithm 1,
described below.
It is provided for compatibility.
You can either install the
.BR sum
program (which has the same content as the
.BR cksum
program file) or create a
.IR gsh (1)
alias that invokes
.BR cksum :
.nf
       alias sum "cksum -o 1"
.fi
.PP
The program options are as follows:
.RS
.IP \fB-o\fR
Use historic algorithms instead of the (superior) default one.
.PP
.RS
Algorithm 1 is the algorithm used by historic
systems as the 
.BR sum (1)
algorithm and by historic
systems as the
.BR sum 
algorithm when using the
.BR -r
option.
This is a 16-bit checksum, with a right rotation before each addition;
overflow is discarded.
.PP
.RS
Algorithm 2 is the algorithm used by historic
systems as the
default
.BR sum 
algorithm.
This is a 32-bit checksum, and is defined as follows:
.nf
      s = sum of all bytes;
      r = s % 2^16 + (s % 2^32) / 2^16;
      cksum = (r % 2^16) + r / 2^16;
.fi
.PP
.RS
Algorithm 3, a 32-bit checksum, is commonly called the
.BR "32bit CRC"
algorithm.
.PP
.RS
Algorithms 1 and 2 write the same fields as
the default algorithm except the size of the file is
expressed in blocks.
For historic reasons, the block size is 1024 for algorithm 1 and 512
for algorithm 2.
Partial blocks are rounded up.
.RE
.PP
The default
.BR CRC
used is based on the polynomial used for
.BR CRC
error checking
in the networking standard
POSIX-3.
The
.BR CRC
checksum encoding is defined by the generating polynomial:
.PP
.nf
G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
     x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
.fi
.PP
Mathematically, the
.BR CRC
value corresponding to a given file is defined by
the following procedure:
.PP
.RS
The
.BR n
bits to be evaluated are considered to be the coefficients of a mod 2
polynomial M(x) of degree
.BR n-1 .
These
.BR n
bits are the bits from the file, with the most significant bit being the most
significant bit of the first octet of the file and the last bit being the least
significant bit of the last octet, padded with zero bits (if necessary) to
achieve an integral number of octets, followed by one or more octets
representing the length of the file as a binary value, least significant octet
first.
The smallest number of octets capable of representing this integer are used.
.PP
.RS
M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by
G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.
The coefficients of R(x) are considered to be a 32-bit sequence.
The bit sequence is complemented and the result is the CRC.
.PP
.RS
The default calculation is identical to that given in pseudo-code
in the
article "Computation of Cyclic Redundancy Checks Via Table Lookup"
by
Dilip V. Sarwate,
.IR "Communications of the ACM" ", August 1988."
.RE
.PP
The
.BR cksum
and
.BR sum
utilities exit 0 on success, and >0 if an error occurs.
.SH VERSION
This manual page documents
.BR cksum
version 2.0.
.SH ATTRIBUTIONS
This command was ported from FreeBSD source code
for distribution with GNO/ME 2.0.6.
.SH HISTORY
A version of
.BR sum
translated from GNU code in 1991 by Marek Pawlowski was distributed with
earlier releases of GNO. Unfortunately, as that version reads files it
translates carriage return characters into new line characters. To use
.BR cksum
to calulate a checksum that matches the old
.BR sum ,
use
.IR tr (1)
to translate the input stream. For example,
.nf
          tr '\\r' '\\n' < filename | cksum -o 1
.fi
.SH STANDARDS
The
.BR cksum
utility is expected to conform to
POSIX-2.