159 lines
4.8 KiB
Groff
159 lines
4.8 KiB
Groff
.\" Copyright (c) 1991, 1991 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to Berkeley by
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.\" the Institute of Electrical and Electronics Engineers, Inc.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)cksum.1 5.5 (Berkeley) 6/27/91
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.\"
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.Dd June 27, 1991
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.Dt CKSUM 1
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.Os
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.Sh NAME
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.Nm cksum
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.Nd display file checksums and block counts
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.Sh SYNOPSIS
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.Nm cksum
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.Op Fl o Op \&1 \&| \&2
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.Op Ar file ...
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.Sh DESCRIPTION
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The
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.Nm cksum
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utility writes to standard output three whitespace separated fields for each
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input file (or the standard input by default).
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These fields are a checksum
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.Tn CRC ,
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the total number of bytes in the file and
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the file name.
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.Pp
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The options are as follows:
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.Bl -tag -width indent
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.It Fl o
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Use historic algorithms instead of the (superior) default one.
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.Pp
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Algorithm 1 is the algorithm used by historic
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.Bx
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systems as the
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.Xr sum 1
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algorithm and by historic
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.At V
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systems as the
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.Xr sum
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algorithm when using the
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.Fl r
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option.
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This is a 16-bit checksum, with a right rotation before each addition;
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overflow is discarded.
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.Pp
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Algorithm 2 is the algorithm used by historic
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.At V
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systems as the
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default
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.Xr sum
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algorithm.
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This is a 32-bit checksum, and is defined as follows:
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.Bd -unfilled -offset indent
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s = sum of all bytes;
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r = s % 2^16 + (s % 2^32) / 2^16;
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cksum = (r % 2^16) + r / 2^16;
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.Ed
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.Pp
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Both algorithm 1 and 2 write to standard output the same fields as
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the default algorithm except that the size of the file in bytes is
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replaced with the size of the file in blocks.
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For historic reasons, the block size is 1024 for algorithm 1 and 512
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for algorithm 2.
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Partial blocks are rounded up.
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.El
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.Pp
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The default
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.Tn CRC
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used is based on the polynomial used for
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.Tn CRC
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error checking
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in the networking standard
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.St -iso8802-3
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The
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.Tn CRC
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checksum encoding is defined by the generating polynomial:
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.Pp
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.Bd -unfilled -offset indent
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G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
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x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
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.Ed
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.Pp
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Mathematically, the
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.Tn CRC
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value corresponding to a given file is defined by
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the following procedure:
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.Bd -filled -offset indent
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The
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.Ar n
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bits of the file are considered to be the coefficients of a mod 2
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polynomial M(x) of degree
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.Ar n Ns \-1 .
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.Pp
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M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by
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G(x) using mod 2 division, producing a remainder R(x) of degree <= 31.
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During the division, each time the intermediate remainder is zero, it
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is changed to the next value from a predefined sequence of
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32-bit integers before completing the division.
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This sequence is long and complex -- see the source code for more
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information.
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.Pp
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The coefficients of R(x) are considered to be a 32-bit sequence.
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.Ed
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.Pp
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The calculation used is identical to that given in pseudo-code in
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the
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.Tn ACM
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article referenced below.
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.Pp
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The
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.Nm cksum
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utility exits 0 on success, and >0 if an error occurs.
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.Sh SEE ALSO
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.Rs
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.%T "Computation of Cyclic Redundancy Checks Via Table Lookup"
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.%A Dilip V. Sarwate
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.%J "Communications of the \\*(tNACM\\*(sP"
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.%D "August 1988"
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.Re
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.Sh STANDARDS
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.Nm Cksum
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is expected to conform to
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.St -p1003.2 .
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.Sh HISTORY
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The
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.Nm cksum
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utility is
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.Ud .
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