xref: /dragonfly/usr.bin/cksum/cksum.1 (revision 67640b13) 
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31.\"	@(#)cksum.1	8.2 (Berkeley) 4/28/95
32.\" $FreeBSD: src/usr.bin/cksum/cksum.1,v 1.19 2005/01/17 07:44:13 ru Exp $
33.\" $DragonFly: src/usr.bin/cksum/cksum.1,v 1.4 2008/05/02 02:05:07 swildner Exp $
34.\"
35.Dd April 28, 1995
36.Dt CKSUM 1
37.Os
38.Sh NAME
39.Nm cksum ,
40.Nm sum
41.Nd display file checksums and block counts
42.Sh SYNOPSIS
43.Nm
44.Op Fl o Ar 1 | 2 | 3
45.Op Ar
46.Nm sum
47.Op Ar
48.Sh DESCRIPTION
49The
50.Nm
51utility writes to the standard output three whitespace separated
52fields for each input file.
53These fields are a checksum
54.Tn CRC ,
55the total number of octets in the file and the file name.
56If no file name is specified, the standard input is used and no file name
57is written.
58.Pp
59The
60.Nm sum
61utility is identical to the
62.Nm
63utility, except that it defaults to using historic algorithm 1, as
64described below.
65It is provided for compatibility only.
66.Pp
67The options are as follows:
68.Bl -tag -width indent
69.It Fl o
70Use historic algorithms instead of the (superior) default one.
71.Pp
72Algorithm 1 is the algorithm used by historic
73.Bx
74systems as the
75.Xr sum 1
76algorithm and by historic
77.At V
78systems as the
79.Xr sum 1
80algorithm when using the
81.Fl r
82option.
83This is a 16-bit checksum, with a right rotation before each addition;
84overflow is discarded.
85.Pp
86Algorithm 2 is the algorithm used by historic
87.At V
88systems as the
89default
90.Xr sum 1
91algorithm.
92This is a 32-bit checksum, and is defined as follows:
93.Bd -unfilled -offset indent
94s = sum of all bytes;
95r = s % 2^16 + (s % 2^32) / 2^16;
96cksum = (r % 2^16) + r / 2^16;
97.Ed
98.Pp
99Algorithm 3 is what is commonly called the
100.Ql 32bit CRC
101algorithm.
102This is a 32-bit checksum.
103.Pp
104Both algorithm 1 and 2 write to the standard output the same fields as
105the default algorithm except that the size of the file in bytes is
106replaced with the size of the file in blocks.
107For historic reasons, the block size is 1024 for algorithm 1 and 512
108for algorithm 2.
109Partial blocks are rounded up.
110.El
111.Pp
112The default
113.Tn CRC
114used is based on the polynomial used for
115.Tn CRC
116error checking
117in the networking standard
118.St -iso8802-3 .
119The
120.Tn CRC
121checksum encoding is defined by the generating polynomial:
122.Bd -unfilled -offset indent
123G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
124     x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
125.Ed
126.Pp
127Mathematically, the
128.Tn CRC
129value corresponding to a given file is defined by
130the following procedure:
131.Bd -ragged -offset indent
132The
133.Ar n
134bits to be evaluated are considered to be the coefficients of a mod 2
135polynomial M(x) of degree
136.Ar n Ns \-1 .
137These
138.Ar n
139bits are the bits from the file, with the most significant bit being the most
140significant bit of the first octet of the file and the last bit being the least
141significant bit of the last octet, padded with zero bits (if necessary) to
142achieve an integral number of octets, followed by one or more octets
143representing the length of the file as a binary value, least significant octet
144first.
145The smallest number of octets capable of representing this integer are used.
146.Pp
147M(x) is multiplied by x^32 (i.e., shifted left 32 bits) and divided by
148G(x) using mod 2 division, producing a remainder R(x) of degree \(<= 31.
149.Pp
150The coefficients of R(x) are considered to be a 32-bit sequence.
151.Pp
152The bit sequence is complemented and the result is the CRC.
153.Ed
154.Sh EXIT STATUS
155.Ex -std cksum sum
156.Sh SEE ALSO
157.Xr md5 1
158.Pp
159The default calculation is identical to that given in pseudo-code
160in the following
161.Tn ACM
162article.
163.Rs
164.%T "Computation of Cyclic Redundancy Checks Via Table Lookup"
165.%A Dilip V. Sarwate
166.%J "Communications of the" Tn ACM
167.%D "August 1988"
168.Re
169.Sh STANDARDS
170The
171.Nm
172utility is expected to conform to
173.St -p1003.2-92 .
174.Sh HISTORY
175The
176.Nm
177utility appeared in
178.Bx 4.4 .
179