CKSUM(1) 386BSD Reference Manual CKSUM(1)
NAME
cksum - display file checksums and block counts
SYNOPSIS
cksum [-o [1 | 2]] [file ...]
DESCRIPTION
The cksum utility writes to standard output three whitespace separated
fields for each input file (or the standard input by default). These
fields are a checksum CRC, the total number of bytes in the file and the
file name.
The options are as follows:
-o Use historic algorithms instead of the (superior) default one.
Algorithm 1 is the algorithm used by historic BSD UNIX systems as
the sum(1) algorithm and by historic AT&T System V UNIX systems
as the sum algorithm when using the -r option. This is a 16-bit
checksum, with a right rotation before each addition; overflow is
discarded.
Algorithm 2 is the algorithm used by historic AT&T System V UNIX
systems as the default sum algorithm. This is a 32-bit checksum,
and is defined as follows:
s = sum of all bytes;
r = s % 2^16 + (s % 2^32) / 2^16;
cksum = (r % 2^16) + r / 2^16;
Both algorithm 1 and 2 write to standard output the same fields
as the default algorithm except that the size of the file in
bytes is replaced with the size of the file in blocks. For
historic reasons, the block size is 1024 for algorithm 1 and 512
for algorithm 2. Partial blocks are rounded up.
The default CRC used is based on the polynomial used for CRC error
checking in the networking standard ISO 8802-3: 1989 The CRC checksum
encoding is defined by the generating polynomial:
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
Mathematically, the CRC value corresponding to a given file is defined by
the following procedure:
The n bits of the file are considered to be the coefficients of a
mod 2 polynomial M(x) of degree n-1.
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. During the division, each time the intermediate remainder
is zero, it is changed to the next value from a predefined sequence
of 32-bit integers before completing the division. This sequence
is long and complex -- see the source code for more information.
The coefficients of R(x) are considered to be a 32-bit sequence.
The calculation used is identical to that given in pseudo-code in the ACM
article referenced below.
The cksum utility exits 0 on success, and >0 if an error occurs.
SEE ALSO
Dilip V. Sarwate, "Computation of Cyclic Redundancy Checks Via Table
Lookup", Communications of the ACM, August 1988.
STANDARDS
Cksum is expected to conform to IEEE Std1003.2 (``POSIX'').
HISTORY
The cksum utility is currently under development.
BSD Experimental June 27, 1991 2