NAME¶
cksum,
sum —
display
file checksums and block counts
SYNOPSIS¶
cksum |
[-o 1 |
2 | 3]
[file ...] |
DESCRIPTION¶
The
cksum utility writes to the standard output three
whitespace separated fields for each input file. These fields are a checksum
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.
The
sum utility is identical to the
cksum
utility, except that it defaults to using historic algorithm 1, as described
below. It is provided for compatibility only.
The options are as follows:
- -o
- Use historic algorithms instead of the (superior) default
one.
Algorithm 1 is the algorithm used by historic BSD
systems as the sum(1) algorithm and by historic
AT&T System V UNIX systems as the
sum(1) 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(1)
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;
Algorithm 3 is what is commonly called the ‘32bit
CRC
’ algorithm. This is a 32-bit checksum.
Both algorithm 1 and 2 write to the 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 to
be evaluated are considered to be the coefficients of a mod 2 polynomial M(x)
of degree
n-1. These
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.
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.
EXIT STATUS¶
The
cksum and
sum utilities exit 0 on
success, and >0 if an error occurs.
SEE ALSO¶
md5(1)
The default calculation is identical to that given in pseudo-code in the
following ACM article.
Dilip V. Sarwate,
Computation of Cyclic Redundancy Checks Via Table
Lookup, Communications of the Tn ACM,
August 1988.
STANDARDS¶
The
cksum utility is expected to conform to
IEEE Std 1003.2-1992 (“POSIX.2”).
HISTORY¶
The
cksum utility appeared in
4.4BSD.