cksum
,
sum
—
display file checksums and block counts
cksum |
[ -o
1 | 2 |
3 ]
[file ... ] |
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.
The
cksum
and
sum
utilities exit 0 on success,
and >0 if an error occurs.
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.
The
cksum
utility is expected to conform to
IEEE Std 1003.2-1992
(“POSIX.2”).
The
cksum
utility appeared in
4.4BSD.