Tribblix: manual page: cksum.1

CKSUM(1) User Commands CKSUM(1)

NAME

cksum - write file checksums and sizes

SYNOPSIS

cksum [file]...

DESCRIPTION

The cksum command calculates and writes to standard output a cyclic
redundancy check (CRC) for each input file, and also writes to
standard output the number of octets in each file.

For each file processed successfully, cksum will write in the
following format:

"%u %d %s\n" <checksum>, <# of octets>, <path name>

If no file operand was specified, the path name and its leading space
will be omitted.

The CRC used is based on the polynomial used for CRC error checking
in the referenced Ethernet standard.

The encoding for the CRC checksum 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:

1. 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 is used.

2. M(x) is multiplied by x ^32 (that is, shifted left 32
bits) and divided by G(x) using mod 2 division, producing
a remainder R(x) of degree <= 31.

3. The coefficients of R(x) are considered to be a 32-bit
sequence.

4. The bit sequence is complemented and the result is the
CRC.

OPERANDS

The following operand is supported:

file
A path name of a file to be checked. If no file operands are
specified, the standard input is used.

USAGE

The cksum command is typically used to quickly compare a suspect file
against a trusted version of the same, such as to ensure that files
transmitted over noisy media arrive intact. However, this comparison
cannot be considered cryptographically secure. The chances of a
damaged file producing the same CRC as the original are
astronomically small; deliberate deception is difficult, but probably
not impossible.

Although input files to cksum can be any type, the results need not
be what would be expected on character special device files. Since
this document does not specify the block size used when doing input,
checksums of character special files need not process all of the data
in those files.

The algorithm is expressed in terms of a bitstream divided into
octets. If a file is transmitted between two systems and undergoes
any data transformation (such as moving 8-bit characters into 9-bit
bytes or changing "Little Endian" byte ordering to "Big Endian"),
identical CRC values cannot be expected. Implementations performing
such transformations may extend cksum to handle such situations.

See largefile(7) for the description of the behavior of cksum when
encountering files greater than or equal to 2 Gbyte ( 2^31 bytes).

ENVIRONMENT VARIABLES

See environ(7) for descriptions of the following environment
variables that affect the execution of cksum: LANG, LC_ALL, LC_CTYPE,
LC_MESSAGES, and NLSPATH.

EXIT STATUS

The following exit values are returned:

0
All files were processed successfully.

>0
An error occurred.

ATTRIBUTES