NAME

cksum - write file checksums and sizes

 SYNOPSIS

cksum [file ...]

 DESCRIPTION

The cksum utility 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. 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) = x³² + x²⁶ + x²³ + x²² + x¹⁶ + x¹² + x¹¹ + x¹⁰ + x⁸ + x⁷ + x⁵ + x⁴ + x² + 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 x³² (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.

 OPTIONS

None.

 OPERANDS

The following operand is supported:

file

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

 STDIN

The standard input is used only if no file operands are specified. See the INPUT FILES section.

 INPUT FILES

The input files can be any file type.

 ENVIRONMENT VARIABLES

The following environment variables affect the execution of cksum:

LANG

Provide a default value for the internationalisation variables that are unset or null. If LANG is unset or null, the corresponding value from the implementation-dependent default locale will be used. If any of the internationalisation variables contains an invalid setting, the utility will behave as if none of the variables had been defined.

LC_ALL

If set to a non-empty string value, override the values of all the other internationalisation variables.

LC_CTYPE

Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single- as opposed to multi-byte characters in arguments).

LC_MESSAGES

Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.

NLSPATH

Determine the location of message catalogues for the processing of LC_MESSAGES .

 ASYNCHRONOUS EVENTS

Default.

 STDOUT

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

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

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

 STDERR

Used only for diagnostic messages.

 OUTPUT FILES

None.

 EXTENDED DESCRIPTION

None.

 EXIT STATUS

The following exit values are returned:

0

All files were processed successfully.

>0

An error occurred.

 CONSEQUENCES OF ERRORS

Default.

 APPLICATION USAGE

The cksum utility 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 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 or on file types not described by the XSH specification. Since this specification 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.

 EXAMPLES

None.

 FUTURE DIRECTIONS

None.

 SEE ALSO

None.