# 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

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 generat-
ing 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 coef-
ficients  of a mod 2 polynomial M(x) of degree n-1. These
n bits are the bits from the file, with the most signifi-
cant  bit  being  the  most  significant bit of the first
octet of the file and the last bit being the least signi-
ficant  bit  of the last octet, padded with zero bits (if
necessary) to achieve an integral number of octets,  fol-
lowed  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 cryp-
tographically secure. The chances of a damaged file  produc-
ing  the  same CRC as the original are astronomically small;
deliberate deception is difficult, but probably not impossi-
ble.

Although input files to cksum can be any type,  the  results
need not be what would be expected on character special dev-
ice 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(5) 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(5) 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

```     See attributes(5) for descriptions of the  following  attri-
butes:

____________________________________________________________
|       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
|_____________________________|_____________________________|
| Availability                | SUNWcsu                     |
|_____________________________|_____________________________|
| Interface Stability         | Standard                    |
|_____________________________|_____________________________|

```

```     sum(1),  attributes(5),  environ(5),   largefile(5),   stan-