# cksum(1posix) [posix man page]

```CKSUM(P)						     POSIX Programmer's Manual							  CKSUM(P)

NAME
cksum - write file checksums and sizes

SYNOPSIS
cksum [file ...]

DESCRIPTION
The cksum utility shall calculate and write to standard output a cyclic redundancy check (CRC) for each input file, and also write to stan-
dard output the number of octets  in  each  file.  The  CRC  used  is  based  on  the  polynomial  used	for  CRC  error  checking  in  the
ISO/IEC 8802-3:1996 standard (Ethernet).

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 shall be 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 shall be 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.

OPTIONS
None.

OPERANDS
The following operand shall be supported:

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

STDIN
The standard input shall be 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 shall affect the execution of cksum:

LANG   Provide a default value for the internationalization variables that  are	unset  or  null.  (See	the  Base  Definitions	volume	of
IEEE Std 1003.1-2001,  Section  8.2,  Internationalization  Variables  for  the precedence of internationalization variables used to
determine the values of locale categories.)

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

LC_CTYPE
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte 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 catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS
Default.

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

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

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

STDERR
The standard error shall be used only for diagnostic messages.

OUTPUT FILES
None.

EXTENDED DESCRIPTION
None.

EXIT STATUS
The following exit values shall be returned:

0     All files were processed successfully.

>0     An error occurred.

CONSEQUENCES OF ERRORS
Default.

The following sections are informative.

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  dam-
aged 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 System Interfaces volume of IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001 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 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.

RATIONALE
The  following  C-language  program can be used as a model to describe the algorithm. It assumes that a char is one octet.  It also assumes
that the entire file is available for one pass through the function. This was done for simplicity in demonstrating  the	algorithm,  rather
than as an implementation model.

static unsigned long crctab[] = {
0x00000000,
0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4,
0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5,
0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07,
0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c,
0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1,
0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b,
0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d,
0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80,
0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a,
0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629,
0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c,
0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a,
0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087,
0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
};

unsigned long memcrc(const unsigned char *b, size_t n)
{
/*  Input arguments:
*  const char*	b == byte sequence to checksum
*  size_t	n == length of sequence
*/

register unsigned   i, c, s = 0;

for (i = n; i > 0; --i) {
c = (unsigned)(*b++);
s = (s << 8) ^ crctab[(s >> 24) ^ c];
}

/* Extend with the length of the string. */
while (n != 0) {
c = n & 0377;
n >>= 8;
s = (s << 8) ^ crctab[(s >> 24) ^ c];
}

return ~s;
}

The  historical	practice  of writing the number of "blocks" has been changed to writing the number of octets, since the latter is not only
more useful, but also since historical implementations have not been consistent in defining what a "block" meant.  Octets are used  instead
of bytes because bytes can differ in size between systems.

The  algorithm  used was selected to increase the operational robustness of cksum. Neither the System V nor BSD sum algorithm was selected.
Since each of these was different and each was the default behavior on those systems, no realistic compromise was available if either  were
selected-some  set  of  historical applications would break. Therefore, the name was changed to cksum. Although the historical sum commands
will probably continue to be provided for many years, programs designed for portability across systems should use the new name.

The algorithm selected is based on that used by the ISO/IEC 8802-3:1996 standard (Ethernet) for the frame check sequence field.	The  algo-
rithm  used  does  not  match  the  technical definition of a checksum; the term is used for historical reasons.  The length of the file is
included in the CRC calculation because this parallels inclusion of a length field by Ethernet in its  CRC,  but  also  because	it  guards
against	inadvertent  collisions between files that begin with different series of zero octets. The chance that two different files produce
identical CRCs is much greater when their lengths are not considered. Keeping the length and the checksum of the file itself separate would
yield  a slightly more robust algorithm, but historical usage has always been that a single number (the checksum as printed) represents the
signature of the file. It was decided that historical usage was the more important consideration.

Early proposals contained modifications to the Ethernet algorithm that involved extracting table values	whenever  an  intermediate  result
became zero. This was demonstrated to be less robust than the current method and mathematically difficult to describe or justify.

The calculation used is identical to that given in pseudo-code in the referenced Sarwate article. The pseudo-code rendition is:

X <- 0; Y <- 0;
for i <- m -1 step -1 until 0 do
begin
T <- X(1) ^ A[i];
X(1) <- X(0); X(0) <- Y(1); Y(1) <- Y(0); Y(0) <- 0;
comment: f[T] and f'[T] denote the T-th words in the
table f and f' ;
X <- X ^ f[T]; Y <- Y ^ f'[T];
end

The  pseudo-code  is  reproduced exactly as given; however, note that in the case of cksum, A[i] represents a byte of the file, the words X
and Y are treated as a single 32-bit value, and the tables f and f' are a single table containing 32-bit values.

The referenced Sarwate article also discusses generating the table.

FUTURE DIRECTIONS
None.