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PROGRAM:

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 standard 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)=x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+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 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 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 if no file operands are specified, and shall be used if a
file operand is '−' and the implementation treats the '−' as meaning standard input.
Otherwise, the standard input shall not be used. 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 POSIX.1‐2008, 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\n", <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 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 System
Interfaces volume of POSIX.1‐2008. Since this volume of POSIX.1‐2008 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,
0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6,
0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac,
0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033,
0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe,
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,
0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698,
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,
0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
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,
0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65,
0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74,
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,
0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d,
0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb,
0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf,
0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
};

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

register size_t i;
register unsigned c, s = 0;

for (i = n; i > 0; −−i) {
c = *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.

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 algorithm 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.

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