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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
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.1‐2017, 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.1‐2017. Since this volume of POSIX.1‐2017 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.
SEE ALSO
The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 CKSUM(1POSIX)