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CKSUM(1P) POSIX Programmer's Manual CKSUM(1P)
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.
cksum — write file checksums and sizes
cksum [file...]
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.
None.
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.
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.
The input files can be any file type.
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.
Default.
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.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 All files were processed successfully.
>0 An error occurred.
Default.
The following sections are informative.
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.
None.
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.
None.
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 CKSUM(1P)
Pages that refer to this page: wc(1p)