cksum.1p (13580B)
- '\" et
- .TH CKSUM "1P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH 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.
- .\"
- .SH NAME
- cksum
- \(em write file checksums and sizes
- .SH SYNOPSIS
- .LP
- .nf
- cksum \fB[\fIfile\fR...\fB]\fR
- .fi
- .SH DESCRIPTION
- The
- .IR 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\(hy3:\|1996 standard (Ethernet).
- .P
- The encoding for the CRC checksum is defined by the generating
- polynomial:
- .sp
- .RS 4
- .nf
- \fIG\fR(\fIx\fR)=\fIx\fR\u\s-332\s+3\d+\fIx\fR\u\s-326\s+3\d+\fIx\fR\u\s-323\s+3\d+\fIx\fR\u\s-322\s+3\d+\fIx\fR\u\s-316\s+3\d+\fIx\fR\u\s-312\s+3\d+\fIx\fR\u\s-311\s+3\d+\fIx\fR\u\s-310\s+3\d+\fIx\fR\u\s-38\s+3\d+\fIx\fR\u\s-37\s+3\d+\fIx\fR\u\s-35\s+3\d+\fIx\fR\u\s-34\s+3\d+\fIx\fR\u\s-32\s+3\d+\fIx\fR+1
- .fi
- .P
- .RE
- .P
- Mathematically, the CRC value corresponding to a given file shall be
- defined by the following procedure:
- .IP " 1." 4
- The
- .IR n
- bits to be evaluated are considered to be the coefficients of a mod 2
- polynomial
- .IR M (\c
- .IR x )
- of degree
- .IR n \-1.
- These
- .IR 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.
- .IP " 2." 4
- .IR M (\c
- .IR x )
- is multiplied by
- .IR x \u\s-332\s+3\d
- (that is, shifted left 32 bits) and divided by
- .IR G (\c
- .IR x )
- using mod 2 division, producing a remainder
- .IR R (\c
- .IR x )
- of degree \(<= 31.
- .IP " 3." 4
- The coefficients of
- .IR R (\c
- .IR x )
- are considered to be a 32-bit sequence.
- .IP " 4." 4
- The bit sequence is complemented and the result is the CRC.
- .SH OPTIONS
- None.
- .SH OPERANDS
- The following operand shall be supported:
- .IP "\fIfile\fR" 10
- A pathname of a file to be checked. If no
- .IR file
- operands are specified, the standard input shall be used.
- .SH STDIN
- The standard input shall be used if no
- .IR file
- operands are specified, and shall be used if a
- .IR file
- operand is
- .BR '\-'
- and the implementation treats the
- .BR '\-'
- as meaning standard input.
- Otherwise, the standard input shall not be used.
- See the INPUT FILES section.
- .SH "INPUT FILES"
- The input files can be any file type.
- .SH "ENVIRONMENT VARIABLES"
- The following environment variables shall affect the execution of
- .IR cksum :
- .IP "\fILANG\fP" 10
- Provide a default value for the internationalization variables that are
- unset or null. (See the Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 8.2" ", " "Internationalization Variables"
- for the precedence of internationalization variables used to determine
- the values of locale categories.)
- .IP "\fILC_ALL\fP" 10
- If set to a non-empty string value, override the values of all the
- other internationalization variables.
- .IP "\fILC_CTYPE\fP" 10
- 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).
- .IP "\fILC_MESSAGES\fP" 10
- .br
- Determine the locale that should be used to affect the format and
- contents of diagnostic messages written to standard error.
- .IP "\fINLSPATH\fP" 10
- Determine the location of message catalogs for the processing of
- .IR LC_MESSAGES .
- .SH "ASYNCHRONOUS EVENTS"
- Default.
- .SH STDOUT
- For each file processed successfully, the
- .IR cksum
- utility shall write in the following format:
- .sp
- .RS 4
- .nf
- "%u %d %s\en", <\fIchecksum\fR>, <\fI# of octets\fR>, <\fIpathname\fR>
- .fi
- .P
- .RE
- .P
- If no
- .IR file
- operand was specified, the pathname and its leading
- <space>
- shall be omitted.
- .SH STDERR
- The standard error shall be used only for diagnostic messages.
- .SH "OUTPUT FILES"
- None.
- .SH "EXTENDED DESCRIPTION"
- None.
- .SH "EXIT STATUS"
- The following exit values shall be returned:
- .IP "\00" 6
- All files were processed successfully.
- .IP >0 6
- An error occurred.
- .SH "CONSEQUENCES OF ERRORS"
- Default.
- .LP
- .IR "The following sections are informative."
- .SH "APPLICATION USAGE"
- The
- .IR 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.
- .P
- Although input files to
- .IR 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\(hy2017. Since this volume of POSIX.1\(hy2017 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.
- .P
- 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
- .IR cksum
- to handle such situations.
- .SH EXAMPLES
- None.
- .SH RATIONALE
- The following C-language program can be used as a model to describe the
- algorithm. It assumes that a
- .BR 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.
- .sp
- .RS 4
- .nf
- 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
- };
- .P
- 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
- */
- .P
- register size_t i;
- register unsigned c, s = 0;
- .P
- for (i = n; i > 0; --i) {
- c = *b++;
- s = (s << 8) \(ha crctab[(s >> 24) \(ha c];
- }
- .P
- /* Extend with the length of the string. */
- while (n != 0) {
- c = n & 0377;
- n >>= 8;
- s = (s << 8) \(ha crctab[(s >> 24) \(ha c];
- }
- .P
- return \(tis;
- }
- .fi
- .P
- .RE
- .P
- 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.
- .P
- The algorithm used was selected to increase the operational robustness
- of
- .IR cksum .
- Neither the System V nor BSD
- .IR 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\(emsome set of historical
- applications would break. Therefore, the name was changed to
- .IR cksum .
- Although the historical
- .IR sum
- commands will probably continue to be provided for many years, programs
- designed for portability across systems should use the new name.
- .P
- The algorithm selected is based on that used by the ISO/IEC\ 8802\(hy3:\|1996 standard (Ethernet)
- for the frame check sequence field. The algorithm used does not match
- the technical definition of a
- .IR 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.
- .P
- 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.
- .P
- The calculation used is identical to that given in pseudo-code in the
- referenced Sarwate article. The pseudo-code rendition is:
- .sp
- .RS 4
- .nf
- X <- 0; Y <- 0;
- for i <- m -1 step -1 until 0 do
- begin
- T <- X(1) \(ha A[i];
- X(1) <- X(0); X(0) <- Y(1); Y(1) <- Y(0); Y(0) <- 0;
- comment: f[T] and f\(aq[T] denote the T-th words in the
- table f and f\(aq ;
- X <- X \(ha f[T]; Y <- Y \(ha f\(aq[T];
- end
- .fi
- .P
- .RE
- .P
- The pseudo-code is reproduced exactly as given; however, note that in
- the case of
- .IR cksum ,
- .BR A[i]
- represents a byte of the file, the words
- .BR X
- and
- .BR Y
- are treated as a single 32-bit value, and the tables
- .BR f
- and
- .BR f'
- are a single table containing 32-bit values.
- .P
- The referenced Sarwate article also discusses generating the table.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "Chapter 8" ", " "Environment Variables"
- .\"
- .SH 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 .
- .PP
- 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 .