'\" et

.TH CKSUM "1P" 2013 "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

\fB

\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 \fR

.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 \(mi1.

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 '\(mi' 

and the implementation treats the

.BR '\(mi' 

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\(hy2008,

.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

\fB

"%u %d %s\en", <\fIchecksum\fR>, <\fI# of octets\fR>, <\fIpathname\fR>

.fi \fR

.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\(hy2008. Since this volume of POSIX.1\(hy2008 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

\fB

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; \(mi\|\(mii) {

        c = *b++;

        s = (s << 8) ^ crctab[(s >> 24) ^ c];

    }

.P

    /* Extend with the length of the string. */

    while (n != 0) {

        c = n & 0377;

        n >>= 8;

        s = (s << 8) ^ crctab[(s >> 24) ^ c];

    }

.P

    return ~s;

}

.fi \fR

.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

\fB

X <\(mi 0; Y <\(mi 0;

for i <\(mi m \(mi1 step \(mi1 until 0 do

    begin

    T <\(mi X(1) ^ A[i];

    X(1) <\(mi X(0); X(0) <\(mi Y(1); Y(1) <\(mi Y(0); Y(0) <\(mi 0;

    comment: f[T] and f'[T] denote the T-th words in the

        table f and f' ;

    X <\(mi X ^ f[T]; Y <\(mi Y ^ f'[T];

    end

.fi \fR

.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\(hy2008,

.IR "Chapter 8" ", " "Environment Variables"

.SH COPYRIGHT

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 .