mirror of https://github.com/mkerrisk/man-pages
352 lines
13 KiB
Groff
352 lines
13 KiB
Groff
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "CKSUM" 1P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" cksum
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.SH NAME
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cksum \- write file checksums and sizes
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.SH SYNOPSIS
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.LP
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\fBcksum\fP \fB[\fP\fIfile\fP \fB...\fP\fB]\fP
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.SH DESCRIPTION
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.LP
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The \fIcksum\fP utility shall calculate and write to standard output
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a cyclic redundancy check (CRC) for each input file, and
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also write to standard output the number of octets in each file. The
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CRC used is based on the polynomial used for CRC error
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checking in the ISO/IEC\ 8802-3:1996 standard (Ethernet).
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.LP
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The encoding for the CRC checksum is defined by the generating polynomial:
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.sp
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.RS
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.nf
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\fIG\fP\fB(\fP\fIx\fP\fB)=\fP\fIx\fP\fB**32+\fP\fIx\fP\fB**26+\fP\fIx\fP\fB**23+\fP\fIx\fP\fB**22+\fP\fIx\fP\fB**16+\fP\fIx\fP\fB**12+\fP\fIx\fP\fB**11+\fP\fIx\fP\fB**10+\fP\fIx\fP\fB**8+\fP\fIx\fP\fB**7+\fP\fIx\fP\fB**5+\fP\fIx\fP\fB**4+\fP\fIx\fP\fB**2+\fP\fIx\fP\fB+1
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\fP
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.fi
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.RE
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.LP
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Mathematically, the CRC value corresponding to a given file shall
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be defined by the following procedure:
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.IP " 1." 4
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The \fIn\fP bits to be evaluated are considered to be the coefficients
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of a mod 2 polynomial \fIM\fP( \fIx\fP) of degree
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\fIn\fP-1. These \fIn\fP bits are the bits from the file, with the
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most significant bit being the most significant bit of the
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first octet of the file and the last bit being the least significant
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bit of the last octet, padded with zero bits (if necessary) to
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achieve an integral number of octets, followed by one or more octets
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representing the length of the file as a binary value, least
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significant octet first. The smallest number of octets capable of
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representing this integer shall be used.
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.LP
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.IP " 2." 4
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\fIM\fP( \fIx\fP) is multiplied by \fIx\fP**32 (that is, shifted left
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32 bits) and divided by
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\fIG\fP( \fIx\fP) using mod 2 division, producing a remainder \fIR\fP(
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\fIx\fP) of degree <= 31.
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.LP
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.IP " 3." 4
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The coefficients of \fIR\fP( \fIx\fP) are considered to be a 32-bit
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sequence.
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.LP
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.IP " 4." 4
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The bit sequence is complemented and the result is the CRC.
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.LP
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.SH OPTIONS
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.LP
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None.
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.SH OPERANDS
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.LP
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The following operand shall be supported:
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.TP 7
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\fIfile\fP
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A pathname of a file to be checked. If no \fIfile\fP operands are
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specified, the standard input shall be used.
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.sp
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.SH STDIN
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.LP
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The standard input shall be used only if no \fIfile\fP operands are
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specified. See the INPUT FILES section.
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.SH INPUT FILES
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.LP
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The input files can be any file type.
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.SH ENVIRONMENT VARIABLES
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.LP
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The following environment variables shall affect the execution of
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\fIcksum\fP:
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.TP 7
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\fILANG\fP
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Provide a default value for the internationalization variables that
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are unset or null. (See the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Section 8.2, Internationalization Variables
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for
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the precedence of internationalization variables used to determine
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the values of locale categories.)
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.TP 7
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\fILC_ALL\fP
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If set to a non-empty string value, override the values of all the
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other internationalization variables.
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.TP 7
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\fILC_CTYPE\fP
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Determine the locale for the interpretation of sequences of bytes
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of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments).
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.TP 7
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\fILC_MESSAGES\fP
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Determine the locale that should be used to affect the format and
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contents of diagnostic messages written to standard
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error.
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.TP 7
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\fINLSPATH\fP
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Determine the location of message catalogs for the processing of \fILC_MESSAGES
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\&.\fP
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.sp
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.SH ASYNCHRONOUS EVENTS
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.LP
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Default.
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.SH STDOUT
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.LP
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For each file processed successfully, the \fIcksum\fP utility shall
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write in the following format:
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.sp
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.RS
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.nf
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\fB"%u %d %s\\n", <\fP\fIchecksum\fP\fB>, <\fP\fI# of octets\fP\fB>, <\fP\fIpathname\fP\fB>
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\fP
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.fi
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.RE
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.LP
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If no \fIfile\fP operand was specified, the pathname and its leading
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<space> shall be omitted.
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.SH STDERR
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.LP
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The standard error shall be used only for diagnostic messages.
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.SH OUTPUT FILES
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.LP
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None.
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.SH EXTENDED DESCRIPTION
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.LP
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None.
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.SH EXIT STATUS
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.LP
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The following exit values shall be returned:
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.TP 7
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\ 0
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All files were processed successfully.
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.TP 7
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>0
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An error occurred.
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.sp
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.SH CONSEQUENCES OF ERRORS
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.LP
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Default.
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.LP
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\fIThe following sections are informative.\fP
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.SH APPLICATION USAGE
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.LP
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The \fIcksum\fP utility is typically used to quickly compare a suspect
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file against a trusted version of the same, such as to
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ensure that files transmitted over noisy media arrive intact. However,
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this comparison cannot be considered cryptographically
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secure. The chances of a damaged file producing the same CRC as the
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original are small; deliberate deception is difficult, but
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probably not impossible.
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.LP
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Although input files to \fIcksum\fP can be any type, the results need
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not be what would be expected on character special device
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files or on file types not described by the System Interfaces volume
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of IEEE\ Std\ 1003.1-2001. Since this volume of
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IEEE\ Std\ 1003.1-2001 does not specify the block size used when doing
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input, checksums of character special files need not
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process all of the data in those files.
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.LP
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The algorithm is expressed in terms of a bitstream divided into octets.
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If a file is transmitted between two systems and
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undergoes any data transformation (such as changing little-endian
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byte ordering to big-endian), identical CRC values cannot be
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expected. Implementations performing such transformations may extend
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\fIcksum\fP to handle such situations.
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.SH EXAMPLES
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.LP
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None.
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.SH RATIONALE
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.LP
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The following C-language program can be used as a model to describe
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the algorithm. It assumes that a \fBchar\fP is one octet.
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It also assumes that the entire file is available for one pass through
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the function. This was done for simplicity in demonstrating
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the algorithm, rather than as an implementation model.
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.sp
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.RS
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.nf
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\fBstatic unsigned long crctab[] = {
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0x00000000,
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0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
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0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6,
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0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
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0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac,
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0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
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0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
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0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
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0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
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0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033,
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0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe,
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0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
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0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4,
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0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
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0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5,
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0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
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0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07,
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0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c,
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0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1,
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0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
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0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b,
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0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698,
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0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d,
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0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
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0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
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0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
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0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80,
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0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
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0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a,
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0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629,
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0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c,
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0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
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0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
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0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65,
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0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
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0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
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0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
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0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
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0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74,
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0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
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0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
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0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a,
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0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087,
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0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
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0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d,
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0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
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0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb,
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0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
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0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
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0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
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0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf,
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0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
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};
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.sp
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unsigned long memcrc(const unsigned char *b, size_t n)
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{
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/* Input arguments:
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* const char* b == byte sequence to checksum
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* size_t n == length of sequence
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*/
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.sp
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register unsigned i, c, s = 0;
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.sp
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for (i = n; i > 0; --i) {
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c = (unsigned)(*b++);
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s = (s << 8) ^ crctab[(s >> 24) ^ c];
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}
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.sp
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/* Extend with the length of the string. */
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while (n != 0) {
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c = n & 0377;
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n >>= 8;
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s = (s << 8) ^ crctab[(s >> 24) ^ c];
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}
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.sp
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return ~s;
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}
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\fP
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.fi
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.RE
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.LP
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The historical practice of writing the number of "blocks" has been
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changed to writing the number of octets, since the latter
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is not only more useful, but also since historical implementations
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have not been consistent in defining what a "block" meant.
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Octets are used instead of bytes because bytes can differ in size
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between systems.
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.LP
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The algorithm used was selected to increase the operational robustness
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of \fIcksum\fP. Neither the System V nor BSD \fIsum\fP
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algorithm was selected. Since each of these was different and each
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was the default behavior on those systems, no realistic
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compromise was available if either were selected-some set of historical
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applications would break. Therefore, the name was changed
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to \fIcksum\fP. Although the historical \fIsum\fP commands will probably
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continue to be provided for many years, programs
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designed for portability across systems should use the new name.
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.LP
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The algorithm selected is based on that used by the ISO/IEC\ 8802-3:1996
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standard (Ethernet) for the frame check sequence
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field. The algorithm used does not match the technical definition
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of a \fIchecksum\fP; the term is used for historical reasons.
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The length of the file is included in the CRC calculation because
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this parallels inclusion of a length field by Ethernet in its
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CRC, but also because it guards against inadvertent collisions between
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files that begin with different series of zero octets. The
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chance that two different files produce identical CRCs is much greater
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when their lengths are not considered. Keeping the length
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and the checksum of the file itself separate would yield a slightly
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more robust algorithm, but historical usage has always been
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that a single number (the checksum as printed) represents the signature
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of the file. It was decided that historical usage was the
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more important consideration.
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.LP
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Early proposals contained modifications to the Ethernet algorithm
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that involved extracting table values whenever an intermediate
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result became zero. This was demonstrated to be less robust than the
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current method and mathematically difficult to describe or
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justify.
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.LP
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The calculation used is identical to that given in pseudo-code in
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the referenced Sarwate article. The pseudo-code rendition
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is:
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.sp
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.RS
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.nf
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\fBX <- 0; Y <- 0;
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for i <- m -1 step -1 until 0 do
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begin
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T <- X(1) ^ A[i];
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X(1) <- X(0); X(0) <- Y(1); Y(1) <- Y(0); Y(0) <- 0;
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comment: f[T] and f'[T] denote the T-th words in the
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table f and f' ;
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X <- X ^ f[T]; Y <- Y ^ f'[T];
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end
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\fP
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.fi
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.RE
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.LP
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The pseudo-code is reproduced exactly as given; however, note that
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in the case of \fIcksum\fP, \fBA[i]\fP represents a byte of
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the file, the words \fBX\fP and \fBY\fP are treated as a single 32-bit
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value, and the tables \fBf\fP and \fBf'\fP are a single
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table containing 32-bit values.
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.LP
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The referenced Sarwate article also discusses generating the table.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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None.
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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