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diff --git a/man-pages-posix-2017/man1p/cksum.1p b/man-pages-posix-2017/man1p/cksum.1p new file mode 100644 index 0000000..73ea9b6 --- /dev/null +++ b/man-pages-posix-2017/man1p/cksum.1p @@ -0,0 +1,375 @@ +'\" 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 . |