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Linux 2.6 - man page for cksum (linux section 1posix)

CKSUM(P)			    POSIX Programmer's Manual				 CKSUM(P)

       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:


       Mathematically, the CRC value corresponding to a given file shall be defined by	the  fol-
       lowing 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  signifi-
	   cant  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  nec-
	   essary) to achieve an integral number of octets, followed by one or more octets repre-
	   senting 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  x**32 (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.


       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 only if no file operands are specified. 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 IEEE Std 1003.1-2001, Section 8.2, Inter-
	      nationalization 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 interna-
	      tionalization variables.

	      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 argu-

	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error.

	      Determine the location of message catalogs for the processing of LC_MESSAGES .


       For  each file processed successfully, the cksum utility shall write in the following for-

	      "%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.



       The following exit values shall be returned:

	0     All files were processed successfully.

       >0     An error occurred.


       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 IEEE Std 1003.1-2001. Since this volume of IEEE Std 1003.1-2001  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 situa-


       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  algo-
       rithm, rather than as an implementation model.

	      static unsigned long crctab[] = {
	      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 char*	b == byte sequence to checksum
	       *  size_t	n == length of sequence

		  register unsigned   i, c, s = 0;

		  for (i = n; i > 0; --i) {
		      c = (unsigned)(*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.  Octets are
       used instead of bytes because bytes can differ in size between systems.

       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  (Ether-
       net)  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
		  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];

       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.



       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 CKSUM(P)

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