cksum(P)                                               cksum(P)





NAME
       cksum - write file checksums and sizes

SYNOPSIS
       cksum [file ...]

DESCRIPTION
       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  poly-
       nomial    used   for   CRC   error   checking   in   the
       ISO/IEC 8802-3:1996 standard (Ethernet).

       The encoding for the CRC checksum is defined by the gen-
       erating polynomial:


              G(x)=x+x+x+x+x+x+x+x+x+x+x+x+x+x+1

       Mathematically,  the  CRC value corresponding to a given
       file shall be defined by the following procedure:

       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
       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 repre-
       senting this integer shall be used.

       M( x) is multiplied by x (that is, shifted left 32 bits)
       and  divided  by G( x) using mod 2 division, producing a
       remainder R( x) of degree <= 31.

       The coefficients of R( x) are considered to be a  32-bit
       sequence.

       The  bit  sequence is complemented and the result is the
       CRC.

OPTIONS
       None.

OPERANDS
       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.


STDIN
       The standard input shall be used only if no  file  oper-
       ands are specified. See the INPUT FILES section.

INPUT FILES
       The input files can be any file type.

ENVIRONMENT VARIABLES
       The  following  environment  variables  shall affect the
       execution of cksum:

       LANG   Provide a default value for the internationaliza-
              tion  variables  that are unset or null. (See the
              Base Definitions volume of  IEEE Std 1003.1-2001,
              Section  8.2,  Internationalization Variables for
              the precedence of internationalization  variables
              used  to  determine  the  values  of locale cate-
              gories.)

       LC_ALL If set to a non-empty string value, override  the
              values  of  all  the  other  internationalization
              variables.

       LC_CTYPE
              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).

       LC_MESSAGES
              Determine  the  locale  that  should  be  used to
              affect the format and contents of diagnostic mes-
              sages written to standard error.

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


ASYNCHRONOUS EVENTS
       Default.

STDOUT
       For each file processed successfully, the cksum  utility
       shall write in the following format:


              "%u %d %s\n", <checksum>, <# of octets>, <pathname>

       If  no  file operand was specified, the pathname and its
       leading <space> shall be omitted.

STDERR
       The standard error shall be  used  only  for  diagnostic
       messages.

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       The following exit values shall be returned:

        0     All files were processed successfully.

       >0     An error occurred.


CONSEQUENCES OF ERRORS
       Default.

       The following sections are informative.

APPLICATION USAGE
       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  con-
       sidered  cryptographically secure. The chances of a dam-
       aged 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 spec-
       ify  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 han-
       dle such situations.

EXAMPLES
       None.

RATIONALE
       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 avail-
       able for one pass through the function.  This  was  done
       for  simplicity  in  demonstrating the algorithm, rather
       than as an implementation model.


              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
              };


              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 con-
       sistent 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  opera-
       tional robustness of cksum. Neither the System V nor BSD
       sum algorithm was selected. Since each of these was dif-
       ferent  and  each was the default behavior on those sys-
       tems, 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 con-
       tinue 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  (Ethernet)  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)  repre-
       sents  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  demon-
       strated  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
                  begin
                  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];
                  end

       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.

FUTURE DIRECTIONS
       None.

SEE ALSO
       None.

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in
       electronic form from  IEEE  Std  1003.1,  2003  Edition,
       Standard  for Information Technology -- Portable Operat-
       ing System Interface (POSIX), The Open Group Base Speci-
       fications Issue 6, Copyright (C) 2001-2003 by the Insti-
       tute 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.open-
       group.org/unix/online.html .



POSIX                         2003                     cksum(P)
