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 From : Vladislav Shikhov                    2:5056/23.11   24 Sep 2001  16:08:45
 To : All
 Subject : Re: crc32
 -------------------------------------------------------------------------------- 
 

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 Yuriy Kaminskiy wrote:
 
 > >  Hello,  Vladislav!
 > >>>>> On 17:29 20/9/2001, Vladislav Shikhov <2:5056/23.11> writes:
 >  IT> ыБЛ ОБЪЩЧБЕФУС РТПЗБ ЛПФПТБС ФЙРБ md5sum, ОП CRC-32 УЮЙФБФШ ХНЕЕФ?
 >  IT> Й ЗДЕ Ег ЧЪСФШ?
 >  VS>  man cksum
 >  уЕТШЕЪОП? щ ЛБЛПЕ ПФОПЫЕОЙЕ cksum ЙНЕЕФ Л crc32?
 
 уБНПЕ РТСНПЕ. юЕМПЧЕЛ ОЕ УЛБЪБМ ЛБЛХА ЙНЕООП ЛПОФТПМШОХА УХННХ ПО ИПЮЕФ
 РПУЮЙФБФШ.
 
 The cksum utility writes to the standard output three whitespace
 separat-
      ed fields for each input file.  These fields are a checksum CRC,
 the to-
      tal number of octets in the file and the file name.  If no file
 name is
      specified, the standard input is used and no file name is written.
 
      The sum utility is identical to the cksum utility, except that it
 de-
      faults to using historic algorithm 1, as described below.  It is
 provided
      for compatibility only.
 
      The options are as follows:
 
      -o      Use historic algorithms instead of the (superior) default
 one.
 
              Algorithm 1 is the algorithm used by historic BSD UNIX
 systems as
              the sum(1) algorithm and by historic AT&T System V UNIX
 systems
              as the sum algorithm when using the -r option.  This is a
 16-bit
              checksum, with a right rotation before each addition;
 overflow is
              discarded.
 
              Algorithm 2 is the algorithm used by historic AT&T System V
 UNIX
        systems as the default sum algorithm.  This is a 32-bit checksum,
              and is defined as follows:
 
                    s = sum of all bytes;
                    r = s % 2^16 + (s % 2^32) / 2^16;
                    cksum = (r % 2^16) + r / 2^16;
 
              Algorithm 3 is what is commonly called the `32bit CRC'
 algorithm.
              This is a 32-bit checksum.
 
              Both algorithm 1 and 2 write to the standard output the
 same
              fields as the default algorithm except that the size of the
 file
              in bytes is replaced with the size of the file in blocks. For
              historic reasons, the block size is 1024 for algorithm 1
 and 512
              for algorithm 2.  Partial blocks are rounded up.
 
      The default CRC used is based on the polynomial used for CRC error
 check-
      ing in the networking standard ISO 8802-3: 1989  The CRC checksum
 encod-
      ing is defined by the generating polynomial:
 
            G(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 +
               x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
       Mathematically, the CRC value corresponding to a given file is
 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
 signif-
            icant 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 val-
            ue, least significant octet first.  The smallest number of
 octets
            capable of representing this integer are used.
 
            M(x) is multiplied by x^32 (i.e., 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.
 
      The cksum and sum utilities exit 0 on success, and >0 if an error
 occurs.
 -- [ччЁх щюц щс-5]
 Best regards,
  Vladislav                              mailto:vladsh@ezmail.ru
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Тема:    Автор:    Дата:    Re: crc32   Vladislav Shikhov   24 Sep 2001 16:08:45   Re: crc32   Igor Tihonov   29 Sep 2001 14:15:49   Re: crc32   yx   29 Sep 2001 21:09:09   Re: crc32   Vladislav Shikhov   01 Oct 2001 11:31:48