Страницы справочника OS X

Эта страница руководства для  версии 10.9 Mac OS X

Читать страницы руководства

crc32(n)                                  Cyclic Redundancy Checks                                  crc32(n)



____________________________________________________________________________________________________________

NAME
       crc32 - Perform a 32bit Cyclic Redundancy Check

SYNOPSIS
       package require Tcl  8.2

       package require crc32  ?1.3?

       ::crc::crc32 ?-format format? ?-seed value? [ -channel chan | -filename file | message ]

       ::crc::Crc32Init ?seed?

       ::crc::Crc32Update token data

       ::crc::Crc32Final token

____________________________________________________________________________________________________________

DESCRIPTION
       This package provides a Tcl implementation of the CRC-32 algorithm based upon information provided at
       http://www.naaccr.org/standard/crc32/document.html If either the critcl package or  the  Trf  package
       are  available then a compiled version may be used internally to accelerate the checksum calculation.

COMMANDS
       ::crc::crc32 ?-format format? ?-seed value? [ -channel chan | -filename file | message ]
              The command takes either string data or a channel or file name and returns  a  checksum  value
              calculated  using  the CRC-32 algorithm. The result is formatted using the format(n) specifier
              provided. The default is to return the value as an unsigned integer (format %u).


OPTIONS
       -channel name
              Return a checksum for the data read from a channel. The command will read data from the  chan-nel channel
              nel  until  the  eof is true. If you need to be able to process events during this calculation
              see the PROGRAMMING INTERFACE section

       -filename name
              This is a convenience option that opens the specified file, sets the encoding  to  binary  and
              then acts as if the -channel option had been used. The file is closed on completion.

       -format string
              Return the checksum using an alternative format template.

       -seed value
              Select  an alternative seed value for the CRC calculation. The default is 0xffffffff. This can
              be useful for calculating the CRC for data  structures  without  first  converting  the  whole
              structure  into a string. The CRC of the previous member can be used as the seed for calculat-ing calculating
              ing the CRC of the next member.  Note that the crc32 algorithm includes a final XOR  step.  If
              incremental  processing  is  desired  then  this must be undone before using the output of the
              algorithm as the seed for further processing. A simpler alternative is to use the  PROGRAMMING
              INTERFACE which is intended for this mode of operation.


PROGRAMMING INTERFACE
       The  CRC-32  package implements the checksum using a context variable to which additional data can be
       added at any time. This is expecially useful in an event based environment such as a  Tk  application
       or  a web server package. Data to be checksummed may be handled incrementally during a fileevent han-dler handler
       dler in discrete chunks. This can improve the interactive nature of a GUI application and can help to
       avoid excessive memory consumption.

       ::crc::Crc32Init ?seed?
              Begins  a new CRC32 context. Returns a token ID that must be used for the remaining functions.
              An optional seed may be specified if required.

       ::crc::Crc32Update token data
              Add data to the checksum identified by token. Calling Crc32Update $token "abcd" is  equivalent
              to calling Crc32Update $token "ab" followed by Crc32Update $token "cb". See EXAMPLES.

       ::crc::Crc32Final token
              Returns  the  checksum  value and releases any resources held by this token. Once this command
              completes the token will be invalid. The result is a 32 bit integer value.


EXAMPLES
       % crc::crc32 "Hello, World!"
       3964322768



       % crc::crc32 -format 0x%X "Hello, World!"
       0xEC4AC3D0



       % crc::crc32 -file crc32.tcl
       483919716



       % set tok [crc::Crc32Init]
       % crc::Crc32Update $tok "Hello, "
       % crc::Crc32Update $tok "World!"
       % crc::Crc32Final $tok
       3964322768


AUTHORS
       Pat Thoyts

BUGS, IDEAS, FEEDBACK
       This document, and the package it describes,  will  undoubtedly  contain  bugs  and  other  problems.
       Please   report   such   in   the   category   crc   of   the   Tcllib  SF  Trackers  [http://source -
       forge.net/tracker/? group_id=12883].  Please also report any ideas for enhancements you may  have  for
       either package and/or documentation.

SEE ALSO
       cksum(n), crc16(n), sum(n)

KEYWORDS
       checksum, cksum, crc, crc32, cyclic redundancy check, data integrity, security

CATEGORY
       Hashes, checksums, and encryption

COPYRIGHT
       Copyright (c) 2002, Pat Thoyts




crc                                                  1.3                                            crc32(n)