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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/stan‐
       dard/crc32/document.html If either the critcl package or the Trf	 pack‐
       age  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	 algo‐
	      rithm.  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 channel 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 calculating 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 use‐
       ful  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  handler  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://sourceforge.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)
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