


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.





crc                            1.3                              1





crc32(n)             Cyclic Redundancy Checks            crc32(n)


       -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






crc                            1.3                              2





crc32(n)             Cyclic Redundancy Checks            crc32(n)


       % 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                              3


