NAME

       crc32 - Perform a 32bit Cyclic Redundancy Check

SYNOPSIS

       package require Tcl  8.2

       package require crc32  ?1.3.2?

       ::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(3tcl)  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 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  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  Trackers  [http://core.tcl.tk/tcllib/reportlist].   Please
       also report any ideas for enhancements you may have for either package and/or documentation.

       When proposing code changes, please provide unified diffs, i.e the output of diff -u.

       Note  further  that  attachments  are strongly preferred over inlined patches. Attachments can be made by
       going to the Edit form of the ticket immediately after its creation, and then using the left-most  button
       in the secondary navigation bar.

SEE ALSO

       cksum(3tcl), crc16(3tcl), sum(3tcl)

KEYWORDS

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

CATEGORY

       Hashes, checksums, and encryption

COPYRIGHT

       Copyright (c) 2002, Pat Thoyts