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 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 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)
