NAME

       crc16 - Perform a 16bit Cyclic Redundancy Check

SYNOPSIS

       package require Tcl  8.2

       package require crc16  ?1.1.2?

       ::crc::crc16 ?-format format? ?-seed value? ?-implementation procname? message

       ::crc::crc16 ?-format format? ?-seed value? ?-implementation procname? -filename file

       ::crc::crc-ccitt ?-format format? ?-seed value? ?-implementation procname? message

       ::crc::crc-ccitt ?-format format? ?-seed value? ?-implementation procname? -filename file

       ::crc::xmodem ?-format format? ?-seed value? ?-implementation procname? message

       ::crc::xmodem ?-format format? ?-seed value? ?-implementation procname? -filename file

_________________________________________________________________

DESCRIPTION

       This  package  provides  a  Tcl-only  implementation  of  the  CRC  algorithms  based upon
       information  provided  at  http://www.microconsultants.com/tips/crc/crc.txt  There  are  a
       number of permutations available for calculating CRC checksums and this package can handle
       all of them. Defaults are set up for the most common cases.

COMMANDS

       ::crc::crc16 ?-format format? ?-seed value? ?-implementation procname? message

       ::crc::crc16 ?-format format? ?-seed value? ?-implementation procname? -filename file

       ::crc::crc-ccitt ?-format format? ?-seed value? ?-implementation procname? message

       ::crc::crc-ccitt ?-format format? ?-seed value? ?-implementation procname? -filename file

       ::crc::xmodem ?-format format? ?-seed value? ?-implementation procname? message

       ::crc::xmodem ?-format format? ?-seed value? ?-implementation procname? -filename file
              The command takes either string data or a file name and returns  a  checksum  value
              calculated  using  the  CRC algorithm. The command used sets up the CRC polynomial,
              initial value and bit ordering for the desired standard checksum  calculation.  The
              result  is  formatted  using  the format(3tcl) specifier provided or as an unsigned
              integer (%u) by default.

              A number of common polynomials are in use with  the  CRC  algorithm  and  the  most
              commonly  used of these are included in this package. For convenience each of these
              has a command alias in the crc namespace.

              It is possible to implement the CRC-32 checksum using this  crc16  package  as  the
              implementation is sufficiently generic to extend to 32 bit checksums. As an example
              this has been done already - however this is not the fastest  method  to  implement
              this algorithm in Tcl and a separate crc32 package is available.

OPTIONS

       -filename name
              Return a checksum for the file contents instead of for parameter data.

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

       -seed value
              Select  an alternative seed value for the CRC calculation. The default is 0 for the
              CRC16 calculation and 0xFFFF for  the  CCITT  version.   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. It is also used for accumulating a checksum
              from fragments of a large message (or file)

       -implementation procname
              This hook is provided to allow users to provide their own implementation (perhaps a
              C  compiled  extension).  The procedure specfied is called with two parameters. The
              first is the data to be checksummed and the second is the seed value. An integer is
              expected as the result.

              The  package  provides  some  implementations  of  standard CRC polynomials for the
              XMODEM, CCITT and the usual CRC-16 checksum. For convenience,  additional  commands
              have been provided that make use of these implementations.

       --     Terminate option processing.

EXAMPLES

              % crc::crc16 "Hello, World!"
              64077

              % crc::crc-ccitt "Hello, World!"
              26586

              % crc::crc16 -format 0x%X "Hello, World!"
              0xFA4D

              % crc::crc16 -file crc16.tcl
              51675

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(3tcl), crc32(3tcl), sum(3tcl)

KEYWORDS

       checksum, cksum, crc, crc16, crc32, cyclic redundancy check, data integrity, security

CATEGORY

       Hashes, checksums, and encryption

COPYRIGHT

       Copyright (c) 2002, Pat Thoyts