| ActiveTcl User Guide |

[ Main table Of Contents | Tcllib Table Of Contents | Tcllib Index ]

# crc32(n) 1.3.2 "Cyclic Redundancy Checks"

## Name

crc32 - Perform a 32bit Cyclic Redundancy Check

## Table Of Contents

## 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(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 Trackers. 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 © 2002, Pat Thoyts

#### Copyright © 2015 for compilation: ActiveState