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crc16 - Cyclic Redundancy Checks
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crc16(n) 1.1.3 tcllib "Cyclic Redundancy Checks"


crc16 - Perform a 16bit Cyclic Redundancy Check


This package provides a Tcl-only implementation of the CRC algorithms based upon information provided at 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.


::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(n) 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.


-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. Please note that using the option termination flag is important when processing data from parameters. If the binary data looks like one of the options given above then the data will be read as an option if this marker is not included. Always use the -- option termination flag before giving the data argument.


% crc::crc16 -- "Hello, World!"
% crc::crc-ccitt -- "Hello, World!"
% crc::crc16 -format 0x%X -- "Hello, World!"
% crc::crc16 -file crc16.tcl


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.

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.


Hashes, checksums, and encryption