Tcl message catalog 

Tclsh Built-In Commands


::msgcat::mc src-string

::msgcat::mclocale ?newLocale?


::msgcat::mcload dirname

::msgcat::mcset locale src-string?translate-string?

::msgcat::mcunknown locale src-string


The msgcat package provides a set of functions that can be used to manage multi-lingual user interfaces. Text strings are defined in a message catalog which is independent from the application, and which can be edited or localized without modifying the application source code. New languages or locales are provided by adding a new file to the message catalog.

Use of the message catalog is optional by any application or package, but is encouraged if the application or package wishes to be enabled for multi-lingual applications.


::msgcat::mc src-string ?arg arg ...

Returns a translation of src-string according to the user's current locale. If additional arguments past src-string are given, the format command is used to substitute the additional arguments in the translation of src-string. ::msgcat::mc will search the messages defined in the current namespace for a translation of src-string; if none is found, it will search in the parent of the current namespace, and so on until it reaches the global namespace. If no translation string exists, ::msgcat::mcunknown is called and the string returned from ::msgcat::mcunknown is returned.

::msgcat::mc is the main function used to localize an application. Instead of using an English string directly, an applicaton can pass the English string through ::msgcat::mc and use the result. If an application is written for a single language in this fashion, then it is easy to add support for additional languages later simply by defining new message catalog entries.

::msgcat::mclocale ?newLocale

This function sets the locale to newLocale. If newLocale is omitted, the current locale is returned, otherwise the current locale is set to newLocale. The initial locale defaults to the locale specified in the user's environment. See LOCALE AND SUBLOCALE SPECIFICATION below for a description of the locale string format.


Returns an ordered list of the locales preferred by the user, based on the user's language specification. The list is ordered from most specific to least preference. If the user has specified LANG=en_US_funky, this procedure would return {en_US_funky en_US en}.

::msgcat::mcload dirname 

Searches the specified directory for files that match the language specifications returned by ::msgcat::mcpreferences. Each file located is sourced. The file extension is .msg. The number of message files which matched the specification and were loaded is returned.

::msgcat::mcset locale src-string ?translate-string

Sets the translation for src-string to translate-string in the specified locale. If translate-string is not specified, src-string is used for both. The function returns translate-string.

::msgcat::mcunknown locale src-string 

This routine is called by ::msgcat::mc in the case when a translation for src-string is not defined in the current locale. The default action is to return src-string. This procedure can be redefined by the application, for example to log error messages for each unknown string. The ::msgcat::mcunknown procedure is invoked at the same stack context as the call to ::msgcat::mc. The return vaue of ::msgcat::mcunknown is used as the return vaue for the call to ::msgcat::mc.


The locale is specified by a locale string. The locale string consists of a language code, an optional country code, and an optional system-specific code, each separated by _. The country and language codes are specified in standards ISO-639 and ISO-3166. For example, the locale en specifies English and en_US specifes U.S. English.

The locale defaults to the value in env(LANG) at the time the msgcat package is loaded. If env(LANG) is not defined, then the locale defaults to C.

When a locale is specified by the user, a best match search is performed during string translation. For example, if a user specifies en_UK_Funky, the locales en_UK_Funky, en_UK, and en are searched in order until a matching translation string is found. If no translation string is available, then ::msgcat::unknown is called.


Strings stored in the message catalog are stored relative to the namespace from which they were added. This allows multiple packages to use the same strings without fear of collisions with other packages. It also allows the source string to be shorter and less prone to typographical error.

For example, executing the code

mcset en hello "hello from ::"
namespace eval foo {mcset en hello "hello from ::foo"}
puts [mc hello]
namespace eval foo {puts [mc hello]}

will print

hello from ::
hello from ::foo

When searching for a translation of a message, the message catalog will search first the current namespace, then the parent of the current namespace, and so on until the global namespace is reached. This allows child namespaces to "inherit" messages from their parent namespace.

For example, executing the code

mcset en m1 ":: message1"
mcset en m2 ":: message2"
mcset en m3 ":: message3"
namespace eval ::foo {
	mcset en m2 "::foo message2"
	mcset en m3 "::foo message3"
namespace eval ::foo::bar {
	mcset en m3 "::foo::bar message3"
puts "[mc m1]; [mc m2]; [mc m3]"
namespace eval ::foo {puts "[mc m1]; [mc m2]; [mc m3]"}
namespace eval ::foo::bar {puts "[mc m1]; [mc m2]; [mc m3]"}

will print

:: message1; :: message2; :: message3
:: message1; ::foo message2; ::foo message3
:: message1; ::foo message2; ::foo::bar message3


Message files can be located in any directory, subject to the following conditions:

  1. All message files for a package are in the same directory.

  2. The message file name is a locale specifier followed by .msg. For example:

    es.msg -- spanish
    en_UK.msg -- UK English
  3. The file contains a series of calls to mcset, setting the necessary translation strings for the language. For example:

    ::msgcat::mcset es "Free Beer!" "Cerveza Gracias!"


If a package is installed into a subdirectory of the tcl_pkgPath and loaded via package require, the following procedure is recommended.


During package installation, create a subdirectory msgs under your package directory.


Copy your *.msg files into that directory.


Add the following command to your package initialization script:

# load language files, stored in msgs subdirectory
::msgcat::mcload [file join [file dirname [info script]] msgs]


It is possible that a message string used as an argument to format might have positionally dependent parameters that might need to be repositioned. For example, it might be syntactically desirable to rearrange the sentence structure while translating.

format "We produced %d units in location %s" $num $city
format "In location %s we produced %d units" $city $num

This can be handled by using the positional parameters:

format "We produced %1\$d units in location %2\$s" $num $city
format "In location %2\$s we produced %1\$d units" $num $city

Similarly, positional parameters can be used with scan to extract values from internationalized strings.


The message catalog code was developed by Mark Harrison.


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


PTC MKS Toolkit for Power Users
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format, namespace, package, scan

PTC MKS Toolkit 10.4 Documentation Build 39.