msgcat man page

msgcat — Tcl message catalog


package require Tcl 8.5

package require msgcat 1.6

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

::msgcat::mcmax ?src-string src-string ...?

::msgcat::mcexists ?-exactnamespace? ?-exactlocale? src-string

::msgcat::mclocale ?newLocale?


::msgcat::mcloadedlocales subcommand ?locale?

::msgcat::mcload dirname

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

::msgcat::mcmset locale src-trans-list

::msgcat::mcflset src-string ?translate-string?

::msgcat::mcflmset src-trans-list

::msgcat::mcunknown locale src-string ?arg arg ...?

::msgcat::mcpackagelocale subcommand ?locale?

::msgcat::mcpackageconfig subcommand option ?value?



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 may be provided by adding a new file to the message catalog.

msgcat distinguises packages by its namespace. Each package has its own message catalog and configuration settings in msgcat.

A locale is a specification string describing a user language like de_ch for Swiss German. In msgcat, there is a global locale initialized by the system locale of the current system. Each package may decide to use the global locale or to use a package specific locale.

The global locale may be changed on demand, for example by a user initiated language change or within a multi user application like a web server.


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

Returns a translation of src-string according to the 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 application 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::mcmax ?src-string src-string ...?
Given several source strings, ::msgcat::mcmax returns the length of the longest translated string. This is useful when designing localized GUIs, which may require that all buttons, for example, be a fixed width (which will be the width of the widest button).
::msgcat::mcexists ?-exactnamespace? ?-exactlocale? src-string
Return true, if there is a translation for the given src-string.

The search may be limited by the option -exactnamespace to only check the current namespace and not any parent namespaces.

It may also be limited by the option -exactlocale to only check the first prefered locale (e.g. first element returned by ::msgcat::mcpreferences if global locale is used).

::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. msgcat stores and compares the locale in a case-insensitive manner, and returns locales in lowercase. The initial locale is determined by the locale specified in the user's environment. See Locale Specification below for a description of the locale string format.

If the locale is set, the preference list of locales is evaluated. Locales in this list are loaded now, if not jet loaded.

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. The list is derived from the current locale set in msgcat by ::msgcat::mclocale, and cannot be set independently. For example, if the current locale is en_US_funky, then ::msgcat::mcpreferences returns {en_us_funky en_us en {}}.
::msgcat:mcloadedlocales subcommand ?locale?
This group of commands manage the list of loaded locales for packages not setting a package locale.

The subcommand get returns the list of currently loaded locales.

The subcommand present requires the argument locale and returns true, if this locale is loaded.

The subcommand clear removes all locales and their data, which are not in the current preference list.

::msgcat::mcload dirname

Searches the specified directory for files that match the language specifications returned by ::msgcat::mcloadedlocales get (or msgcat::mcpackagelocale preferences if a package locale is set) (note that these are all lowercase), extended by the file extension “.msg”. Each matching file is read in order, assuming a UTF-8 encoding. The file contents are then evaluated as a Tcl script. This means that Unicode characters may be present in the message file either directly in their UTF-8 encoded form, or by use of the backslash-u quoting recognized by Tcl evaluation. The number of message files which matched the specification and were loaded is returned.

In addition, the given folder is stored in the msgcat package configuration option mcfolder to eventually load message catalog files required by a locale change.

::msgcat::mcset locale src-string ?translate-string?
Sets the translation for src-string to translate-string in the specified locale and the current namespace. If translate-string is not specified, src-string is used for both. The function returns translate-string.
::msgcat::mcmset locale src-trans-list
Sets the translation for multiple source strings in src-trans-list in the specified locale and the current namespace. src-trans-list must have an even number of elements and is in the form {src-string translate-string ?src-string translate-string ...?} ::msgcat::mcmset can be significantly faster than multiple invocations of ::msgcat::mcset. The function returns the number of translations set.
::msgcat::mcflset src-string ?translate-string?
Sets the translation for src-string to translate-string in the current namespace for the locale implied by the name of the message catalog being loaded via ::msgcat::mcload. If translate-string is not specified, src-string is used for both. The function returns translate-string.
::msgcat::mcflmset src-trans-list
Sets the translation for multiple source strings in src-trans-list in the current namespace for the locale implied by the name of the message catalog being loaded via ::msgcat::mcload. src-trans-list must have an even number of elements and is in the form {src-string translate-string ?src-string translate-string ...?} ::msgcat::mcflmset can be significantly faster than multiple invocations of ::msgcat::mcflset. The function returns the number of translations set.
::msgcat::mcunknown locale src-string ?arg arg ...?

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 passed by format if there are any arguments. 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 value of ::msgcat::mcunknown is used as the return value for the call to ::msgcat::mc.

Note that this routine is only called if the concerned package did not set a package locale unknown command name.

The calling package clears all its state within the msgcat package including all settings and translations.

Locale Specification

The locale is specified to msgcat by a locale string passed to ::msgcat::mclocale. 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” specifies U.S. English.

When the msgcat package is first loaded, the locale is initialized according to the user's environment. The variables env(LC_ALL), env(LC_MESSAGES), and env(LANG) are examined in order. The first of them to have a non-empty value is used to determine the initial locale. The value is parsed according to the XPG4 pattern


to extract its parts. The initial locale is then set by calling ::msgcat::mclocale with the argument


On Windows and Cygwin, if none of those environment variables is set, msgcat will attempt to extract locale information from the registry. From Windows Vista on, the RFC4747 locale name "lang-script-country-options" is transformed to the locale as "lang_country_script" (Example: sr-Latn-CS -> sr_cs_latin). For Windows XP, the language id is transformed analoguously (Example: 0c1a -> sr_yu_cyrillic). If all these attempts to discover an initial locale from the user's environment fail, msgcat defaults to an initial locale of “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_GB_Funky, the locales “en_gb_funky”, “en_gb”, “en” and “” (the empty string) are searched in order until a matching translation string is found. If no translation string is available, then the unknown handler is called.

Namespaces and Message Catalogs

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

::msgcat::mcset en hello "hello from ::"
namespace eval foo {
    ::msgcat::mcset en hello "hello from ::foo"
puts [::msgcat::mc hello]
namespace eval foo {puts [::msgcat::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 (in the “en” locale) the code

::msgcat::mcset en m1 ":: message1"
::msgcat::mcset en m2 ":: message2"
::msgcat::mcset en m3 ":: message3"
namespace eval ::foo {
    ::msgcat::mcset en m2 "::foo message2"
    ::msgcat::mcset en m3 "::foo message3"
namespace eval ::foo::bar {
    ::msgcat::mcset en m3 "::foo::bar message3"
namespace import ::msgcat::mc
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

Location and Format of Message Files

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

All message files for a package are in the same directory.
The message file name is a msgcat locale specifier (all lowercase) followed by “.msg”. For example:
es.msg    — spanish
en_gb.msg — United Kingdom English

Exception: The message file for the root locale “” is called “ROOT.msg”. This exception is made so as not to cause peculiar behavior, such as marking the message file as “hidden” on Unix file systems.

The file contains a series of calls to mcflset and mcflmset, setting the necessary translation strings for the language, likely enclosed in a namespace eval so that all source strings are tied to the namespace of the package. For example, a short es.msg might contain:
namespace eval ::mypackage {
    ::msgcat::mcflset "Free Beer" "Cerveza Gratis"

Positional Codes for Format and Scan Commands

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. Note that it is not necessary to pass the output of ::msgcat::mc to format directly; by passing the values to substitute in as arguments, the formatting substitution is done directly.

msgcat::mc {Produced %1$d at %2$s} $num $city
# ... where that key is mapped to one of the
# human-oriented versions by msgcat::mcset

Package private locale

A package using msgcat may choose to use its own package private locale and its own set of loaded locales, independent to the global locale set by ::msgcat::mclocale.

This allows a package to change its locale without causing any locales load or removal in other packages and not to invoke the global locale change callback (see below).

This action is controled by the following ensemble:

::msgcat::mcpackagelocale set ?locale?
Set or change a package private locale. The package private locale is set to the given locale if the locale is given. If the option locale is not given, the package is set to package private locale mode, but no locale is changed (e.g. if the global locale was valid for the package before, it is copied to the package private locale).

This command may cause the load of locales.

::msgcat::mcpackagelocale get
Return the package private locale or the global locale, if no package private locale is set.
::msgcat::mcpackagelocale preferences
Return the package private preferences or the global preferences, if no package private locale is set.
::msgcat::mcpackagelocale loaded
Return the list of locales loaded for this package.
::msgcat::mcpackagelocale isset
Returns true, if a package private locale is set.
::msgcat::mcpackagelocale unset
Unset the package private locale and use the globale locale. Load and remove locales to adjust the list of loaded locales for the package to the global loaded locales list.
::msgcat::mcpackagelocale present locale
Returns true, if the given locale is loaded for the package.
::msgcat::mcpackagelocale clear
Clear any loaded locales of the package not present in the package preferences.

Changing package options

Each package using msgcat has a set of options within msgcat. The package options are described in the next sectionPackage options. Each package option may be set or unset individually using the following ensemble:

::msgcat::mcpackageconfig get option
Return the current value of the given option. This call returns an error if the option is not set for the package.
::msgcat::mcpackageconfig isset option
Returns 1, if the given option is set for the package, 0 otherwise.
::msgcat::mcpackageconfig set option value
Set the given option to the given value. This may invoke additional actions in dependency of the option. The return value is 0 or the number of loaded packages for the option mcfolder.
::msgcat::mcpackageconfig unset option
Unsets the given option for the package. No action is taken if the option is not set for the package. The empty string is returned.

Package options

The following package options are available for each package:


This is the message folder of the package. This option is set by mcload and by the subcommand set. Both are identical and both return the number of loaded message catalog files.

Setting or changing this value will load all locales contained in the preferences valid for the package. This implies also to invoke any set loadcmd (see below).

Unsetting this value will disable message file load for the package.

This callback is invoked before a set of message catalog files are loaded for the package which has this property set.

This callback may be used to do any preparation work for message file load or to get the message data from another source like a data base. In this case, no message files are used (mcfolder is unset).

See section callback invocation below. The parameter list appended to this callback is the list of locales to load.

If this callback is changed, it is called with the preferences valid for the package.

This callback is invoked when a default local change was performed. Its purpose is to allow a package to update any dependency on the default locale like showing the GUI in another language.

See the callback invocation section below. The parameter list appended to this callback is mcpreferences. The registered callbacks are invoked in no particular order.

Use a package locale mcunknown procedure instead of the standard version supplied by the msgcat package (msgcat::mcunknown).

The called procedure must return the formatted message which will finally be returned by msgcat::mc.

A generic unknown handler is used if set to the empty string. This consists in returning the key if no arguments are given. With given arguments, format is used to process the arguments.

See section callback invocation below. The appended arguments are identical to ::msgcat::mcunknown.

Callback invocation

A package may decide to register one or multiple callbacks, as described above.

Callbacks are invoked, if:

1. the callback command is set,

2. the command is not the empty string,

3. the registering namespace exists.

If a called routine fails with an error, the bgerror routine for the interpreter is invoked after command completion. Only exception is the callback unknowncmd, where an error causes the invoking mc-command to fail with that error.


Packages which display a GUI may update their widgets when the global locale changes. To register to a callback, use:

namespace eval gui {
    msgcat::mcpackageconfig changecmd updateGUI

    proc updateGui args {
        puts "New locale is '[lindex $args 0]'."
% msgcat::mclocale fr
% New locale is 'fr'.

If locales (or additional locales) are contained in another source like a data base, a package may use the load callback and not mcload:

namespace eval db {
    msgcat::mcpackageconfig loadcmd loadMessages

    proc loadMessages args {
        foreach locale $args {
            if {[LocaleInDB $locale]} {
                msgcat::mcmset $locale [GetLocaleList $locale]

The clock command implementation uses msgcat with a package locale to implement the command line parameter -locale. Here are some sketches of the implementation:

First, a package locale is initialized and the generic unknown function is desactivated:

msgcat::mcpackagelocale set
msgcat::mcpackageconfig unknowncmd ""

As an example, the user requires the week day in a certain locale as follows:

clock format clock seconds -format %A -locale fr

clock sets the package locale to fr and looks for the day name as follows:

msgcat::mcpackagelocale set $locale
return [lindex [msgcat::mc DAYS_OF_WEEK_FULL] $day]
### Returns "mercredi"

Within clock, some message-catalog items are heavy in computation and thus are dynamically cached using:

proc ::tcl::clock::LocalizeFormat { locale format } {
    set key FORMAT_$format
    if { [::msgcat::mcexists -exactlocale -exactnamespace $key] } {
        return [mc $key]
    #...expensive computation of format clipped...
    mcset $locale $key $format
    return $format


The message catalog code was developed by Mark Harrison.

See Also

format(n), scan(n), namespace(n), package(n)


internationalization, i18n, localization, l10n, message, text, translation

Referenced By

clock(n), package(n).

Explore man page connections for msgcat(n).

msgcat 1.5 Tcl Bundled Packages