White Paper: Internationalization in GTK+

Internationalization in GTK+

Figure 1. Current architecture of internationalization in GTK+.

Manipulating Text
The most basic task that GTK+ (and applications using GTK+) have to handle when dealing with international text is manipulating strings. The strings in the GTK+ interfaces are handled in the multi-byte encoding for the locale. This allows good compatibility with existing applications that aren't explicitely enabled for multi-byte support. Internally, GTK+ converts these strings to wide-character strings for easier manipulation, and the conversion routines are also available for applications that need these facilties.

Input
Internationalized input in GTK+ is done using the X Input Method extension (XIM). The X Input Method extension is an interface between X Window System applications and input methods. An input method handles translating keystrokes into characters. The X libraries include a simple built in input method that does compose-key handling for European languages. The more complicated input method handling for Asian languages is typically done by an external program.

You can see the basic architecture of XIM in Figure 1 in the section called Internationalization in GTK+. GTK+ forwards the keystrokes it receives to the input method via Xlib, and when a complete input string is received, it is displayed to the user. From the point of view of an application which is just using the standard GTK+ Text and Entry widgets, this is all done transparently behind the scenes, and the application only sees the final strings.

The X Input Method extension provides a complete set of facilities, and input methods are available for Chinese, Japanese and Korean. However, the programming interface that XIM provides has a number of disadvantages. For one thing, strings are communicated using the native encoding for the current locale and input methods are also selected by looking at the current locale. This makes it difficult to use XIM to do input in multiple languages at once. Also, the functioning of XIM is very closely tied to the event handling and text string models of X. An application that renders strings itself (for instance, an illustration program) will have a hard time using XIM.

Output
GTK+ handles output of strings in different scripts using font sets. A font set is a list of X fonts for different character sets. When drawing, for example, mixed Roman and CJK text, then the two different fonts needed are extracted with multiple character sets.

The font to use for a particular widget is generally determined in GTK+ using resource configuration (RC) files. There is a system-wide file that the system admininstrator can set up, and additionally, each user can override the settings by creating a file in his home directory. This mechanism has been extended to deal with getting the correct fonts for each locale, even for user's that switch between different locales. When GTK+ reads in a RC file --- for example, the file .gtkrc in the user's home directory --- it also looks for the same file with an extension corresponding to the current locale. If the locale is ja_JP then GTK+ will check for the file .gtkrc.ja.

GTK+ ships with gtkrc files for Japanese, Korean, and Russian, and a system administrator can easily create them for additional languages as needed.