About X Keyboard Support

About X Keyboard Support

This section provides some background that may help make the following sections easier to understand.

Keys, Keycodes, and Keysyms

When a client connects to the X server, the server announces its range of keycodes and exports a table of keysyms. Each key event the client receives has a single byte keycode, which directly represents a physical key, and a single byte state, which represents currently engaged modifier keys, such as Shift or Alt.

Note: The mapping of state bits to modifiers is done by another table acquired from the server. Keysyms are well defined, and there has been an attempt to have a keysym for every engraving one might possibly find on any keyboard, anywhere. (An engraving is the image imprinted on a physical key.) These are contained in /usr/include/X11/keysymdef.h. Keysyms represent the engravings on the actual keys, but not their meanings. The server's idea of the keysym table can be changed by clients, and clients may receive KeyMap events when this remapping happens, but such events don't happen often.

When a client receives a Key event, it asks Xlib to use the keycode to index into its keysym table to find a list of keysyms. (This list is usually very short. Most keys have only one or two engravings on them.) Using the state byte, Xlib chooses a keysym from the list to find out what was engraved on the key the user pressed.

At this point, the client can choose to act on the keysym itself (if, for instance, it was a backspace) or it can ask for a character string represented by the keysym (or both). Generating such a string is tricky; it is discussed in "Input Methods (IMs)," below.

Details on X keyboard support can be found in X Window System, Third Edition, from Digital Press. Details on input methods are also available in that book, as well as in the Xlib Programming Manual, Volume One.

Composed Characters

There are two ways to compose characters that do not exist on a keyboard: explicit and implicit. It is common for an application to be modal and switch between the two. For example, Japanese input of kana is often done via implicit composition.

Users switch between a mode where input is interpreted as romaji (Latin characters) and a mode where input is translated to kana.

Furthermore, both styles may operate simultaneously. While an application is supporting implicit composition of certain characters, other characters may be composable via explicit composition.

Not every keystroke produces a character, even if the associated keysym normally implies character text. The event-to-string translation routines figure out what result a given set of keystrokes should produce (see "Using XLookupString(), XwcLookupString(), and XmbLookupString()" in this section).

Character composition from the user's aspect is discussed in the compose(5) and composetable(5) reference pages.

Explicit Composition

Explicit composition is requested when the user presses the Compose key and then types a key sequence that corresponds to the desired character. For example, to compose the character ñ under some keymaps, you might press the Compose key and then type ~n.

Note: The xmodmap(1) reference page tells how to map the XK_Multi_key keysym onto whatever key you want to use as Compose.

Implicit Composition

Implicit composition mimics many existing European typewriters that have "dead" keys: keys that type a character but do not advance the carriage. When a special "dead" key is struck, the system attempts to compose a character using the next character struck. For example, on a keyboard that had a diaeresis (�) and an O, but no Ö, you would strike � and then O to compose Ö.

Implicit composition support usually comes with some specified way to leave characters uncomposed.

Supported Keyboards

IRIX currently supports 12 keyboard layouts: American, Belgian, Danish, English, French, German, Italian, Norwegian, Portuguese, Spanish, Swedish, and Swiss. All are representable in Latin 1; the American keyboard needs only ASCII.