208 lines
8.0 KiB
Markdown
208 lines
8.0 KiB
Markdown
# Introduction to XKB {#xkb-intro}
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__XKB__ stands for “X Keyboard Extension”. It may refer to either:
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- a [protocol](@ref xkb-the-protocol)
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- a [keyboard layout configuration](@ref xkb-the-config)
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- a [text format](@ref xkb-the-text-format)
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## XKB the protocol {#xkb-the-protocol}
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A __protocol__ for the [X Windows System], that extends the core protocol.
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_xkbcommon’s_ API is somehow derived from this API, but has been
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substantially reworked to function as a library instead of a protocol,
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and exposes fewer internal details to clients.
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_xkbcommon_ does not depend on a particular windows system; for instance
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it is used by the [Wayland] protocol.
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_xkbcommon_ provides the <code>[xkbcommon-x11]</code> module to interface
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a client with an X server using the XKB protocol. Relevant links:
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- [The X Window System Protocol][X Protocol]
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- [The X Keyboard Extension: Protocol Specification][XKB Protocol]
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- [xkbcommon-x11]
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## XKB the keyboard keymap configuration {#xkb-the-config}
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In order to use [the protocol](@ref xkb-the-protocol), one must first load a
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[complete keymap]. The keymap usually comes from the OS _layout database_,
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which is commonly [xkeyboard-config]. Since keymaps may have definitions in
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common, the database actually stores their basic components separately to allow
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maximum composability and coherence. A recipe to compose a keymap from its
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components is called a _keymap configuration_.
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In XKB, there are several ways to define a keymap configuration. They all aim to
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produce a [complete keymap]. The following diagram presents an overview.
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Then they are presented hereinafter, ordered from end user to low-level
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implementation.
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@dotfile xkb-configuration "XKB keymap configurations"
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<dl>
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<dt>
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RMLVO: <u>R</u>ules, <u>M</u>odel, <u>L</u>ayout, <u>V</u>ariant,
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<u>O</u>ptions @anchor RMLVO-intro
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</dt>
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<dd>
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This is the configuration the end user usually faces in the UI.
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The idea is to expose high level concepts such as [keyboard model] and
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[keyboard layout] to the user, then to _map_ them to the corresponding set
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of low-level configuration files (see [KcCGST]).
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@note The RMLVO configurations actually available to the end user is managed
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by the `xkbregistry`. It uses an XML file, the _registry_, which exposes and
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documents the set of RMLVO settings in the layout database.
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The RMLVO configuration consists of the following components:
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<dl>
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<dt>Rules</dt>
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<dd>
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The rules define the _mapping_ from high to low level components.
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The rules _component_ is the file containing the set of rules to use.
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It is usually implicit and set by the system.
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See the [rules file format](doc/rules-format.md) for further details.
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</dd>
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<dt>Model</dt>
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<dd>
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The name of the model of the keyboard hardware in use.
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It may depend on:
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- The _location_ and _language_ of the user, because languages may
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require [specific keys][language input keys] for their input methods,
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such as the _muhenkan_ key on Japanese keyboard and the _Hanja_ key
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for Korean keyboards. The keyboard are usually classified by the
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[standard][keyboard standard] it is based on, e.g. ANSI, ISO, JIS,
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ABNT.
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- The keyboard _vendor:_ keyboard may have a set of keys that are not
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standard, or may be specific to an OS.
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</dd>
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<dt>Layout</dt>
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<dd>
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The identifier of the general layout to use. It usually refers to a
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country or a language.
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</dd>
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<dt>Variant</dt>
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<dd>
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Any minor variants on the general layout. It may be national variants
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</dd>
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<dt>Options</dt>
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<dd>
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Set of extra options to customize the standard layouts.
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Examples: switch modifiers keys, location of the compose key, etc.
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</dd>
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</dl>
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</dd>
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<dt>
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KcCGST: <u>K</u>ey<u>c</u>odes, <u>C</u>ompat, <u>G</u>eometry,
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<u>S</u>ymbols, <u>T</u>ypes @anchor KcCGST-intro
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</dt>
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<dd>
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This is the low-level configuration of XKB and how the files are actually
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organized in the _layout database_.
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It is not really intuitive or straight-forward for the uninitiated.
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@note _xkbcommon_ [does not offer an API for KcCGST](@ref KcCGST-support):
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it is considered an implementation detail.
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Instead, [RMLVO] is the preferred way for the user to configure XKB.
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The KcCGST configuration consists of the following components:
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<dl>
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<dt>Key codes</dt>
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<dd>
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A translation of the raw [key codes] from the keyboard into
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symbolic names.
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</dd>
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<dt>Compatibility</dt>
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<dd>
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A specification of what internal actions modifiers and various
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special-purpose keys produce.
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</dd>
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<dt>Geometry</dt>
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<dd>
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A description of the physical layout of a keyboard.
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@attention This legacy feature is [not supported](@ref geometry-support)
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by _xkbcommon_.
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</dd>
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<dt>Key symbols</dt>
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<dd>
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A translation of symbolic key codes into actual [key symbols] (keysyms).
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</dd>
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<dt>Key types</dt>
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<dd>
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Types describe how a pressed key is affected by active [modifiers]
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such as Shift, Control, Alt, etc.
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</dd>
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</dl>
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</dd>
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<dt>Complete Keymap @anchor keymap-intro</dt>
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<dd>
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A complete keymap is a _self-contained_ text file with all the [KcCGST]
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components needed to configure a keyboard. This is the result of the
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_resolution_ of the [RMLVO] and [KcCGST] configurations. This is also the
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format used by X11 and Wayland when prompted to _serialize_ the keymap in use.
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@note This is a low-level configuration. [RMLVO] is the preferred way for the
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end user to configure XKB, but some _power users_ may need it for _avanced_
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configurations.
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See the [XKB text format] for further details.
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</dd>
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</dl>
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@note Layout making use of dead keys require a [Compose](@ref compose) file. The
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same applies when if using a [Compose key].
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[key codes]: @ref keycode-def
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[key symbols]: @ref keysym-def
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[levels]: @ref level-def
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[modifiers]: @ref modifier-def
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[RMLVO]: @ref RMLVO-intro
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[KcCGST]: @ref KcCGST-intro
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[complete keymap]: @ref keymap-intro
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[Compose key]: https://en.wikipedia.org/wiki/Compose_key
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[XKB text format]: @ref xkb-the-text-format
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## XKB the text format {#xkb-the-text-format}
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A __text format__ to define keyboard keymaps. XKB 1.0 is the specification
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implemented in current X servers. The format supported by _xkbcommon_
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is very close to XKB 1.0, with some removals and additions. See the
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[compatibility] page for further details.
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The format supported by _xkbcommon_ is documented at the page
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“[The XKB keymap text format, V1][keymap-format-text-v1]”.
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The documentation of the _original_ XKB 1.0 format is much more scarce than
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for the protocol. Some priceless resources are:
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- [Ivan Pascal's XKB documentation][ivan-pascal]
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- [An Unreliable Guide to XKB Configuration][unreliable-guide]
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- [ArchWiki XKB page][arch-wiki]
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[X Windows System]: https://en.wikipedia.org/wiki/X_Window_System
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[X Protocol]: https://www.x.org/releases/current/doc/xproto/x11protocol.html#Keyboards
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[XKB Protocol]: https://www.x.org/releases/current/doc/kbproto/xkbproto.html
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[xkbcommon-x11]: @ref x11-overview
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[Wayland]: https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_keyboard
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[compatibility]: @ref xkb-v1-compatibility
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[keymap-format-text-v1]: doc/keymap-format-text-v1.md
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[ivan-pascal]: https://web.archive.org/web/20190724015820/http://pascal.tsu.ru/en/xkb/
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[unreliable-guide]: https://www.charvolant.org/doug/xkb/html/index.html
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[arch-wiki]: https://wiki.archlinux.org/index.php/X_keyboard_extension
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[keyboard model]: https://en.wikipedia.org/wiki/Computer_keyboard
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[keymap]: https://en.wikipedia.org/wiki/Keyboard_layout
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[keyboard layout]: https://en.wikipedia.org/wiki/Keyboard_layout
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[xkeyboard-config]: https://gitlab.freedesktop.org/xkeyboard-config/xkeyboard-config
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[keyboard standard]: https://en.wikipedia.org/wiki/Computer_keyboard#Types_and_standards
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[language input keys]: https://en.wikipedia.org/wiki/Language_input_keys
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@todo Explain how to configure XKB, with examples
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