kc3-lang/libxkbcommon/doc/introduction-to-xkb.md

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Introduction to XKB {#xkb-intro}

XKB stands for “X Keyboard Extension”. It may refer to either:

XKB the protocol {#xkb-the-protocol}

A protocol for the X Windows System, that extends the core protocol.

xkbcommon’s API is somehow derived from this API, but has been substantially reworked to function as a library instead of a protocol, and exposes fewer internal details to clients.

xkbcommon does not depend on a particular windows system; for instance it is used by the Wayland protocol.

xkbcommon provides the <code>[xkbcommon-x11]</code> module to interface a client with an X server using the XKB protocol. Relevant links:

XKB the keyboard keymap configuration {#xkb-the-config}

In order to use the protocol, one must first load a [complete keymap]. The keymap usually comes from the OS layout database, which is commonly xkeyboard-config. Since keymaps may have definitions in common, the database actually stores their basic components separately to allow maximum composability and coherence. A recipe to compose a keymap from its components is called a keymap configuration.

In XKB, there are several ways to define a keymap configuration. They all aim to produce a [complete keymap]. The following diagram presents an overview. Then they are presented hereinafter, ordered from end user to low-level implementation.

@dotfile xkb-configuration “XKB keymap configurations”

RMLVO: Rules, Model, Layout, Variant, Options @anchor RMLVO-intro
This is the configuration the end user usually faces in the UI. The idea is to expose high level concepts such as [keyboard model] and [keyboard layout] to the user, then to _map_ them to the corresponding set of low-level configuration files (see [KcCGST]). @note The RMLVO configurations actually available to the end user is managed by the `xkbregistry`. It uses an XML file, the _registry_, which exposes and documents the set of RMLVO settings in the layout database. The RMLVO configuration consists of the following components:
Rules
The rules define the _mapping_ from high to low level components. The rules _component_ is the file containing the set of rules to use. It is usually implicit and set by the system. See the [rules file format](@ref rule-file-format) for further details.
Model
The name of the model of the keyboard hardware in use. It may depend on: - The _location_ and _language_ of the user, because languages may require [specific keys][language input keys] for their input methods, such as the _muhenkan_ key on Japanese keyboard and the _Hanja_ key for Korean keyboards. The keyboard are usually classified by the [standard][keyboard standard] it is based on, e.g. ANSI, ISO, JIS, ABNT. - The keyboard _vendor:_ keyboard may have a set of keys that are not standard, or may be specific to an OS.
Layout
The identifier of the general layout to use. It usually refers to a country or a language.
Variant
Any minor variants on the general layout. It may be national variants
Options
Set of extra options to customize the standard layouts. Examples: switch modifiers keys, location of the compose key, etc.
KcCGST: Keycodes, Compat, Geometry, Symbols, Types @anchor KcCGST-intro
This is the low-level configuration of XKB and how the files are actually organized in the _layout database_. It is not really intuitive or straight-forward for the uninitiated. @note _xkbcommon_ [does not offer an API for KcCGST](@ref KcCGST-support): it is considered an implementation detail. Instead, [RMLVO] is the preferred way for the user to configure XKB. The KcCGST configuration consists of the following components:
Key codes
A translation of the raw [key codes] from the keyboard into symbolic names.
Compatibility
A specification of what internal actions modifiers and various special-purpose keys produce.
Geometry
A description of the physical layout of a keyboard. @attention This legacy feature is [not supported](@ref geometry-support) by _xkbcommon_.
Key symbols
A translation of symbolic key codes into actual [key symbols] (keysyms).
Key types
Types describe how a pressed key is affected by active [modifiers] such as Shift, Control, Alt, etc.
Complete Keymap @anchor keymap-intro
A complete keymap is a _self-contained_ text file with all the [KcCGST] components needed to configure a keyboard. This is the result of the _resolution_ of the [RMLVO] and [KcCGST] configurations. This is also the format used by X11 and Wayland when prompted to _serialize_ the keymap in use. @note This is a low-level configuration. [RMLVO] is the preferred way for the end user to configure XKB, but some _power users_ may need it for _avanced_ configurations. See the [XKB text format] for further details.

@note Layout making use of dead keys require a Compose file. The same applies when if using a Compose key.

[key codes]: @ref keycode-def [key symbols]: @ref keysym-def [levels]: @ref level-def [modifiers]: @ref modifier-def [RMLVO]: @ref RMLVO-intro [KcCGST]: @ref KcCGST-intro [complete keymap]: @ref keymap-intro

[XKB text format]: @ref xkb-the-text-format

XKB the text format {#xkb-the-text-format}

A text format to define keyboard keymaps. XKB 1.0 is the specification implemented in current X servers. The format supported by xkbcommon is very close to XKB 1.0, with some removals and additions. See the [compatibility] page for further details.

The format supported by xkbcommon is documented at the page “[The XKB keymap text format, V1][keymap-text-format-v1]”.

The documentation of the original XKB 1.0 format is much more scarce than for the protocol. Some priceless resources are:

[xkbcommon-x11]: @ref x11-overview

[compatibility]: @ref xkb-v1-compatibility [keymap-text-format-v1]: @ref keymap-text-format-v1

@todo Explain how to configure XKB, with examples

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