xref: /aosp_15_r20/external/libxkbcommon/doc/user-configuration.md (revision 2b949d0487e80d67f1fda82db69e101e761f8064)

# User-configuration

This page describes how to add a custom layout or option so that it will be
parsed by libxkbcommon.

**The below requires libxkbcommon as keymap compiler and does not work in X**.

## Data locations

libxkbcommon searches the following paths for XKB configuration files:
- `$XDG_CONFIG_HOME/xkb/`, or `$HOME/.config/xkb/` if the `$XDG_CONFIG_HOME`
  environment variable is not defined
- `$HOME/.xkb/`
- `$XKB_CONFIG_EXTRA_PATH` if set, otherswise `<sysconfdir>/xkb` (on most
  distributions this is `/etc/xkb`)
- `$XKB_CONFIG_ROOT` if set, otherwise `<datadir>/X11/xkb/` (path defined by the
  `xkeyboard-config` package, on most distributions this is
  `/usr/share/X11/xkb`)

A keymap created with `xkb_keymap_new_from_names()` will look up those paths in
order until the required data is found.

**Note: Where libxkbcommon runs in a privileged context, only the system
(datadir) path is available.**

Each directory should have one or more of the following subdirectories:
- `compat`
- `geometry` (libxkbcommon ignores this directory)
- `keycodes`
- `rules`
- `symbols`
- `types`

The majority of user-specific configuration involve modifying key symbols and
this is what this document focuses on. For use-cases where a user may need to
add new key types or compat entries the general approach remains the same. A
detailed description for how to add those types or compat entries is out of
scope for this document.

You should never need to add user-specific keycodes. Where a keycode is missing,
the addition should be filed in the upstream xkeyboard-config project.

## RMLVO vs KcCGST

Due to how XKB is configured, there is no such thing as a "layout" in XKB
itself, or, indeed, any of the rules, models, variant, options (RMLVO) decribed
in `struct xkb_rule_names`. RMLVO names are merely lookup keys in the
rules file provided by xkeyboard-config to map to the correct keycode, compat,
geometry (ignored by libxkbcommon), symbols and types (KcCGST). The KcCGST data
is the one used by XKB and libxbkcommon to map keys to actual symbols.

For example, a common RMLVO configuration is layout "us", variant "dvorak" and
option "terminate:ctrl_alt_bksp". Using the default rules file and model
this maps into the following KcCGST components:

```
xkb_keymap {
	xkb_keycodes  { include "evdev+aliases(qwerty)"	};
	xkb_types     { include "complete"	};
	xkb_compat    { include "complete"	};
	xkb_symbols   { include "pc+us(dvorak)+inet(evdev)+terminate(ctrl_alt_bksp)"	};
	xkb_geometry  { include "pc(pc105)"	};
};
```

A detailed explanation of how rules files convert RMLVO to KcCGST is out of
scope for this document. See [the rules file](md_doc_rules-format.html) page
instead.


## Adding a layout

Adding a layout requires that the user adds **symbols** in the correct location.

The default rules files (usually `evdev`) have a catch-all to map a layout, say
"foo", and a variant, say "bar", into the "bar" section in the file
`$xkb_base_dir/symbols/foo`.
This is sufficient to define a new keyboard layout. The example below defines
the keyboard layout "banana" with an optional variant "orange"

```
$ cat $XDG_CONFIG_HOME/xkb/symbols/banana
// Like a US layout but swap the top row so numbers are on Shift
default partial alphanumeric_keys
xkb_symbols "basic" {
    include "us(basic)"
    name[Group1]= "Banana (US)";

    key <AE01> { [ exclam,          1]     };
    key <AE02> { [ at,              2]     };
    key <AE03> { [ numbersign,      3]     };
    key <AE04> { [ dollar,          4]     };
    key <AE05> { [ percent,         5]     };
    key <AE06> { [ asciicircum,     6]     };
    key <AE07> { [ ampersand,       7]     };
    key <AE08> { [ asterisk,        8]     };
    key <AE09> { [ parenleft,       9]     };
    key <AE10> { [ parenright,      0]     };
    key <AE11> { [ underscore,      minus] };
    key <AE12> { [ plus,            equal] };
};

// Same as banana but map the euro sign to the 5 key
partial alphanumeric_keys
xkb_symbols "orange" {
    include "banana(basic)"
    name[Group1] = "Banana (Eurosign on 5)";
    include "eurosign(5)"
};
```

The `default` section is loaded when no variant is given. The first example
sections uses ``include`` to populate with a symbols list defined elsewhere
(here: section `basic` from the file `symbols/us`, aka. the default US keyboard
layout) and overrides parts of these
symbols. The effect of this section is to swap the numbers and symbols in the
top-most row (compared to the US layout) but otherwise use the US layout.

The "orange" variant uses the "banana" symbols and includes a different section
to define the eurosign. It does not specificially override any symbols.

The exact details of how `xkb_symbols` work is out of scope for this document.

## Adding an option

For technical reasons, options do **not** have a catch-all to map option names
to files and sections and must be specifically mapped by the user. This requires
a custom rules file. As the `evdev` ruleset is hardcoded in many clients, the
custom rules file must usually be named `evdev`.

```
$ cat $XDG_CONFIG_HOME/xkb/rules/evdev
! option = symbols
  custom:foo    = +custom(bar)
  custom:baz    = +other(baz)

! include %S/evdev
```

This rules file maps the RMLVO option "custom:foo" to the "bar" section in the
`symbols/custom` file and the "custom:baz" option to the "baz" section in the
`symbols/other` file. Note how the RMLVO option name may be different to the
file or section name.

The `include` statement includes the system-provided `evdev` ruleset. This
allows users to only override those options they need.

The files themselves are similar to the layout examples in the previous section:

```
$ cat $XDG_CONFIG_HOME/xkb/symbols/custom
// map the Tilde key to nothing on the first shift level
partial alphanumeric_keys
xkb_symbols "bar" {
    key <TLDE> {        [      VoidSymbol ]       };
};

$ cat $XDG_CONFIG_HOME/xkb/symbols/other
// map first key in bottom row (Z in the US layout) to k/K
partial alphanumeric_keys
xkb_symbols "baz" {
    key <AB01> {        [      k, K ]       };
};
```

With these in place, a user may select any layout/variant together with
the "custom:foo" and/or "custom:baz" options.

## Discoverable layouts

**The below requires libxkbregistry as XKB lookup tool and does not work where
clients parse the XML file directly**.

The above sections apply only to the data files and require that the user knows
about the existence of the new entries. To make custom entries discoverable by
the configuration tools (e.g. the GNOME Control Center), the new entries must
also be added to the XML file that is parsed by libxkbregistry. In most cases,
this is the `evdev.xml` file in the rules directory. The example below shows the
XML file that would add the custom layout and custom options as outlined above
to the XKB registry:

```
$ cat $XDG_CONFIG_HOME/xkb/rules/evdev.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xkbConfigRegistry SYSTEM "xkb.dtd">
<xkbConfigRegistry version="1.1">
  <layoutList>
    <layout>
      <configItem>
        <name>banana</name>
        <shortDescription>ban</shortDescription>
        <description>Banana</description>
      </configItem>
      <variantList>
        <variant>
          <configItem>
            <name>orange</name>
            <shortDescription>or</shortDescription>
            <description>Orange (Banana)</description>
          </configItem>
        </variant>
      </variantList>
    </layout>
  </layoutList>
  <optionList>
    <group allowMultipleSelection="true">
      <configItem>
        <name>custom</name>
        <description>Custom options</description>
      </configItem>
      <option>
      <configItem>
        <name>custom:foo</name>
        <description>Map Tilde to nothing</description>
      </configItem>
      </option>
      <option>
      <configItem>
        <name>custom:baz</name>
        <description>Map Z to K</description>
      </configItem>
      </option>
    </group>
  </optionList>
</xkbConfigRegistry>
```

The default behavior of libxkbregistry ensures that the new layout and options
are added to the system-provided layouts and options.

For details on the XML format, see DTD in `<datadir>/X11/xkb/rules/xkb.dtd`
and the system-provided XML files in `<datadir>/X11/xkb/rulies/xkb.dtd`.