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input-remapper/HELP.md

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The problems with overwriting keys

If you had one keyboard layout for your mouse that writes SHIFT keys on keycode 10, and one for your keyboard that is normal and writes 1/! on keycode 10, then you would not be able to write ! by pressing that mouse button and that keyboard button at the same time. Keycodes may not clash.

The first idea was to write special keycodes known only to key-mapper (256 - 511) into an input device in /dev/input, and map those to SHIFT and such, whenever a button is clicked. A mapping would have existed to prevent the original keycode 10 from writing a 1. But X seems to ignore anything greater than 255, or even crash in some cases, for regular keyboard events. Mouse buttons can use those though, but they cannot be remapped, which I guess is another indicator of that.

The second idea is to create a new input device that uses 8 - 255, just like other layouts, and key-mapper always tries to use the same keycodes for SHIFT as already used in the system default. The pipeline is like this:

  1. A human thumb presses an extra-button of the device "mouse"
  2. key-mapper uses evdev to get the event from "mouse", sees "ahh, it's a 10, I know that one and will now write 50 into my own device". 50 is the keycode for SHIFT on my regular keyboard, so it won't clash anymore with alphanumeric keys and such.
  3. X has key-mappers configs for the key-mapper device loaded and checks in it's keycodes config file "50, that would be <50>", then looks into it's symbols config "<50> is mapped to SHIFT", and then it actually presses the SHIFT down to modify all other future buttons.
  4. X has another config for "mouse" loaded, which prevents any system default mapping to print the overwritten key "1" into the session.

How I would have liked it to be

setxkbmap -layout ~/.config/key-mapper/mouse -device 13

config looks like:

10 = a, A
11 = Shift_L

done. Without crashing X. Without printing generic useless errors. If it was that easy, an app to map keys would have already existed.

Folder Structure of Key Mapper in /usr

Stuff has to be placed in /usr/share/X11/xkb to my knowledge.

Every user gets a path within that /usr/... directory which is very unconventional, but it works. This way the presets of multiple users don't clash.

Presets

  • /usr/share/X11/xkb/symbols/key-mapper/<user>/<device>/<preset>

This is how a single preset is stored.

Defaults

  • /usr/share/X11/xkb/symbols/key-mapper/<user>/default

This is where key-mapper stores the defaults. They are generated from the parsed output of xmodmap and used to keep the unmapped keys at their system defaults.

Keycodes

  • /usr/share/X11/xkb/keycodes/key-mapper

Because the concept of "reasonable symbolic names" (www.x.org) doesn't apply when mouse buttons are all over the place, an identity mapping to make generating "symbols" files easier/possible exists. A keycode of 10 will be known as "<10>" in symbols configs. This has the added benefit that keycodes reported by xev can be identified in the symbols file.