one of my keyboards (motospeed ck62) doesn't have a normal fn key, it toggles layers instead, so I made this tool that captures all keyboard input through evdev and emulates a fn key by forwarding modified inputs to a virtual keyboard created with uinput. it has basically zero latency

you can reuse stupidlayers.c as a mini library to make your own keyboard remapper

keybinds:

caps lock is esc
esc is `
esc acts as a normal fn key if held down
esc + q is `
esc + shift + q is ~
esc + e is ~
no esc keypress is sent on release if you used it as fn key

usage

use evtest to figure out which /dev/input/event* device is your keyboard. for me it's /dev/input/event3

gcc main.c -o stupidlayers
sudo ./stupidlayers /dev/input/event3

if you run this by hand on the keyboard that gets captured you might end up with the enter key getting stuck from missing the release event. just press enter again to stop it. it's better if you put this in your xinitrc though

example of finding a device by name (what I have in my xinitrc):

sudo killall stupidlayers
for x in /dev/input/event*; do
  devname=$(cat $(echo $x | sed 's|dev|sys/class|g')/device/name)
  if [ "$devname" = "SONiX USB DEVICE" ]; then
    sudo stupidlayers $x &
    break
  fi
done

keep in mind that if you put this in .xinitrc you must have no password required on sudo

venam/stupidlayers

usage