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

ideally you want a udev rule that automatically runs stupidlayers when the keyboard is plugged in, here's an example for void linux (assumes you copied stupidlayers to /usr/bin)

note that the rules.d directory might be different in other distros

sudo tee /usr/lib/udev/rules.d/30-stupidlayers.rules << "EOF"
ACTION=="add", \
KERNEL=="event[0-9]*", \
ATTRS{name}=="SONiX USB DEVICE", \
RUN+="/bin/sh -c 'echo /usr/bin/stupidlayers /dev/input/%k | at now'"
EOF

sudo xbps-install at
sudo ln -s /etc/sv/at /var/service
sudo sv start at
sudo udevadm control --reload
sudo udevadm control --reload-rules

you can also just make a script that finds the device and run it in your xinitrc or something, but it won't stick if you unplug the keyboard

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