Alt F......n Ctrl

The assumed plan was to remap a triple sized Fn key of a Logitech K345 keyboard to a missing Meta key. That however, might not be possible as the Fn key is private to the keyboard (this external keyboard), i.e. it causes the keyboard to send altered scancodes when other keys are pressed, but does not send a scancode for itself.

This is a toy program that creates global keyboard hooks, and can be used to detect keyboard events for reference:

• CGEvent's at the Quartz window system layer,
• IOHIDManager events at the kernel devices layer.

Installation

make build

An executable listenkey will be placed into a bin folder inside the program directory.

Usage

• listenkey --scancode listens for scancodes the keyboard sends to the OS.
• listenkey --keycode listens for virtual key codes for keystrokes the window system dispatches.

Answering to the remapping problem

Mapping the Fn key of an external keyboard might not be possible by software. To map other keys, macOS has hidutil command; to do it programmatically, use the IOKit/hidsystem APIs (see this). A hidden file "$HOME"/Library/Preferences/ByHost/.GlobalPreferences.*.plist might be responsible for persisting the mapping upon restarts. plutil -convert xml1 -o foo.xml foo.plist can be used to unpack this file. The mapping is stored under the key: com.apple.keyboard.modifiermapping.{idVendor}-{idProduct}-0. idVendor and idProduct are reported by ioreg -p IOUSB -c IOUSBDevice. The value stored is an array of items, each item looks like:

<dict>
    <key>HIDKeyboardModifierMappingDst</key>
    <integer>30064771298</integer>
    <key>HIDKeyboardModifierMappingSrc</key>
    <integer>30064771299</integer>
</dict>

The structure is self explanatory, and the numbers are offsetted scancodes of keystrokes.

30064771298=0x7000000e2=0x700000000|0xe2=0x700000000+226
30064771299=0x7000000e3=0x700000000|0xe3=0x700000000+227

226 is left Alt, and 227 is left GUI (source) which by default is mapped as Cmd; this can be confirmed by this program as well (scancodes are not altered before being listened).

Another way of persisting this remapping configuration is using defaults.

defaults -currentHost read -g com.apple.keyboard.modifiermapping.{idVendor}-{idProduct}-0

defaults -currentHost delete -g com.apple.keyboard.modifiermapping.{idVendor}-{idProduct}-0

defaults -currentHost write -g \
com.apple.keyboard.modifiermapping.{idVendor}-{idProduct}-0 \
-array "<dict>\
<key>HIDKeyboardModifierMappingDst</key>\
<integer>30064771076</integer>\
<key>HIDKeyboardModifierMappingSrc</key>\
<integer>30064771298</integer>\
</dict>"

The write would map the left Alt to a, effective upon next login. Pressing Alt yields scancode 226 and key code 0, also the CGEvent's type changes from NSEventTypeFlagsChanged to NSEventTypeKeyDown; which means the Alt key is no longer a modifier key.