Map scancodes to keycodes

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Scancodes are the lowest identification numbers for a key, it is the value that a keyboard sends to a computer. Scancodes are mapped to keycodes, which are then mapped to keysyms depending on used keyboard layout. Mapping scancodes to keycodes is universal and not specific to Linux console or Xorg, which means that changes to this mapping will be effective in both.

See Extra Keyboard Keys for more information.

There are two ways of mapping scancodes to keycodes:

  • Using udev
  • Using setkeycodes

The preferred method is to use udev because it uses hardware information (which is a quite reliable source) to choose the keyboard model in a database. It means that if your keyboard model has been found in the database, your keys are recognized out of the box.

Identifying scancodes

You need to know the scancodes of keys you wish to remap. There are multiple ways to do that.

Using showkey

The universal way to get a scancode is to use the showkey utility. showkey waits for a key to be pressed and if none is during 10 seconds it quits, which is the only way to exit the program. To execute showkey you need to be in a virtual console, not in a graphical environment. Run the following command

# showkey --scancodes

and try to push your hotkeys, you should see scancodes being printed to the output.

Using dmesg

Note: This method does not provide scancodes for all keys, it only identifies the unknown keys.

You can get the scancode of a key by pressing the desired key and looking the output of dmesg command. For example, if you get:

Unknown key pressed (translated set 2, code 0xa0 on isa0060/serio0

then the scancode you need is 0xa0.

Using udev

udev provides a builtin function called hwdb to maintain the hardware database index in /etc/udev/hwdb.bin. The database is compiled from files with .hwdb extension located in directories /usr/lib/udev/hwdb.d/, /run/udev/hwdb.d/ and /etc/udev/hwdb.d/. The default scancodes-to-keycodes mapping file is /usr/lib/udev/hwdb.d/60-keyboard.hwdb. See man udev for details.

The .hwdb file can contain multiple blocks of mappings for different keyboards, or one block can be applied to multiple keyboards. The keyboard: prefix is used to match a block against a hardware, the following hardware matches are supported:

  • USB keyboards identified by the usb kernel modalias:
    keyboard:usb:v<vendor_id>p<product_id>*
    where <vendor_id> and <product_id> are the 4-digit hex uppercase vendor and product IDs (you can find those by running the lsusb command).
  • AT keyboard DMI data matches:
    keyboard:dmi:bvn*:bvr*:bd*:svn<vendor>:pn<product>:pvr*
    where <vendor> and <product> are the firmware-provided strings exported by the kernel DMI modalias.
  • Platform driver device name and DMI data match:
    keyboard:name:<input_device_name>:dmi:bvn*:bvr*:bd*:svn<vendor>:pn*
    where <input_device_name> is the name device specified by the driver and <vendor> is the firmware-provided string exported by the kernel DMI modalias.

The format of each line in the block body is KEYBOARD_KEY_<scancode>=<keycode>. The value of <scancode> is hexadecimal, but without the leading 0x (i.e. specify a0 instead of 0xa0), whereas the value of <keycode> is the lower-case keycode name string as listed in /usr/include/linux/input.h (see the KEY_<KEYCODE> variables), a sorted list is available at [1]. It is not possible to specify decimal value in <keycode>.

Here is an example to match all USB and AT keyboards:

/etc/udev/hwdb.d/90-custom-keyboard.hwdb
keyboard:usb:v*p*
keyboard:dmi:bvn*:bvr*:bd*:svn*:pn*:pvr*
 KEYBOARD_KEY_10=suspend
 KEYBOARD_KEY_a0=search

After changing the configuration files, the hardware database index needs to be rebuilt. This is done on reboot, or manually using the following command:

# udevadm hwdb --update

Using setkeycodes

setkeycodes is a tool to load scancodes-to-keycodes mapping table into Linux kernel. Its usage is:

# setkeycodes scancode keycode ...

It is possible to specify multiple pairs at once. Scancodes are given in hexadecimal, keycodes in decimal.