/dev/usb/kbd

/dev/usb/kbd is the interface to the USB keyboard driver.

Reads, seeks and ioctlvs always fail. Writes are used to send commands. Ioctls are used to receive events.

The device can be opened and closed even without a keyboard connected. Performing an ioctl on the device in this state will simply block forever.

Write

Writes are used the send commands to the driver. This is for controlling the keyboard LEDs. Writes are always 5 bytes in length.

Available commands:

Write data	Function
00 00 00 00 80	Turn off keyboard LEDs.
?? ?? ?? ?? 00	Unknown
?? ?? ?? ?? 00	Unknown
?? ?? ?? ?? 00	Unknown
?? ?? ?? ?? 00	Unknown

Ioctl

Ioctls are used to read keyboard events. All parameters except the output buffer are ignored. The buffer should be 16 bytes in size and will be filled with the event data. Note that the ioctl is blocking.

Event data format

Offset	Length	Contents
0	4	Message type
4	4	Unknown (keyboard identifier?)
8	1	Modifiers
9	1	Unknown
10	6	Pressed keys

Message type

Message type is one of the following:

Hex value	Description
00 00 00 00	Keyboard connect
00 00 00 01	Keyboard disconnect
00 00 00 02	Event

You will always be properly notified of available keyboards. This means that when the device is opened after the keyboard is connected, a keyboard connect event is still received as the very first message.

Modifiers

The modifiers are a bitmask of the following:

Bit no. Hex value Description 1 01 Left control 2 02 Left shift 3 04 Left alt / left meta 4 08 Left super / left windows key

Bit no. Hex value Description 5 10 Right control 6 20 Right shift 7 40 Right alt / right meta / alt gr 8 80 Right super / right windows key

Key data

The key data is an array of bytes with key codes. The codes are probably passed directly from the keyboard as they correspond to the table in the USB HID Usage Tables Chapter 10 (Keyboard/Keypad Page), available here.

Each byte represents a pressed key. When a key is pressed, a new event will arrive with the key added to the array. Once a key is released, a new event will arrive with the key simply removed from the array.

The array is 6 bytes long, so no more than 6 simultaneous key presses can be detected. On top of that, most keyboards will barf on certain combinations. If an error occurs, a new event will arrive with all elements of the array set to 0x01.

Special keys that some keyboards provide seem to be ignored.

The following table of key codes was pulled from a Logitech Internet Navigator keyboard, which is a 105 key US layout keyboard with some special keys:

Dec	Hex	Description
4	04	a
5	05	b
6	06	c
7	07	d
8	08	e
9	09	f
10	0a	g
11	0b	h
12	0c	i
13	0d	j
14	0e	k
15	0f	l
16	10	m
17	11	n
18	12	o
19	13	p
20	14	q
21	15	r
22	16	s
23	17	t
24	18	u
25	19	v
26	1a	w
27	1b	x
28	1c	y
29	1d	z

Dec	Hex	Description
31	1f	1
32	20	2
33	21	3
34	22	4
35	23	5
36	24	6
37	25	7
38	26	8
39	27	9
40	28	Enter
41	29	Escape
42	2a	Backspace
43	2b	Tab
44	2c	Spacebar
45	2d	-
46	2e	=
47	2f	[
48	30	]
49	31	Unknown
50	32	Right \
51	33	;
52	34	'
53	35	~
54	36	,
55	37	.
56	38	/
57	39	Caps Lock

Dec	Hex	Description
58	3a	F1
59	3b	F2
60	3c	F3
61	3d	F4
62	3e	F5
63	3f	F6
64	40	F7
65	41	F8
66	42	F9
67	43	F10
68	44	F11
69	45	F12
70	46	Print screen
71	47	Scroll lock
72	48	Pause / Break
73	49	Insert
74	4a	Home
75	4b	Page Up
76	4c	Delete
77	4d	End
78	4e	Page Down
79	4f	Arrow Right
80	50	Arrow Left
81	51	Arrow Down
82	52	Arrow Up

Dec	Hex	Description
83	53	Numlock
84	54	Numpad /
85	55	Numpad *
86	56	Numpad -
87	57	Numpad +
88	58	Numpad Enter
89	59	Numpad 1
90	5a	Numpad 2
91	5b	Numpad 3
92	5c	Numpad 4
93	5d	Numpad 5
94	5e	Numpad 6
95	5f	Numpad 7
96	60	Numpad 8
97	61	Numpad 9
98	62	Numpad 0
99	63	Numpad .
100	64	Left \
101	65	Menu key

I think the table above is wrong. My iMac keyboard gives 30 for '1' through 39 for '0'. Also, I get 39 for backslash. My F13, F14, and F15 keys are 104, 105, and 106. Everything else seems the same. [aksommerville]