xref: /linux/Documentation/input/notifier.rst (revision b8d312aa075f33282565467662c4628dae0a2aff) 
1=================
2Keyboard notifier
3=================
4
5One can use register_keyboard_notifier to get called back on keyboard
6events (see kbd_keycode() function for details).  The passed structure is
7keyboard_notifier_param:
8
9- 'vc' always provide the VC for which the keyboard event applies;
10- 'down' is 1 for a key press event, 0 for a key release;
11- 'shift' is the current modifier state, mask bit indexes are KG_*;
12- 'value' depends on the type of event.
13
14- KBD_KEYCODE events are always sent before other events, value is the keycode.
15- KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.
16  value is the keycode.
17- KBD_UNICODE events are sent if the keycode -> keysym translation produced a
18  unicode character. value is the unicode value.
19- KBD_KEYSYM events are sent if the keycode -> keysym translation produced a
20  non-unicode character. value is the keysym.
21- KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.
22  That permits one to inspect the resulting LEDs for instance.
23
24For each kind of event but the last, the callback may return NOTIFY_STOP in
25order to "eat" the event: the notify loop is stopped and the keyboard event is
26dropped.
27
28In a rough C snippet, we have::
29
30    kbd_keycode(keycode) {
31	...
32	params.value = keycode;
33	if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)
34	    || !bound) {
35		notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);
36		return;
37	}
38
39	if (unicode) {
40		param.value = unicode;
41		if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)
42			return;
43		emit unicode;
44		return;
45	}
46
47	params.value = keysym;
48	if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)
49		return;
50	apply keysym;
51	notifier_call_chain(KBD_POST_KEYSYM,&params);
52    }
53
54.. note:: This notifier is usually called from interrupt context.
55