xref: /linux/Documentation/input/notifier.rst (revision a36e9f5cfe9eb3a1dce8769c7058251c42705357) 
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 (see <linux/keyboard.h>):
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- 'ledstate' is the current LED state;
13- 'value' depends on the type of event.
14
15- KBD_KEYCODE events are always sent before other events, value is the keycode.
16- KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.
17  value is the keycode.
18- KBD_UNICODE events are sent if the keycode -> keysym translation produced a
19  unicode character. value is the unicode value.
20- KBD_KEYSYM events are sent if the keycode -> keysym translation produced a
21  non-unicode character. value is the keysym.
22- KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.
23  That permits one to inspect the resulting LEDs for instance.
24
25For each kind of event but the last, the callback may return NOTIFY_STOP in
26order to "eat" the event: the notify loop is stopped and the keyboard event is
27dropped.
28
29In a rough C snippet, we have::
30
31    kbd_keycode(keycode) {
32	...
33	params.value = keycode;
34	if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)
35	    || !bound) {
36		notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);
37		return;
38	}
39
40	if (unicode) {
41		param.value = unicode;
42		if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)
43			return;
44		emit unicode;
45		return;
46	}
47
48	params.value = keysym;
49	if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)
50		return;
51	apply keysym;
52	notifier_call_chain(KBD_POST_KEYSYM,&params);
53    }
54
55.. note:: This notifier is usually called from interrupt context.
56