================= Keyboard notifier

One can use register_keyboard_notifier to get called back on keyboard events (see kbd_keycode() function for details). The passed structure is keyboard_notifier_param (see <linux/keyboard.h>):

• 'vc' always provide the VC for which the keyboard event applies;

• 'down' is 1 for a key press event, 0 for a key release;

• 'shift' is the current modifier state, mask bit indexes are KG_*;

• 'ledstate' is the current LED state;

• 'value' depends on the type of event.

• KBD_KEYCODE events are always sent before other events, value is the keycode.

• KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym. value is the keycode.

• KBD_UNICODE events are sent if the keycode -> keysym translation produced a unicode character. value is the unicode value.

• KBD_KEYSYM events are sent if the keycode -> keysym translation produced a non-unicode character. value is the keysym.

• KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms. That permits one to inspect the resulting LEDs for instance.

For each kind of event but the last, the callback may return NOTIFY_STOP in order to "eat" the event: the notify loop is stopped and the keyboard event is dropped.

In a rough C snippet, we have::

kbd_keycode(keycode) {
...
params.value = keycode;
if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)
    || !bound) {
	notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);
	return;
}

if (unicode) {
	param.value = unicode;
	if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)
		return;
	emit unicode;
	return;
}

params.value = keysym;
if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)
	return;
apply keysym;
notifier_call_chain(KBD_POST_KEYSYM,&params);
}

.. note:: This notifier is usually called from interrupt context.