=================Keyboard notifier================= One can use register_keyboard_notifier to get called back on keyboardevents (see kbd_keycode() function for details).  The passed structure iskeyboard_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 inorder to "eat" the event: the notify loop is stopped and the keyboard event isdropped. 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.