Kernel-3.10.0-957.el7_notifier

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:

• ‘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_*;

• ‘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.