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