TOMOYO Linux Cross Reference
Linux/Documentation/input/notifier.rst

   =================
   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.

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