LXR linux/Documentation/input/notifier.rst

   1=================
   2Keyboard notifier
   3=================
   4
   5One can use register_keyboard_notifier to get called back on keyboard
   6events (see kbd_keycode() function for details).  The passed structure is
   7keyboard_notifier_param (see <linux/keyboard.h>):
   8
   9- 'vc' always provide the VC for which the keyboard event applies;
  10- 'down' is 1 for a key press event, 0 for a key release;
  11- 'shift' is the current modifier state, mask bit indexes are KG_*;
  12- 'ledstate' is the current LED state;
  13- 'value' depends on the type of event.
  14
  15- KBD_KEYCODE events are always sent before other events, value is the keycode.
  16- KBD_UNBOUND_KEYCODE events are sent if the keycode is not bound to a keysym.
  17  value is the keycode.
  18- KBD_UNICODE events are sent if the keycode -> keysym translation produced a
  19  unicode character. value is the unicode value.
  20- KBD_KEYSYM events are sent if the keycode -> keysym translation produced a
  21  non-unicode character. value is the keysym.
  22- KBD_POST_KEYSYM events are sent after the treatment of non-unicode keysyms.
  23  That permits one to inspect the resulting LEDs for instance.
  24
  25For each kind of event but the last, the callback may return NOTIFY_STOP in
  26order to "eat" the event: the notify loop is stopped and the keyboard event is
  27dropped.
  28
  29In a rough C snippet, we have::
  30
  31    kbd_keycode(keycode) {
  32        ...
  33        params.value = keycode;
  34        if (notifier_call_chain(KBD_KEYCODE,&params) == NOTIFY_STOP)
  35            || !bound) {
  36                notifier_call_chain(KBD_UNBOUND_KEYCODE,&params);
  37                return;
  38        }
  39
  40        if (unicode) {
  41                param.value = unicode;
  42                if (notifier_call_chain(KBD_UNICODE,&params) == NOTIFY_STOP)
  43                        return;
  44                emit unicode;
  45                return;
  46        }
  47
  48        params.value = keysym;
  49        if (notifier_call_chain(KBD_KEYSYM,&params) == NOTIFY_STOP)
  50                return;
  51        apply keysym;
  52        notifier_call_chain(KBD_POST_KEYSYM,&params);
  53    }
  54
  55.. note:: This notifier is usually called from interrupt context.
  56