3.Keyboard notifier¶

One can use register_keyboard_notifier to get called back on keyboardevents (seekbd_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 aunicode character. value is the unicode value.

• KBD_KEYSYM events are sent if the keycode -> keysym translation produced anon-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);}