pub type ble_gap_event_fn = Option<unsafe extern "C" fn(event: *mut ble_gap_event, arg: *mut c_void) -> c_int>;
enum ble_gap_event_fn { None, Some(unsafe extern "C" fn(_: *mut ble_gap_event, _: *mut c_void) -> i32), }
No value.
Some value of type T.
T