Function bufferevent_getcb 

pub unsafe extern "C" fn bufferevent_getcb(
    bufev: *mut bufferevent,
    readcb_ptr: *mut bufferevent_data_cb,
    writecb_ptr: *mut bufferevent_data_cb,
    eventcb_ptr: *mut bufferevent_event_cb,
    cbarg_ptr: *mut *mut c_void,
)