Function enif_select 

pub unsafe extern "C" fn enif_select(
    env: *mut ErlNifEnv,
    e: ErlNifEvent,
    flags: ErlNifSelectFlags,
    obj: *const c_void,
    pid: *const ErlNifPid,
    eref: ERL_NIF_TERM,
) -> c_int

See enif_select in the Erlang docs.