pub unsafe extern "C" fn ip4_output_if_opt(
    p: *mut pbuf,
    src: *const ip4_addr_t,
    dest: *const ip4_addr_t,
    ttl: u8_t,
    tos: u8_t,
    proto: u8_t,
    netif: *mut netif,
    ip_options: *mut c_void,
    optlen: u16_t
) -> err_t