Function us_socket_context_on_open 

pub unsafe extern "C" fn us_socket_context_on_open(
    ssl: c_int,
    context: *mut us_socket_context_t,
    on_open: Option<unsafe extern "C" fn(s: *mut us_socket_t, is_client: c_int, ip: *mut c_char, ip_length: c_int) -> *mut us_socket_t>,
)