Function us_socket_context_on_data 

pub unsafe extern "C" fn us_socket_context_on_data(
    ssl: c_int,
    context: *mut us_socket_context_t,
    on_data: Option<unsafe extern "C" fn(s: *mut us_socket_t, data: *mut c_char, length: c_int) -> *mut us_socket_t>,
)