pub type __io_read_fn = Option<unsafe extern "C" fn(__cookie: *mut c_void, __buf: *mut c_char, __nbytes: usize) -> __ssize_t>;
pub enum __io_read_fn { None, Some(unsafe extern "C" fn(*mut c_void, *mut i8, usize) -> i32), }
No value.
Some value of type T.
T