Function ungetwc 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn ungetwc(
    wc: wint_t,
    stream: &mut FILE,
) -> wint_t

See https://pubs.opengroup.org/onlinepubs/9799919799/functions/ungetwc.html.

Push wide character wc back onto stream so it’ll be read next