Function setvbuf 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn setvbuf(
    stream: *mut FILE,
    buf: *mut c_char,
    mode: c_int,
    size: size_t,
) -> c_int

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

Reset stream to use buffer buf of size size If this isn’t the meaning of unsafe, idk what is