Function setvbuf 

pub unsafe extern "C" fn setvbuf(
    __stream: *mut FILE,
    __buf: *mut c_char,
    __modes: c_int,
    __n: usize,
) -> c_int

Make STREAM use buffering mode MODE. If BUF is not NULL, use N bytes of it for buffering; else allocate an internal buffer N bytes long.