Function open_keyed 

pub fn open_keyed(
    dd: Option<&Dd>,
    mode: OpenMode,
    format: Format,
    keying: Keying,
    record_len: usize,
    page: &'static CodePage,
) -> Result<Open>

Opens a file held in memory. With no DD (an OPTIONAL file) or no file at the DD’s path, it starts empty; OUTPUT always starts empty. record_len is the longest record.