If s is a NULL pointer, the wcrtomb function is equivalent to the
    call:

       wcrtomb (buf, L'\0', ps)

    where buf is an internal buffer.

    If s is not a NULL pointer, the wcrtomb function determines the
    number of bytes needed to represent the multibyte character that
    corresponds to the wide character specified by wc (including any
    shift sequences), and stores the resulting bytes in the array
    whose first element is pointed to by s. At most MB_CUR_MAX bytes
    are stored.

    If wc is a null wide character, a null byte is stored preceded by
    any shift sequence needed to restore the initial shift state. The
    resulting state described is the initial conversion state.