When using the ungetwc function, the wide character is pushed
    back onto the file indicated by file_ptr.

    One push-back is guaranteed, even if there has been no previous
    activity on the file. If a file positioning function (such as
    fseek) is called before the pushed back character is read, the
    bytes representing the pushed back character are lost.

    If the character to be pushed back is WEOF, the operation fails,
    the input stream is left unchanged, and WEOF is returned.

    See also getwc.