Description

   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.