The ungetwc function is the wide-character equivalent of the ungetc function. It pushes back a wide character onto stream and returns it.
If wc is WEOF, it returns WEOF. If wc is an invalid wide character, it sets errno to EILSEQ and returns WEOF.
If wc is a valid wide character, it is pushed back onto the stream and thus becomes available for future wide character read operations. The file-position indicator is decremented by one or more. The end-of-file indicator is cleared. The backing storage of the file is not affected.
Note: wc need not be the last wide character read from the stream; it can be any other valid wide character.