The ungetwc function is the wide-character equivalent of the
ungetc function. It pushes back a wide character onto stream and
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.
If the implementation supports multiple push-back operations in a row, the
pushed-back wide characters will be read in reverse order; however, only one
level of push-back is guaranteed.