The ungetwc function pushes the wide character wc (converted to an
.Vt wchar_t ) back onto the input stream pointed to by stream. The pushed-backed wide characters will be returned by subsequent reads on the stream (in reverse order). A successful intervening call, using the same stream, to one of the file positioning functions fseek(3), fsetpos(3), or rewind(3) will discard the pushed back wide characters.
One wide character of push-back is guaranteed, but as long as there is sufficient memory, an effectively infinite amount of pushback is allowed.
If a character is successfully pushed-back, the end-of-file indicator for the stream is cleared.
The ungetwc function returns the wide character pushed-back after the conversion, or WEOF if the operation fails. If the value of the argument c character equals WEOF, the operation will fail and the stream will remain unchanged.