#include <wchar.h> wint_t ungetwc(wint_t wc, FILE *stream);
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.