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UNGETWC(3) Library Functions Manual UNGETWC(3)

un-get wide character from input stream

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

The ungetwc() function pushes the wide character wc (converted to an 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.

fseek(3), getwc(3), ungetc(3)

The ungetwc() function conforms to ISO/IEC 9899:1999 (“ISO C99”).

The current implementation uses a fixed sized ungetwc-buffer.
December 1, 2017 OpenBSD-current