This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for  Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical  and  Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE  and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                       UNGETWC(3POSIX)

       The  functionality  described  on  this  reference  page is aligned with the ISO C standard. Any conflict
       between the requirements described  here  and  the  ISO C  standard  is  unintentional.  This  volume  of
       POSIX.1‐2017 defers to the ISO C standard.

       The  ungetwc() function shall push the character corresponding to the wide-character code specified by wc
       back onto the input stream pointed to by  stream.   The  pushed-back  characters  shall  be  returned  by
       subsequent  reads  on  that  stream  in the reverse order of their pushing. A successful intervening call
       (with the stream pointed to by stream) to a file-positioning function (fseek(), fseeko(),  fsetpos(),  or
       rewind())  or  fflush()  shall  discard  any  pushed-back characters for the stream. The external storage
       corresponding to the stream is unchanged.

       At least one character of push-back shall be provided. If ungetwc() is called too many times on the  same
       stream without an intervening read or file-positioning operation on that stream, the operation may fail.

       If  the value of wc equals that of the macro WEOF, the operation shall fail and the input stream shall be
       left unchanged.

       A successful call to ungetwc() shall clear the end-of-file indicator for the stream.  The  value  of  the
       file-position  indicator  for the stream after all pushed-back characters have been read, or discarded by
       calling fseek(), fseeko(), fsetpos(), or rewind() (but not fflush()), shall be the same as it was  before
       the  characters  were  pushed  back.  The file-position indicator is decremented (by one or more) by each
       successful call to ungetwc(); if its value was 0 before a call, its value is unspecified after the call.

       The ungetwc() function may fail if:

       EILSEQ An invalid character sequence is detected, or a wide-character code does not correspond to a valid
              character.

       Thefollowingsectionsareinformative.

       None.

       None.

       ungetwc — push wide-character code back into the input stream

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       Upon successful completion, ungetwc() shall return the wide-character code corresponding to  the  pushed-
       back character. Otherwise, it shall return WEOF.

Section2.5, StandardI/OStreams, fseek(), fsetpos(), read(), rewind(), setbuf()

       The Base Definitions volume of POSIX.1‐2017, <stdio.h>, <wchar.h>

       #include <stdio.h>
       #include <wchar.h>

       wint_t ungetwc(wint_t wc, FILE *stream);