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                                        UNGETC(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  ungetc()  function  shall push the byte specified by c (converted to an unsignedchar) back onto the
       input stream pointed to by stream.  The pushed-back bytes 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 bytes for the stream. The external storage corresponding to the stream shall be
       unchanged.

       One byte of push-back shall be provided. If ungetc() 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 c equals that of the macro EOF, the operation shall fail and the input stream shall be
       left unchanged.

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

       No errors are defined.

       Thefollowingsectionsareinformative.

       None.

       None.

       ungetc — push byte back into 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, ungetc() shall return the byte pushed back after  conversion.  Otherwise,  it
       shall return EOF.

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

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

       #include <stdio.h>

       int ungetc(int c, FILE *stream);