UNGETC(3)                  POSIX Programmer's Manual                 UNGETC(3)



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

NAME
       ungetc -- push byte back into input stream

SYNOPSIS
       #include <stdio.h>

       int ungetc(int _c, FILE *stream);

DESCRIPTION
       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 unsigned char) 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.

RETURN VALUE
       Upon successful completion, ungetc() shall return the byte pushed back
       after conversion. Otherwise, it shall return EOF.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.5, Standard I/O Streams, fseek(), getc(), fsetpos(), read(),
       rewind(), setbuf()

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

COPYRIGHT
       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(3)
