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.
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.1‐2008 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.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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
IEEE/The Open Group 2013 UNGETC(3P)