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.
#include <stdio.h> #include <wchar.h> wint_t ungetwc(wint_t wc, FILE *stream);
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.
Upon successful completion, ungetwc() shall return the wide-character code corresponding to the pushed-back character. Otherwise, it shall return WEOF.
The ungetwc() function may fail if:
An invalid character sequence is detected, or a wide-character code does not correspond to a valid character.
The following sections are informative.
Section 2.5, Standard I/O Streams, fseek(), fsetpos(), read(), rewind(), setbuf()
The Base Definitions volume of POSIX.1-2017, <stdio.h>, <wchar.h>
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 .
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