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(), fsetpos(), or
rewind()) discards 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
reading or discarding all pushed-back characters 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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 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