The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


ungetwc - push wide-character code back into the input stream


#include <stdio.h>
#include <wchar.h>

wint_t ungetwc(wint_t
wc, FILE *stream);


[CX] [Option Start] 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. [Option End]

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(), [CX] [Option Start]   fseeko(), [Option End] fsetpos(), or rewind()) [CX] [Option Start] or fflush() [Option End]  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(), [CX] [Option Start]   fseeko(), [Option End] fsetpos(), or rewind() [CX] [Option Start] (but not fflush()), [Option End]  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:

[CX] [Option Start] An invalid character sequence is detected, or a wide-character code does not correspond to a valid character. [Option End]

The following sections are informative.










Standard I/O Streams, fseek, fsetpos, read, rewind, setbuf

XBD <stdio.h>, <wchar.h>


First released in Issue 4. Derived from the MSE working draft.

Issue 5

The Optional Header (OH) marking is removed from <stdio.h>.

Issue 6

The [EILSEQ] optional error condition is marked CX.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0690 [87,93], XSH/TC1-2008/0691 [87], and XSH/TC1-2008/0692 [14] are applied.

End of informative text.


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