NAME

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

SYNOPSIS

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

wint_t ungetwc(wint_t wc, FILE *stream);


DESCRIPTION

^[CX] 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.1-2024 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(), ^[CX]  fseeko(), fsetpos(), or rewind()) ^[CX] 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 a successful call to ungetwc() is unspecified until all pushed-back wide characters are read or discarded; its value after all pushed-back wide characters have been read shall be the same as it was before the wide characters were pushed back.

RETURN VALUE

Upon successful completion, ungetwc() shall return the wide-character code corresponding to the pushed-back character. Otherwise, it shall return WEOF.

ERRORS

The ungetwc() function may fail if:

[EILSEQ]

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

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

The ISO C standard includes the text "The value of the file position indicator for the stream after reading or discarding all pushed-back wide characters shall be the same as it was before the wide characters were pushed back." POSIX.1 omits "or discarding" from this because it is redundant—in the ISO C standard the discarding is done by file positioning functions and does not affect the position set by those functions. In particular, a relative seek using fseek() or fseeko() with SEEK_CUR adjusts the position relative to the position on entry to the function, not the position after the pushed-back wide characters have been discarded. POSIX.1 also requires fflush() to discard pushed back wide characters in situations where the ISO C standard says the behavior of fflush() is undefined.

FUTURE DIRECTIONS

None.

SEE ALSO

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

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

CHANGE HISTORY

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.

Issue 8

Austin Group Defect 701 is applied, clarifying how the file-position indicator for the stream is updated.

Austin Group Defect 1374 is applied, correcting a conflict with the ISO C standard regarding the value of the file-position indicator for the stream after a successful call to ungetwc().