Issue 0438: ungetc / ungetwc and file position after discarding push back problems

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Austin Group, Nick Stoughton (US)
Date: 2013-06-19
Reference document: N1720, Austin Group Defect #701
Submitted against: C11 / C17
Status: Fixed
Fixed in: C17
Converted from: n2396.htm

Summary

C11 (and C99 before it) state for both ungetc() and ungetwc() that

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 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.

(7.21.7.10 p2 & p5, with similar at 7.29.3.10 p2 & p5). The "or discarding" phrasing in p5 makes no sense: all of the listed functions which discard the push back also _set_ the file position. The file position will end up as whatever the function sets it to, not "the same as it was before the characters were pushed back".

Suggested Change

Change

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.

to

The value of the file position indicator for the stream after all pushed-back characters have been read shall be the same as it was before the characters were pushed back.

Comment from WG14 on 2017-11-03:

Oct 2013 meeting

Committee Discussion

• This issue arises because of the different architectures of a buffered stream which can hold pushed back characters, and that of the underlying filesystem which honors seek independent of the buffering.
• The committee sentiment is that the suggested changes are nearly correct and have requested that the project editor suggest better wording, subject to further discussion.

Apr 2014 meeting

Committee Discussion

The Standard is correct as written because the intent is that the specified file position indicator is an intermediate state inside the file positioning function after the pushed-back characters are discarded but before the actual seek. That gives you a reliable file position from which to do the seek. It's not intended that the file positioning function doesn't set the file position indicator.

Proposed Technical Corrigendum

Add a footnote to 7.21.7.10 paragraph 5, second sentence:

Note that a file positioning function may further modify the file position indicator after discarding any pushed-back characters.

Add a footnote to 7.29.3.10 paragraph 5, second sentence:

Note that a file positioning function may further modify the file position indicator after discarding any pushed-back wide characters.