Submitter: Joseph Myers
Submission Date: 2013-07-21
Source: WG 14
Reference Document: N1730
Date: April 2014
Subject: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 3
SummarySome issues with floating point in C11 have been identified as part of the UK review of the N1711 draft of TS 18661-1. While such issues relate to the general area of C bindings to IEC 60559:2011, and so could be addressed in the TS on that basis, since the issues also apply to C11 as-is it may be more appropriate to address some or all of these issues as Defect Reports with a view to having a normative fix in a future TC to C11 rather than only having a fix in conjunction with the new bindings.
Issue 3: Floating-point exceptions and 6.5#5
C11 6.5#5 says "If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), the behavior is undefined.". When the Annex F bindings are in effect, it must be intended that floating-point exceptions do not produce such undefined behavior (instead, behavior such as evaluating to a NaN must be defined). But no normative text actually says that.
A suitable fix would be:
Append to 6.5#5: For implementations defining __STDC_IEC_559__, this does not apply to exceptional conditions where the behavior (such as raising a floating-point exception and returning a NaN) is defined by Annex F, directly or by reference to IEC 60559.
If an exceptional condition occurs during the evaluation of an expression (that is, if the result is not mathematically defined or not in the range of representable values for its type), and the behaviour is not defined in this annex or by reference to IEC 60559, 6.5p5 applies and the behaviour is undefined.
Apr 2014 meeting
Further correspondence with the author and excerpted in N1804 has identified the core issue as being a simple misunderstanding of the applicability of normative annexes to the standard.
Proposed Committee Response
WG14 treats normative annexes such as Annex F as if they were linear extensions of the standard itself. When Annex F is requested via definition of __STDC_IEC_559__ then 6.5#5 is superseded and floating point exceptions become well defined.
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