Defect report #443

Defect Report #443

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Submitter: Joseph Myers
Submission Date: 2013-07-21
Source: WG 14
Reference Document: N1730
Version: 1.1
Date: April 2014
Subject: Floating-point issues in C11 from PDTS 18661-1 UK review, Issue 4

Summary

Some 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 4: Floating-point state not being an object

The description of the floating-point environment in C11 fails to make sufficiently clear what is or is not an object (C11 footnote 205 is not normative, and so cannot be used to that effect); it uses terms such as "system variable" without saying what that is. Simply moving that footnote to normative text would fix this issue:

Move the contents of footnote 205 (C11 subclause 7.6) to the end of 5.1.2.3#2.

Oct 2013 meeting

Committee Discussion

The committee agrees that the floating point environment is not an object and must be stated in normative terms instead of in non-normative footnote 205.
The environment may be modified more than once in an expression without inducing undefined behavior, this is just "how it works".
The floating-point environment is described in the standard using an otherwise undefined term "system variable" having 7.6p2 thread storage duration, implying that it is an object, and so this is perhaps the actual manifestation of the defect that requires correction.
The floating-point environment is technically an actor, not an object, having a model of its behavior defined purely by messages as its internal "piece of state" is not directly visible. In this case the messages are the macros and functions, as are the electronic signals from the floating-point computational units to consult and honor the desired floating point modal behaviors, and the electronic signals that record "auxiliary information" in the model for floating point exceptions.
In any case, the floating point environment implementation is not described by the abstract C machine, only its operations.
The committee prefers that this be cleared up in 7.6, by moving the second sentence and possibly the first sentence into normative text.

Apr 2014 meeting

Committee Discussion

The committee discusses this issue further and could not see an actual defect: there are no misinterpretations stated or implied.

Proposed Committee Response

Since operations on the floating point environment are well defined there is no need to normatively define anything further about its implementation. The footnote adds clarity and should remain as is.

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