Defect Report #287

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Submitter: Fred Tydeman (USA)
Submission Date: 2003-04-13
Source: WG 14
Reference Document: ISO/IEC WG14 C90 DR 087
Version: 1.3
Date: 2006-03-29
Subject: Floating-point status flags and sequence points


The problem is, 6.5 Expressions (which existed in C90) was not updated when the floating-point status flags were added to C99. Also, the response to C90 DR 087 was not incorporated into C99.

Consider the expression: a = x*y + w*z; where all variables are of type double and each of the sub-expressions x*y and w*z raises the floating-point overflow exception, which sets the floating-point overflow status flag as a side-effect. The model used by C99 ( Program execution, in particular, paragraph 2 and footnote 11; as well as, 7.6 Floating-point environment <fenv.h>, in particular, paragraph 1; 7.6.2 Floating-point exceptions; and F.7.1 Environment management) and IEC 60599 / IEEE-754 is that the status flags are sticky and may be set multiple times as side effects of floating-point operations between sequence points. Setting the same floating-point status flag multiple times is well defined: it is set.

Consider the expression: b = (feclearexcept)(FE_OVERFLOW) + (feraiseexcept)(FE_OVERFLOW); which both clears and sets the same floating-point status flag between two sequence points by the use of functions (not macros). If the execution of the two functions is allowed to overlap, then this is undefined behaviour (as the same object is being modified to two different values at the "same" time (between the same pair of sequence points)). If functions are atomic (not allowed to overlap execution), then, each function evaluation is considered a sequence point, and the two modifies are not between the same two sequence points. That means, there is no undefined behaviour, but it is unspecified as to which of the two function calls is done first/last. I understand that draft C89 had words similar to "Function calls are allowed to overlap.", but that they were removed before C89 became a standard, and that only those who know that bit of history know that C99 does not allow functions to overlap execution. C90 Defect Report 087 had as part of its response "function calls do not overlap", but those words are not in C99.

The same problem exists for ERRNO. Consider the expression: b = (log)(-1.0) + (exp)(DBL_MAX); in which log sets errno to EDOM, while exp sets errno to ERANGE.

Modifying the same status flag twice between two sequence points is a direct contradiction of 6.5 Expressions, paragraph 2: "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression." C99 needs to allow for multiple updates to the same floating-point status flag.

Suggested Technical Corrigendum

Add to 6.5 Expressions, paragraph 2, after the first sentance: An exception to this shall be permitted if the object is a floating-point status flag and the modification sets the flag.

Add to 6.5 Expressions, paragraph 3, after the last sentance: Function calls, in the same expression-statement, do not overlap. Another possible place to add this could be Function calls, paragraph 10. Possible wording issue: recursive function calls.

Committee Discussion

Flags are not objects, thus the constraint on modifying objects doesn't apply to flags.

Committee Response

As noted in the response to DR 087, function calls in the same expression do not overlap. This has not changed for C99.

Technical Corrigendum

Add a footnote to 6.5 Expressions, paragraph 2, after the first sentence.
*A floating-point status flag is not an object and can be set more than once within an expression.
Add a footnote to 7.6 Floating-point environment, paragraph 1, after the third sentence.
*A floating-point status flag is not an object and can be set more than once within an expression.

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