Submitter: Clive Feather (UK)
Submission Date: 2000-04-04
Source:
Reference Document: N/A
Version: 1.2
Date: 2001-09-18 15:58:00
Subject: Signs of non-numeric floating point values
Summary
There is an implication at various points in the standard,
notably the copysign function, that infinities and NaNs have
signs. This is not the case in all implementations, and this
needs to be allowed for.
Suggested Technical Corrigendum
Add a new paragraph to 5.2.4.2.2, preferably after [#3]:
[#3a] An implementation may give zero and non-numeric values (such as infinities and NaNs) a sign or may leave them unsigned. Wherever such values are unsigned, any requirement in this International Standard to retrieve the sign shall act as if the value were positive, and any requirement to set the sign shall be ignored.or:
[...]
to retrieve the sign shall produce an unspecified sign, and any requirement to set the sign shall be ignored.
Committee Response
In addition to the following Technical Corrigendum, add to the
Rationale section that discusses 5.2.4.2.2 of the C
Standard:
The committee has been made aware of at least one implementation (VAX and Alpha in VAX mode) whose floating-point format does not support signed zeros. The hardware representation that one thinks would represent -0.0 is in fact treated as a non-numeric value similar to a NaN. Therefore, copysign(+0.0,-1.0) returns +0.0, not the expected -0.0, on this implementation. Some places that mention (or might have) signed zero results and the sign might be different than you expect:The complex functions, in particular with branch cuts;
ceil() conj() copysign() fmod() modf() fprintf() (Footnote 233 is OK)fwprintf() (Footnote 273 is OK)nearbyint() nextafter() nexttoward() remainder() (Footnote 201 does not need to be changed)remquo() rint() round() signbit() strtod() (Footnote 249 is OK)trunc() wcstod() (Footnote 285 is OK)Underflow: In particular: ldexp(), scalbn(), scalbln().
Technical Corrigendum
Add a new paragraph to 5.2.4.2.2, after [#3]:
[#3a] An implementation may give zero and non-numeric values (such as infinities and NaNs) a sign or may leave them unsigned. Wherever such values are unsigned, any requirement in this International Standard to retrieve the sign shall produce an unspecified sign, and any requirement to set the sign shall be ignored.