Issue 0CFP.15: P3: Characteristic macros for non-arithmetic formats

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

Authors: WG14, Jim Thomas
Date: 2017-10-25
Reference document: N2171
Submitted against: Floating-point TS 18661 (C11 version, 2014-2016)
Status: Fixed
Fixed in: C23
Converted from: n2397.htm

Summary

This DR addresses an issue related to DR 501 and IEC 60559 non-arithmetic interchange formats.

TS 18661-3 attempted to change the definition of DECIMAL_DIG so that it would apply to all supported IEC 60559 formats. DR 501 shows that this change is problematic. After several unsuccessful attempts at a suitable TC, WG 14 and the CFP group have agreed to obsolesce DECIMAL_DIG in favor of the type_DECIMAL_DIG macros.

The type_DECIMAL_DIG macros are helpful in using conversions between binary floating-point formats and decimal character sequences. TS 18661-3 specifies such conversions for non-arithmetic binary interchange formats, but neglects to include the macros for such formats.

Likewise, the type_DIG macros too are helpful in using conversions between binary floating-point formats and decimal character sequences, though TS 18661-3 does not specify them for non-arithmetic formats.

The suggested TC below extends the set of FLTN_DECIMAL_DIG and FLTN_DIG macros to include ones for IEC 60559 non-arithmetic binary interchange formats.

Suggested Technical Corrigendum

In TS 18661-3 5.3, in the text for 5.2.4.2.2#6c, after the list for supported types _FloatN, insert:

for IEC 60559 non-arithmetic binary interchange formats of width N:

FLTN_DECIMAL_DIG     FLTN_DIG

In TS 18661-3 clause 7, in the text for 5.2.4.2.2a#1, change:

The prefix FLTN_ indicates a binary interchange floating type of width N.

to:

The prefix FLTN_ indicates a binary interchange floating type or a non-arithmetic binary interchange format of width N.

Change the last sentence of  5.2.4.2.2a#1 from:

Conversely, for each such type that the implementation does not provide, <float.h> shall not define the associated macros in the following lists.

to:

Conversely, for each such type that the implementation does not provide, <float.h> shall not define the associated macros in the following list, except, the implementation shall define the macros FLTN_DECIMAL_DIG and FLTN_DIG if it supports IEC 60559 non-arithmetic binary interchange formats of width N by providing the encoding-to-encoding conversion functions in <math.h> and the string-to-encoding and string-from-encoding functions in <stdlib.h>.

Comment from WG14 on 2018-10-18:

Oct 2017 meeting

Committee Discussion

The committee agreed with this issue and its proposed clarification.

Proposed Technical Clarification

In TS 18661-3 5.3, in the text for 5.2.4.2.2#6c, after the list for supported types _FloatN, insert:

for IEC 60559 non-arithmetic binary interchange formats of width N:

FLTN_DECIMAL_DIG     FLTN_DIG

In TS 18661-3 clause 7, in the text for 5.2.4.2.2a#1, change:

The prefix FLTN_ indicates a binary interchange floating type of width N.

to:

The prefix FLTN_ indicates a binary interchange floating type or a non-arithmetic binary interchange format of width N.

Change the last sentence of  5.2.4.2.2a#1 from:

Conversely, for each such type that the implementation does not provide, <float.h> shall not define the associated macros in the following lists.

to:

Conversely, for each such type that the implementation does not provide, <float.h> shall not define the associated macros in the following list, except, the implementation shall define the macros FLTN_DECIMAL_DIG and FLTN_DIG if it supports IEC 60559 non-arithmetic binary interchange formats of width N by providing the encoding-to-encoding conversion functions in <math.h> and the string-to-encoding and string-from-encoding functions in <stdlib.h>.