Document: WG14 N1377

xxx_DECIMAL_DIG macros for <float.h>


Submitter: Fred Tydeman (USA)
Submission Date: 2009-03-31
Previous version of paper: N1303, N1317, N1352
Related WG14 documents: N1151, N1171
Subject: xxx_DECIMAL_DIG macros for <float.h>

Existing practice: C99 has DECIMAL_DIG with the similar meaning as LDBL_DECIMAL_DIG.

This is a rewrite based upon feedback from the committee.

Changes to C1x

Add new bullets to 5.2.4.2.2 Characteristics of floating types <float.h>

[bullet near DECIMAL_DIG] number of decimal digits, n, such that any floating-point number with p radix b digits can be rounded to a floating-point number with n decimal digits and back again without change to the value, 

 p log10 b             if b is power of 10
 ceil(1 + p log10 b)   otherwise

[Note to editor: WG14 C1x WP for DECIMAL_DIG has correct symbols/fonts for the above two math expressions.] 

FLT_DECIMAL_DIG    6
DBL_DECIMAL_DIG   10
LDBL_DECIMAL_DIG  10

[paragraph 14, example 1] Add 

FLT_DECIMAL_DIG   9
after FLT_RADIX

[paragraph 15, example 2] Add 

FLT_DECIMAL_DIG    9
DBL_DECIMAL_DIG   17
before DECIMAL_DIG

Words for Rationale:

[add to 5.2.4.2.2 section] For applications that need to print non-decimal FP values as decimal values, and read them later, can use xxx_DECIMAL_DIG macros as the precision to make sure that the original value is kept.