Issue 0210: fprintf %a and %A conversions recommended practice

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

Authors: WG 14, Fred Tydeman (US)
Date: 1999-10-20
Submitted against: C99
Status: Fixed
Fixed in: C99 TC1
Converted from: summary-c99.htm, dr_210.htm

Summary

What should fprintf do when it is printing a floating-point number using %a and %A conversions and it is exactly representable in the given precision? That is, what is the result of: fprintf("%a", 1.0);? The current wording appears to say that it be converted into either 0x1.0p0+DBL_EPSILON or 0x1.0p0-DBL_EPSILON/FLT_RADIX, instead of the correct value 0x1.0p0 (this appears to be an oversight that forgot about exactly representable values).

What should the strtod family of functions do when converting a hexadecimal form of input and the result is exactly representable? That is, what is the result of: strtod("0x1.0p0", (char **)NULL);? The current wording appears to say that it be converted into either 1.0+DBL_EPSILON or 1.0-DBL_EPSILON/FLT_RADIX, instead of the correct value 1.0 (this appears to be an oversight that forgot about exactly representable values).

Comment from WG14 on 2001-01-22:

Technical Corrigendum

7.19.6.1 The fprintf function:

Page 279, Paragraph 12 should be changed from:

If FLT_RADIX is not a power of 2, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

to:

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

7.20.1.3 The strtod ... functions:

Page 308, paragraph 8 should be changed from:

If the subject sequence has the hexadecimal form and FLT_RADIX is not a power of 2, the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error should have a correct sign for the current rounding direction.

to:

If the subject sequence has the hexadecimal form, FLT_RADIX is not a power of 2 and the result is not exactly representable, the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error should have a correct sign for the current rounding direction.

7.24.2.1 The fwprintf function:

Page 354, Paragraph 12 should be changed from:

If FLT_RADIX is not a power of 2, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

to:

For a and A conversions, if FLT_RADIX is not a power of 2 and the result is not exactly representable in the given precision, the result should be one of the two adjacent numbers in hexadecimal floating style with the given precision, with the extra stipulation that the error should have a correct sign for the current rounding direction.

7.24.4.1.1 The wcstod ... functions:

Page 372, paragraph 8 should be changed from:

If the subject sequence has the hexadecimal form and FLT_RADIX is not a power of 2, the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error should have a correct sign for the current rounding direction.

to:

If the subject sequence has the hexadecimal form, FLT_RADIX is not a power of 2 and the result is not exactly representable, the result should be one of the two numbers in the appropriate internal format that are adjacent to the hexadecimal floating source value, with the extra stipulation that the error should have a correct sign for the current rounding direction.