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When a float or double is streamed to output in fixed-point format, rounding up occurs well before the precision specified by the stream. 5.55555555e-17, when streamed to a stream with the scientific flag set and a precision of 25 will print 5.55555555[#'s-up-to-20]e-17. When streamed with the fixed flag set, it will print out 0.0000000000000000560000000. If the number is especially small (e-20+), it will print out as all zeroes. I looked at the code in 3.2.3 and 3.4 versions of g++, and saw that it was because the max_digits is being set from the numeric_limits<double>::digits10 field. The precision is then set to be less than or equal to the max_digits. So on my platform, where digits10 is 16 (+2), the precision will get set to 18, max. That works fine for scientific values, but I would argue that if I ask a stream for 25 places of precision, and put a float/double into it that should be represented by 0.0000000000000000555555555, that I would see that value, not a rounded version. My understanding is that the numeric_limits<double>::digits10 value for a float/double amounts to the significant digits if it were in scientific notation. If that is the case, then the max_digits for fixed number would be digits10 + the number of preceding zeroes, which is probably max_exponent10. This was seen on Solaris 8 and Irix 6.5.22.
Confirmed. Actually, basing on the standard (22.2.2.2.2, p8-11-13), I think that simply the precision passed down to the printing code must not be clipped at all.
Subject: Bug 14220 CVSROOT: /cvs/gcc Module name: gcc Branch: hammer-3_3-branch Changes by: aj@gcc.gnu.org 2004-02-22 15:00:40 Modified files: libstdc++-v3/include/bits: locale_facets.tcc libstdc++-v3 : ChangeLog.hammer Log message: 2004-02-22 Paolo Carlini <pcarlini@suse.de> PR libstdc++/14220 * include/bits/locale_facets.tcc (num_put<>::_M_convert_float): Don't clip the precision passed down to __convert_from_v: 22.2.2.2.2 nowhere says so. 2004-02-22 Paolo Carlini <pcarlini@suse.de> * include/bits/locale_facets.tcc (_M_extract_float, _M_extract_int): Remove a wrong comment and simplify the corresponding code using char_traits::find(). Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/bits/locale_facets.tcc.diff?cvsroot=gcc&only_with_tag=hammer-3_3-branch&r1=1.82.2.11&r2=1.82.2.12 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.hammer.diff?cvsroot=gcc&only_with_tag=hammer-3_3-branch&r1=1.1.2.3&r2=1.1.2.4
Subject: Bug 14220 CVSROOT: /cvs/gcc Module name: gcc Branch: hammer-3_3-branch Changes by: aj@gcc.gnu.org 2004-02-22 15:33:00 Modified files: libstdc++-v3/include/bits: locale_facets.tcc libstdc++-v3 : ChangeLog.hammer Log message: 2004-02-22 Paolo Carlini <pcarlini@suse.de> PR libstdc++/14220 * include/bits/locale_facets.tcc (num_put<>::_M_convert_float): Fix previous commit: in case of non-fixed outputs __max_digits is still needed in order to upper bound the size of the buffer. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/bits/locale_facets.tcc.diff?cvsroot=gcc&only_with_tag=hammer-3_3-branch&r1=1.82.2.12&r2=1.82.2.13 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.hammer.diff?cvsroot=gcc&only_with_tag=hammer-3_3-branch&r1=1.1.2.4&r2=1.1.2.5
Subject: Bug 14220 CVSROOT: /cvs/gcc Module name: gcc Branch: gcc-3_3-branch Changes by: paolo@gcc.gnu.org 2004-02-22 17:57:21 Modified files: libstdc++-v3 : ChangeLog libstdc++-v3/include/bits: locale_facets.tcc Log message: 2004-02-22 Paolo Carlini <pcarlini@suse.de> PR libstdc++/14220 * include/bits/locale_facets.tcc (num_put<>::_M_convert_float): Don't clip the precision passed down to __convert_from_v: 22.2.2.2.2 nowhere says so. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_3-branch&r1=1.1464.2.181&r2=1.1464.2.182 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/bits/locale_facets.tcc.diff?cvsroot=gcc&only_with_tag=gcc-3_3-branch&r1=1.82.4.18&r2=1.82.4.19
*** Bug 14969 has been marked as a duplicate of this bug. ***
Subject: Bug 14220 CVSROOT: /cvs/gcc Module name: gcc Branch: gcc-3_4-branch Changes by: paolo@gcc.gnu.org 2004-04-30 17:46:52 Modified files: libstdc++-v3 : ChangeLog libstdc++-v3/include/bits: locale_facets.tcc Added files: libstdc++-v3/testsuite/22_locale/num_put/put/char: 14220.cc libstdc++-v3/testsuite/22_locale/num_put/put/wchar_t: 14220.cc Log message: 2004-04-30 Paolo Carlini <pcarlini@suse.de> PR libstdc++/14220 * include/bits/locale_facets.tcc (num_put<>::_M_insert_float): Don't clip the precision passed down to __convert_from_v: 22.2.2.2.2 nowhere says so. * testsuite/22_locale/num_put/put/char/14220.cc: New. * testsuite/22_locale/num_put/put/wchar_t/14220.c: Likewise. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.2224.2.93&r2=1.2224.2.94 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/bits/locale_facets.tcc.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.166.2.10&r2=1.166.2.11 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/testsuite/22_locale/num_put/put/char/14220.cc.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=NONE&r2=1.1.2.1 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/testsuite/22_locale/num_put/put/wchar_t/14220.cc.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=NONE&r2=1.1.2.1
Marked as 3.5 only, so retargeting at 3.5.0.
Actually, this is already fixed for 3.4.1. I want to put the fix on hold for 3.5 only because with Benjamin we have a plan to branch a separate 7.0 branch where a pending invasive patch will interfere with this fix...
Really retargeting at 3.5.0 this time.
Subject: Bug 14220 CVSROOT: /cvs/gcc Module name: gcc Changes by: paolo@gcc.gnu.org 2004-07-29 22:11:05 Modified files: libstdc++-v3 : ChangeLog libstdc++-v3/include/bits: locale_facets.tcc Added files: libstdc++-v3/testsuite/22_locale/num_put/put/char: 14220.cc libstdc++-v3/testsuite/22_locale/num_put/put/wchar_t: 14220.cc Log message: 2004-07-29 Paolo Carlini <pcarlini@suse.de> PR libstdc++/14220 * include/bits/locale_facets.tcc (num_put<>::_M_insert_float): Don't clip the precision passed down to __convert_from_v: 22.2.2.2.2 nowhere says so. * testsuite/22_locale/num_put/put/char/14220.cc: New. * testsuite/22_locale/num_put/put/wchar_t/14220.c: Likewise. Patches: http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/ChangeLog.diff?cvsroot=gcc&r1=1.2592&r2=1.2593 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/include/bits/locale_facets.tcc.diff?cvsroot=gcc&r1=1.196&r2=1.197 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/testsuite/22_locale/num_put/put/char/14220.cc.diff?cvsroot=gcc&r1=1.1&r2=1.2 http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/libstdc++-v3/testsuite/22_locale/num_put/put/wchar_t/14220.cc.diff?cvsroot=gcc&r1=1.1&r2=1.2
Fixed everywhere.