libc6 printf "alternate format" %#.g prints wrong number of digits

[Wirawan Purwanto]

Binary package hint: libc6

I used Ubuntu 7.04 on x86 system (Pentium M), and updated the libc package to version 2.6: (2.6-3ubuntu2). The error described below also appliled to glibc 2.5, IIRC.

I found that the following C program won't print the correct number of digits:

#include <stdio.h>
int main(int argc, char *argv[], char *env[])
{
    printf("%#.4g\n", 912.98);
    return 0;
}

The output should be "913.0", but my Ubuntu box shows "913." (no trailing zero). The alternate format, according to the C standard, should print out the trailing zeros after decimal point.

This error is shameful and severe! I tried on different systems (Fedora 5, Fedora 6, SunOS, CentOS), none of them show this sickness.

Wirawan

[Wirawan Purwanto (wirawan0)]

This, as it turns out, did not happen if we use libc.a in libc6-dev_2.5-0ubuntu14_i386.deb . So what happens? Perhaps the patch was missing or wrong in the newer libc?

Wirawan

[Wirawan Purwanto (wirawan0)]

Please use the following testcase:

#include <stdio.h>
int main(int argc, char *argv[], char *env[])
{
    printf("%#.4g\n", 912.98);
    return 0;
}

This code should print "913.0", but instead it only prints "913." . It lacks the
trailing zeros, which should be there according to the standard C specs.

I believe that this bug is reproducible on the stock version of glibc 2.6 . I
have established the bug by building the 2.6 library on my local machine, then
linking the program above against the newly built libc.a .

[Wirawan Purwanto (wirawan0)]

I submitted the corresponding bug report in sources.redhat.com, as bug #4858 . I found out that even the stock glibc 2.6 has it.

Wirawan

[Jakub Jelinek (jakub-redhat)]

This is a regression caused by the BZ#4070 fix, particularly the
+ else if (__builtin_expect (spec == 'g' && type == 'f' && info->alt,
+ 0))
+ /* This is a special case: the rounded number is 1.0,
+ the format is 'g' or 'G', and the alternative format
+ is selected. This means the result must be "1.". */
+ --added_zeros;
hunk. It is IMHO only valid when the the integral part is all nines (or empty),
i.e. when we'll need more int digits, but we don't know this at this point.
For the "%#.0g", 0.956 case which got fixes that is true and similarly for
"%#.4g", 999.98, but not for this "%#.4g", 912.98.

[Drepper-fsp (drepper-fsp)]

Fixed in cvs.

[Matthias Klose (doko)]

glibc (2.6-5ubuntu1) gutsy; urgency=low

  * Merge with Debian.
    - cvs-printf_fp.c.diff: new patch to print the wrong number of
       digits with %#g. LP: #128355.

 -- Matthias Klose <email address hidden> Sun, 29 Jul 2007 20:39:57 +0200

[Drepper-fsp (drepper-fsp)]

*** Bug 4869 has been marked as a duplicate of this bug. ***

[Cvs-commit (cvs-commit)]

Subject: Bug 4858

CVSROOT: /cvs/glibc
Module name: libc
Branch: glibc-2_6-branch
Changes by: <email address hidden> 2007-07-31 12:42:39

Modified files:
 . : ChangeLog
 stdio-common : printf_fp.c tfformat.c

Log message:
 2007-07-31 Jakub Jelinek <email address hidden>

 * stdio-common/tfformat.c (sprint_doubles): Add 12 new tests.

 2007-07-28 Ulrich Drepper <email address hidden>

 [BZ #4858]
 * stdio-common/printf_fp.c (___printf_fp): Fix special case of
 #.0g and value rounded to 1.0.
 * stdio-common/tfformat.c (sprint_doubles): Add two new tests.

Patches:
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/ChangeLog.diff?cvsroot=glibc&only_with_tag=glibc-2_6-branch&r1=1.10640.2.1&r2=1.10640.2.2
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/stdio-common/printf_fp.c.diff?cvsroot=glibc&only_with_tag=glibc-2_6-branch&r1=1.63&r2=1.63.2.1
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/stdio-common/tfformat.c.diff?cvsroot=glibc&only_with_tag=glibc-2_6-branch&r1=1.10&r2=1.10.2.1

[Cvs-commit (cvs-commit)]

Subject: Bug 4858

CVSROOT: /cvs/glibc
Module name: libc
Branch: glibc-2_5-branch
Changes by: <email address hidden> 2007-07-31 12:44:28

Modified files:
 . : ChangeLog
 stdio-common : printf_fp.c tfformat.c

Log message:
 2007-07-31 Jakub Jelinek <email address hidden>

 * stdio-common/tfformat.c (sprint_doubles): Add 12 new tests.

 2007-07-28 Ulrich Drepper <email address hidden>

 [BZ #4858]
 * stdio-common/printf_fp.c (___printf_fp): Fix special case of
 #.0g and value rounded to 1.0.
 * stdio-common/tfformat.c (sprint_doubles): Add two new tests.

Patches:
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/ChangeLog.diff?cvsroot=glibc&only_with_tag=glibc-2_5-branch&r1=1.10362.2.96&r2=1.10362.2.97
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/stdio-common/printf_fp.c.diff?cvsroot=glibc&only_with_tag=glibc-2_5-branch&r1=1.58.2.3&r2=1.58.2.4
http://sourceware.org/cgi-bin/cvsweb.cgi/libc/stdio-common/tfformat.c.diff?cvsroot=glibc&only_with_tag=glibc-2_5-branch&r1=1.7.8.2&r2=1.7.8.3