[Bug math/29556] New: ecvt(999999999999.9, 3, ...) is broken

["gabravier at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29556

            Bug ID: 29556
           Summary: ecvt(999999999999.9, 3, ...) is broken
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: gabravier at gmail dot com
  Target Milestone: ---

int main()
{
    int decpt, sign;
    char *result = ecvt(999999999999.9, 3, &decpt, &sign);

    printf("ecvt(999999999999.9, 3, &[%d], &[%d]) = '%s'\n", decpt, sign,
result);
}

I believe the following program should output either:

ecvt(999999999999.9, 3, &[13], &[0]) = '100'

or:

ecvt(999999999999.9, 3, &[12], &[0]) = '999'

glibc instead outputs this:

ecvt(999999999999.9, 3, &[13], &[0]) = '1000'

which seems like it can't possibly be correct considering the call explicitly
asked for 3 characters to be printed.

PS: This seems to be caused by different in rounding between what `ecvt`
determines and what `fcvt` determines in the inner call to it by `ecvt`

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