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

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

[gabravier at gmail dot com]

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

[gabravier at gmail dot com]

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

Gabriel Ravier <gabravier at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|unspecified                 |2.36
           Keywords|                            |std-posix

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[Bug math/29556] ecvt rounding in edge-cases is broken

[gabravier at gmail dot com]

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

Gabriel Ravier <gabravier at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|ecvt(999999999999.9, 3,     |ecvt rounding in edge-cases
                   |...) is broken              |is broken

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[Bug math/29556] ecvt rounding in edge-cases is broken

[gabravier at gmail dot com]

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

--- Comment #1 from Gabriel Ravier <gabravier at gmail dot com> ---
PS: I've tested this on:
- the original BSD implementation of ecvt
- Cosmopolitan's ecvt 
- musl's ecvt (which happens to have the same issue as glibc but in fcvt
instead :p)
- OpenBSD's ecvt
- Windows's ecvt
- ReactOS's ecvt
- UNIX v10's ecvt
- Illumos's ecvt
- ELKS's ecvt

and they all returned either 100 or 999

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[Bug math/29556] ecvt rounding in edge-cases is broken

[schwab@linux-m68k.org]

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

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
fcvt (9.999999999999, 2,,) has the same problem, that is what ecvt eventually
calls.

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[Bug math/29556] ecvt rounding in edge-cases is broken

[gabravier at gmail dot com]

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

--- Comment #3 from Gabriel Ravier <gabravier at gmail dot com> ---
Hmmmm, I'm not sure fcvt's behavior is by itself a bug, as POSIX leaves the
rounding behavior of `fcvt` implementation-defined.

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