[MPIWG Fortran] MPI-3 ticket 349: Fortran question

[Bill Long]

On 1/2/14 10:36 AM, Dave Goodell (dgoodell) wrote:
> On Dec 24, 2013, at 11:36 AM, Jim Dinan <james.dinan at gmail.com> wrote:
>
>> Given that MPI_Aint will be at least the same size as ptrdiff_t, do we
>> actually have a problem with subtracting MPI_Aints to find pointer diffs?
>> It seems like this may actually be ok.
>
> There's still a problem.  I don't think that ptrdiff_t has anything to do with it since that normally only comes into play when you subtract two _pointers_, not two integers acting sort of like pointers.
>

>
> The overflow occurs because "(a - b)" is really "(-9223372036854775808 - 9223372036854775804)" in signed integer arithmetic.  Note that most implementations will give you the expected value if you don't set '-ftrapv', but that's *undefined behavior* according to the C standard and should not be relied upon (see http://www.airs.com/blog/archives/120).
>
> This is all just the C side of things.  I have no idea whether Fortran integer overflow is defined.
>

Fortran rule:

The execution of any numeric operation whose result is not defined by 
the arithmetic used by the processor is prohibited.

Cheers,
Bill


> -Dave
>
> _______________________________________________
> mpiwg-fortran mailing list
> mpiwg-fortran at lists.mpi-forum.org
> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-fortran
>

-- 
Bill Long                                           longb at cray.com
Fortran Technical Support    &                 voice: 651-605-9024
Bioinformatics Software Development            fax:   651-605-9142
Cray Inc./Cray Plaza, Suite 210/380 Jackson St./St. Paul, MN 55101