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

Bill Long longb at cray.com
Thu Jan 2 10:46:39 CST 2014

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.


> -Dave
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Bill Long                                           longb at cray.com
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