[Mpi-forum] why is MPI_Status not opaque?

[Jeff Hammond]

Yes, but every time the MPI Forum breaks backwards compatibility, an angel loses its wings, so we can’t fix design flaws like this.  Everyone must suffer forever.

More seriously, Lisandro pointer out that we’d need allocate and deallocate functions for status if they were handles to hidden state, so we can’t just fix this by making Status opaque.

I guess we have no choice but to standardize the MPI_Status object size and layout.

Jeff

> On 21Nov 2022, at 2:43 PM, Skjellum, Anthony <tony-skjellum at utc.edu> wrote:
> 
> It should be an opaque object :-) 
> 
> Anthony Skjellum, PhD
> 205-807-4968
> 
> 
> On Nov 21, 2022, at 7:29 AM, Jeff Hammond via mpi-forum <mpi-forum at lists.mpi-forum.org> wrote:
> 
> 
> I assume that MPI_Status is not opaque because somebody asserted that function call overhead was too much for some use cases.  Was there more to it than this?
> 
> Why does the standard say there is an opaque part for elements and cancelled, but not make those visible?  The lack of consistency here doesn't make a lot of sense to me.
> 
> MPI_Status not being opaque was a horrible mistake but I would like to be less mad about it by learning what possible reasons for it existed in 1995.
> 
> Thanks,
> 
> Jeff 
> 
> -- 
> Jeff Hammond
> jeff.science at gmail.com <mailto:jeff.science at gmail.com>
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