[Mpi-forum] Fortran change for discussion

N.M. Maclaren nmm1 at cam.ac.uk
Tue Sep 11 07:30:29 CDT 2012

On Sep 11 2012, Jeremiah Willcock wrote:
>On Tue, 11 Sep 2012, Jeff Squyres (jsquyres) wrote:
>> FWIW, we chose the copying language not because of the c language, but 
>> rather because we have to explain that the MPI data type is applied, and 
>> that seemed far easier to explain against a (virtual) contiguous buffer.
>Maybe the issue is that I'm not a Fortran programmer, but the copying 
>language has always been confusing to me (its effect on what user-defined 
>datatypes need to look like is another thing I asked about before).  Am I 
>to understand the interface as something like this?
>    For purposes of interpreting derived datatypes and other information
>    in communication operations, array slices are treated as if they are
>    stored contiguously in memory.  I.e., it is "as if" some kind of
>    magic memory remapping occurs by the MPI implementation that maps the
>    user's non-contiguous array into a contiguous one without copying the
>    elements (in either direction).

No, that's not the reason, because your wording is considerably closer
to the Fortran model than the current wording.  What you have said is
a fairly precise paraphrase of what sequence association is.

Realistically, I agree with your concerns - this is confusing.  Your
addendum would help, as would adding the phrase "at an unspecified time
between the initial call and the wait".  Personally, I would do both.

Nick Maclaren.

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