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From: Robert Ramey (ramey_at_[hidden])
Date: 2005-11-23 00:00:13

Ian McCulloch wrote:

> You need to construct an MPI 'datatype', which is conceptually a
> record of how the type is laid out as a sequence of (offset,
> nested_datatype) pairs.


> The 'usual' way (if there is such a thing; datatypes seem to be not
> used
> much in MPI) of constructing datatypes is using offsetof(), or just
> knowing what the layout is for compiler X, and constructing the
> datatype by hand.

> I don't know what mechanism Dave is thinking of to construct the
> datatype,
> it sounds unlikely that it could be done via a usual serialization
> function (but maybe if you could do member pointer arithmetic to
> replace
> offsetof() ?). I don't understand the restriction to PODs with no
> pointers though, as pointers are no problem - at least in principle -
> you just recursively follow the pointer when constructing the
> typemap. There is a difference though, as a datatype for an object
> without nested pointers has fixed offsets, whereas for a type
> containing pointers a new datatype needs
> to be constructed for each object that is serialized.

Sounds like a lot of work. But if one want's to use all this stuff - then
what's the point of using the serialization library at all? I mean
I can see using one or the other - but what is the point of both?

Robert Ramey

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