I'm not sure your example is relevant for a Fortran API though. In
Fortran we like to be safer than using NULL as a valid reference
point :-).
Seriously though, we have been discussing using the Fortran intrinsic
C_LOC to obtain the address of a Fortran object to pass to C. In the
description of C_LOC is requires that the variable "shall either be a
contiguous variable with interoperable type and type parameters, or
be a scalar, ...".
I believe your example:
struct foo {
char c;
double d;
}
then passing the address of foo_instance[0].c should work fine in
Fortran as well under the following restraints:
1. foo_instance in a contiguous array
2. foo in of interoperable type
Cheers,
Craig
On Mar 18, 2008, at 7:16 PM, Jeff Squyres wrote:
> On Mar 18, 2008, at 11:02 AM, Craig Rasmussen wrote:
>
>>> No.
>>
>> Yes! (sorry, couldn't resist Jeff :-)
>>
>> What you say below is as I expected. If you have an array of foo,
>> then foo must be contiguous in order for you to be able to stride
>> though it to get at each (noncontiguous) char c. I don't think we
>> disagree here. In Fortran, you can take a slice of an array so that
>> the sliced elements aren't contiguous anymore. The compiler handles
>> this by modifying the array meta-data (stride information), or if
>> necessary when going to C, make a copy. So in Fortran we have to be
>> a little more careful what we mean by contiguous (and I'm sure I
>> don't fully understand it yet).
>
>
> There is absolutely no requirement for contiguousness of user buffers
> in the C bindings for MPI.
>
> Indeed, you can construct an MPI datatype (map) based off an arbitrary
> reference point in memory (e.g., NULL) and use relative memory offsets
> to describe the layout of the data that you want to access. Hence,
> the pointer that you pass to MPI_SEND may actually have a memory map
> that spans all over actual memory.
>
> --
> Jeff Squyres
> Cisco Systems
>
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