Isn't there likely to be a "hole" between c and d? On Mar 19, 2008, at 12:22 PM, Craig Rasmussen wrote:

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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-- Jeff Squyres Cisco Systems