On May 28, 2009, at 2:03 PM, Aleksandar Donev wrote:

But the point here is to find something for MPI. In most MPI codes, I am sure people will not check the integer handles and in fact simply do illegal things like Jeff mentioned: pass the first element of an array and that way later refer to the whole array. Also, the C interface already uses void*, and trying to do "better" than that seems pointless. I though the decision was not to "modernize" the MPI interface but rather try to emulate the C interface as closely as possible.

I wish it were this clear. :-) 1. The (void*) extra_state/cookie/whatever_you_want_to_call it mechanism is very widely used in C in a large number of interfaces -- MPI was just re-using a very popular mechanism there (in C). 2. The *intent* for the Fortran callback is to provide whatever is natural for Fortran programmers to effect some kind of handle/token/ whatever_you_want_to_call_it functionality. It does not have to mirror the C interface; it should mirror the intent from the language neutral bindings. This is in the scope of MPI-3, so changing the prototype of both MPI_COMM_CREATE_KEYVAL and the associated callback routines is permissible. If a "more modern" prototype is the Right way to go (and is readily available in most/all existing Fortran compilers), then great. If something like C_PTR/C_LOC is a better compromise, then so be it. I guess this really comes down to the MPI community asking the Fortran community: what is the Right Thing to do here / what does the Fortran community want to effect this intent? Does that make sense? -- Jeff Squyres Cisco Systems