(j3.2006) MPI usage problems in Fortran

[Aleksandar Donev]

Craig,

You are mixing two things up: Either your buffer is an regular (typed) Fortran 
array and has the ASYNC attribute, *or* it is a "TYPE(C_PTR), VALUE". Don't 
mix the two as it makes no sense. The pointer itself is neither volatile nor 
asynchronous, it is the target of the pointer that is.

> 1. VALUE and ASYNCHRONOUS lead to a compile error (is this correct?)
Yes it is correct, but don't ask me why.

> 2. buf doesn't have the TARGET attribute
Definitely a bad idea---you are taking and retaining a pointer to it. It must 
have TARGET.

> 3. How long is buf tainted as Bill suggests?
Ask Bill :-) The standard says nothing that I can understand...

Here is what it should look like:

--------------
interface
? ? ?function MPI_Isend(buf, count, datatype, &
? ? ? ? ? ? ? ? ? ? ? ? dest, tag, comm, request) 
       BIND(C, ?name="MPI_Isend") result(err)
? ? ? ?use, intrinsic :: ISO_C_BINDING
? ? ? ?import :: MPI_HANDLE
? ? ? ?implicit none
? ? ? ?type(C_PTR), value :: buf
       ...
? ? ?end function MPI_Isend
end interface

use, intrinsic :: ISO_C_BINDING
use MPI3
type(MPI_HANDLE) :: request
integer, volatile, target :: buf(10)
integer(C_INT) :: err

err = MPI_Isend(C_LOC(buf), 10, MPI_INTEGER, 1, 1, MPI_COMM_WORLD, ?
request)
----------------

Note that buf is volatile and target. Maybe asynchronous and target would also 
work, but I think the standard reserves asynchronous for internal Fortran I/O 
use and not for things outside of the standard. I don't think it will make 
any difference---don't compilers treat the two identically?

This will kill optimization of anything done with buf in any program unit 
where it has both the volatile and the target attribute.

Bill and the misguided Note in the standard suggest that maybe this was some 
intended use of asynchronous:

--------------
interface
     function MPI_Isend(buf, count, datatype, &
                        dest, tag, comm, request) 
       BIND(C,  name="MPI_Isend") result(err)
       use, intrinsic :: ISO_C_BINDING
       import :: MPI_HANDLE
       implicit none
       real(c_double), dimension(*), asynchronous :: buf ! An array
       ...
     end function MPI_Isend
end interface

use, intrinsic :: ISO_C_BINDING
use MPI3
type(MPI_HANDLE) :: request
integer, target :: buf(10) ! Should/must this have ASYNC attribute???
integer(C_INT) :: err

err = MPI_Isend(buf, ...,  request)
----------------

but again I don't think the standard holds water in making this actually make 
sense, yet alone work.

Best,
Aleks