[MPI3 Fortran] Fortran extra_state argument to MPIattributefunctions

[Aleksandar Donev]

Bill Long wrote:
> Besides the more formal methods mentioned by Aleks below, there is a 
> common programming style approach to solve this problem: put the 
> variable extra in a module.
This will work too, although it is not quite as guaranteed to work. This 
is because when you first pass extra as an extra_state, you need to 
somehow retain its address so you can refer to it later. Using LOC is 
not conforming, and C_LOC requires TARGET. Using plain old argument 
passing requires *matching* TARGETs on both the dummy and the actual to 
be guaranteed to avoid copy in/out and thus work (this goes for FFTW as 
well, though most codes, including mine, often cheat). Of course, if you 
don't try to pass the address to extra but some "token" (Nick's word) 
that means "look at extra", then the callback is free to modify the 
variable extra and a compiler has to assume the worst.
Best,
Aleks