I have been drafting a design for Fortran coarrays based on MPI, and have hit a couple of rather critical missing primitives. One is that I need to do a blocking wait (of the form that will yield to other threads and processes, not a spin loop) that will terminate as soon as either a message comes in (i.e. like MPI_Probe) or one of the outstanding non-blocking messages completes (i.e. like MPI_Waitall). Another is the ability to interrupt one of those primitives (or, I suppose, MPI_Probe or one of the MPI_Waits) from another thread in the same process in the case that MPI_THREAD_MULTIPLE is NOT set. No message needed - just a way of breaking out. The first could easily be added by a pseudo-request that was satisfied by the existence of an incoming message - obviously, the status would be full of the usual 'unset' values - or by adding a separate function. I doubt that it would be any trickier to implement than MPI_Waitany. The second is easy to specify. It would need a three-state return value: success, not in a wait, or failure. Allowing the last would make it reliably implementable, though not ideal for my purposes :-( I can see how to implement it in most cases, so that opt-out should rarely be used. But there are circumstances when it would be needed. Any comments? Including that I have been an idiot and missed something obvious :-) Regards, Nick Maclaren.