​From a user point of view, I do not like that behaviour. I agree that the MPI implementation is free to allocate as much space it is convenient, but the size it reports back after a query is a different story. Does the MPI standard requires that the result query of the query is the exact allocated size? If not, I see more convenient to return the requested-at-allocation- time size the the allocated one. What's the use case of returning the exact allocated size? After all, I guess users cannot rely on "extra space" being available at the end of the buffer.