This is absolutely not true. Open MPI supports zero length collective operations (all of them actually), but if their arguments are correctly shaped.
What you're asking for is a free ticket to write MPI calls that do not follow the MPI requirements when a special value for count is given.
While zero-length arrays/sequence/containers do appears in real code, they are not equal to NULL. If they are NULL, that means they do not contain any useful data, and they don't need to be source or target of any kind of [collective or point-to-point] communications.
On Feb 11, 2010, at 11:53 , Lisandro Dalcin wrote:
> Well, I have to disagree... I understand you (as an MPI implementor)
> think that Reduce(count=0) could be meaningless and add complexity to
> the implementation of MPI_Reduce()... But Reduce(count=0) could save
> user code of special-casing the count==0 situation... after all,
> zero-length arrays/sequences/containers do appear in actual codes...