Is it allowed to communicate messages with (buffer=NULL,count=0,type=MPI_DATATYPE_NULL)? George thinks MPI_DATATYPE_NULL cannot be used even when count=0, which I think is ridiculous, particularly when one makes the analogy to buffer=NULL. See http://www.open-mpi.org/community/lists/users/2016/01/28255.php for the thread in question. There is example code in MPI that uses this behavior. George and I agree that example text is non-normative, but I cannot imagine how this example came to exist without the Forum believing it to be valid. Jeff -- Jeff Hammond [email protected] http://jeffhammond.github.io/