There seems to be some ambiguity in the standard with respect to usage of the MPI_DATATYPE_NULL variable. It would seem to me that if an MPI routine such as MPI_Send or MPI_ALLTOALLW has a data count argument of 0 then MPI_DATATYPE_NULL should be an acceptable value for the datatype. Here is a discussion from several years ago with openmpi developers. It references a discussion in the standards forum but I can no longer find it. http://lists.mpi-forum.org/mpi-forum/2016/01/3152.php Would it be possibly to clarify this case in the next iteration of the standard? -- Jim Edwards CESM Software Engineer National Center for Atmospheric Research Boulder, CO