I have a different recollection. The link is broken, here is one that works for me https://lists.mpi-forum.org/pipermail/mpi-forum/2016-January/003152.html TL;DR from Bill Gropp: "The standard says that the null handles are invalid for input unless explicitly permitted. There’s no exemption for a datatype argument where the associated count is zero." IIRC there was no objection to updating the standard in order to allow this, but unless this has been completed, I am afraid MPI_DATATYPE_NULL is not a valid datatype here Gilles ----- Original Message ----- You are correct. The MPI_DATATYPE_NULL is an acceptable datatype for cases where the target processor sends or receives no data. The standard uses MPI_DATATYPE_NULL in 2 instances (in example 5.23) to highlight no data movement, but fails to make a clear reference to the fact that this is an acceptable use. George. On Thu, Dec 8, 2022 at 5:31 PM Jim Edwards via mpi-comments <mpi- [email protected]> wrote: 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 _______________________________________________ mpi-comments mailing list [email protected] https://lists.mpi-forum.org/mailman/listinfo/mpi-comments