Null processes

In many instances, it is convenient to specify a ``dummy'' source or destination for communication. This simplifies the code that is needed for dealing with boundaries, for example, in the case of a non-circular shift done with calls to send-receive.

The special value MPI_PROC_NULL can be used instead of a rank wherever a source or a destination argument is required in a call. A communication with process MPI_PROC_NULL has no effect. A send to MPI_PROC_NULL succeeds and returns as soon as possible. A receive from MPI_PROC_NULL succeeds and returns as soon as possible with no modifications to the receive buffer. When a receive with source = MPI_PROC_NULL is executed then the status object returns source = MPI_PROC_NULL, tag = MPI_ANY_TAG and count = 0.