Intra-communicators bring together the concepts of group and context. To
support
implementation-specific optimizations, and application topologies
(defined in the next chapter, Chapter Process Topologies
), communicators may
also ``cache'' additional information (see Section Caching
). MPI
communication operations reference communicators to determine the scope and
the ``communication universe'' in which a point-to-point or collective
operation is to operate.
Each communicator contains a group of valid participants; this group always includes the local process. The source and destination of a message is identified by process rank within that group.
For collective communication, the intra-communicator specifies the set of processes that participate in the collective operation (and their order, when significant). Thus, the communicator restricts the ``spatial'' scope of communication, and provides machine-independent process addressing through ranks.
Intra-communicators are represented by opaque intra-communicator objects, and hence cannot be directly transferred from one process to another.