## 5.2.3. Intra-Communicators

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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.

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