Context Identifiers

Context IDs are of type IMPI_Uint8 and are collectively unique. Collectively unique means the context ID for a communicator is the same for each process in the communicator and no other communicator of which the process is a member has the same context ID.

Advice to implementors.Mandating a collectively unique context ID may be a burden on some implementations that use memory addresses to segregate message contexts. Such implementations may choose to let the agent handle the mapping between context IDs and memory addresses and not impact the performance of the intra-implementation communication protocols.(End of advice to implementors.)

The context identifier of MPI_COMM_WORLD is negotiated at startup time. Please refer to the

No separate context IDs are reserved for IMPI collective communication. Negative tags are used internally to distinguish between the user's point-to-point messages and the messages generated by the collective routines.

Advice to implementors.Error checking for negative tags in MPI point-to-point calls made in the implementation of interoperable collectives must be disabled. The collective algorithms below assume this.(End of advice to implementors.)