Communicators (see
below) provide a ``caching'' mechanism that allows one to
associate new attributes with communicators, on a par with MPI built-in
features. This can be used by advanced users to adorn communicators further,
and by MPI to implement some communicator functions. For example, the
virtual-topology functions described in
Chapter 6 are likely to be supported this way.
MPI-Standard for MARMOT