process groupgroup A group is an ordered set of process identifiers (henceforth processes); processes are implementation-dependent objects. Each process in a group is associated with an integer rank. Ranks are contiguous and start from zero. rankprocess rank Groups are represented by opaque group objects, and hence cannot be directly transferred from one process to another.
There is a special pre-defined group: MPI_GROUP_EMPTY, which is MPI_GROUP_EMPTY a group with no members. The predefined constant MPI_GROUP_NULL is the value used for invalid group handles. MPI_GROUP_NULL MPI_GROUP_EMPTY, which is a valid handle to an empty group, MPI_GROUP_EMPTY should not be confused with MPI_GROUP_NULL, which is MPI_GROUP_NULL an invalid handle. The former may be used as an argument to group operations; the latter, which is returned when a group is freed, in not a valid argument.
Group operations are discussed in Section 
.