## 5.6.2. Inter-communicator Operations

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This section introduces four blocking inter-communicator operations. MPI_INTERCOMM_CREATE is used to bindtwo intra-communicators into an inter-communicator; the function MPI_INTERCOMM_MERGE creates an intra-communicator by merging the local and remote groups of an inter-communicator. The functions
MPI_COMM_DUP and
MPI_COMM_FREE, introduced previously, duplicate and free an inter-communicator, respectively.

Overlap of local and remote groups that are bound into an inter-communicator is prohibited. If there is overlap, then the program is erroneous and is likely to deadlock. (If a process is multithreaded, and MPI calls block only a thread, rather than a process, then ``dual membership'' can be supported. It is then the user's responsibility to make sure that calls on behalf of the two ``roles'' of a process are executed by two independent threads.)

The function MPI_INTERCOMM_CREATE can be used to create an inter-communicator from two existing intra-communicators, in the following situation: At least one selected member from each group (the ``group leader'') has the ability to communicate with the selected member from the other group; that is, a ``peer'' communicator exists to which both leaders belong, and each leader knows the rank of the other leader in this peer communicator (the two leaders could be the same process). Furthermore, members of each group know the rank of their leader.

Construction of an inter-communicator from two intra-communicators requires separate collective operations in the local group and in the remote group, as well as a point-to-point communication between a process in the local group and a process in the remote group.

In standard MPI implementations (with static process allocation at initialization), the MPI_COMM_WORLD communicator (or preferably a dedicated duplicate thereof) can be this peer communicator. In dynamic MPI implementations, where, for example, a process may spawn new child processes during an MPI execution, the parent process may be the ``bridge'' between the old communication universe and the new communication world that includes the parent and its children.

The application topology functions described in chapter Process Topologies do not apply to inter-communicators. Users that require this capability should utilize MPI_INTERCOMM_MERGE to build an intra-communicator, then apply the graph or cartesian topology capabilities to that intra-communicator, creating an appropriate topology-oriented intra-communicator. Alternatively, it may be reasonable to devise one's own application topology mechanisms for this case, without loss of generality.

MPI_INTERCOMM_CREATE(local_comm, local_leader, peer_comm, remote_leader, tag,
newintercomm)
[ IN local_comm ] local intra-communicator (handle)
[ IN local_leader] rank of local group leader in local_comm (integer)
[ IN peer_comm] ``peer'' communicator; significant only at the local_leader (handle)
[ IN remote_leader] rank of remote group leader in peer_comm; significant only at the local_leader (integer)
[ IN tag] ``safe'' tag (integer)
[ OUT newintercomm ] new inter-communicator (handle)

`int MPI_Intercomm_create(MPI_Comm local_comm, int local_leader, MPI_Comm peer_comm, int remote_leader, int tag, MPI_Comm *newintercomm)
`

`MPI_INTERCOMM_CREATE(LOCAL_COMM, LOCAL_LEADER, PEER_COMM, REMOTE_LEADER, TAG, NEWINTERCOMM, IERROR)
INTEGER LOCAL_COMM, LOCAL_LEADER, PEER_COMM, REMOTE_LEADER, TAG, NEWINTERCOMM, IERROR
`

This call creates an inter-communicator. It is collective over the union of the local and remote groups. Processes should provide identical local_comm and local_leader arguments within each group. Wildcards are not permitted for remote_leader, local_leader, and tag.

This call uses point-to-point communication with communicator peer_comm, and with tag tag between the leaders. Thus, care must be taken that there be no pending communication on peer_comm that could interfere with this communication.

[] Advice to users.

We recommend using a dedicated peer communicator, such as a duplicate of MPI_COMM_WORLD, to avoid trouble with peer communicators. ( End of advice to users.)
MPI_INTERCOMM_MERGE(intercomm, high, newintracomm)
[ IN intercomm] Inter-Communicator (handle)
[ IN high] (logical)
[ OUT newintracomm] new intra-communicator (handle)

`int MPI_Intercomm_merge(MPI_Comm intercomm, int high, MPI_Comm *newintracomm)
`

`MPI_INTERCOMM_MERGE(INTERCOMM, HIGH, INTRACOMM, IERROR)
INTEGER INTERCOMM, INTRACOMM, IERROR
LOGICAL HIGH
`

This function creates an intra-communicator from the union of the two groups that are associated with intercomm. All processes should provide the same high value within each of the two groups. If processes in one group provided the value high = false and processes in the other group provided the value high = true then the union orders the ``low'' group before the ``high'' group. If all processes provided the same high argument then the order of the union is arbitrary. This call is blocking and collective within the union of the two groups.

[] Advice to implementors. The implementation of MPI_INTERCOMM_MERGE,
MPI_COMM_FREE and MPI_COMM_DUP are similar to the implementation of
MPI_INTERCOMM_CREATE, except that contexts private to the input inter-communicator are used for communication between group leaders rather than contexts inside a bridge communicator. ( End of advice to implementors.)

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