# 18.2.234. MPI_Intercomm_create

MPI_Intercomm_create — Creates an intercommunicator from two intracommunicators.

## 18.2.234.1. SYNTAX

### 18.2.234.1.1. C Syntax

    #include <mpi.h>

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

### 18.2.234.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_INTERCOMM_CREATE(LOCAL_COMM, LOCAL_LEADER, PEER_COMM,
                 REMOTE_LEADER, TAG, NEWINTERCOMM, IERROR)
         INTEGER LOCAL_COMM, LOCAL_LEADER, PEER_COMM, REMOTE_LEADER
         INTEGER TAG, NEWINTERCOMM, IERROR

### 18.2.234.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Intercomm_create(local_comm, local_leader, peer_comm, remote_leader,
                 tag, newintercomm, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: local_comm, peer_comm
         INTEGER, INTENT(IN) :: local_leader, remote_leader, tag
         TYPE(MPI_Comm), INTENT(OUT) :: newintercomm
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.234.2. INPUT PARAMETERS

- `local_comm`: The communicator containing the process that initiates the inter-communication (handle).

- `local_leader`: Rank of local group leader in local_comm (integer).

- `peer_comm`: “Peer” communicator; significant only at the local_leader (handle).

- `remote_leader`: Rank of remote group leader in peer_comm; significant only at the local_leader (integer).

- `tag`: Message tag used to identify new intercommunicator (integer).

## 18.2.234.3. OUTPUT PARAMETERS

- `newintercomm`: Created intercommunicator (handle).

- `ierror`: Fortran only: Error status (integer).

## 18.2.234.4. DESCRIPTION

This call creates an intercommunicator. 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.

If multiple MPI_Intercomm_creates are being made, they should use different tags (more precisely, they should ensure that the local and remote leaders are using different tags for each MPI_intercomm_create).

## 18.2.234.5. NOTES

We recommend using a dedicated peer communicator, such as a duplicate of MPI_COMM_WORLD, to avoid trouble with peer communicators.

The MPI 1.1 Standard contains two mutually exclusive comments on the input intracommunicators. One says that their respective groups must be disjoint; the other that the leaders can be the same process. After some discussion by the MPI Forum, it has been decided that the groups must be disjoint. Note that the reason given for this in the standard is not the reason for this choice; rather, the other operations on intercommunicators (like MPI_Intercomm_merge ) do not make sense if the groups are not disjoint.

## 18.2.234.6. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Intercomm_merge

- MPI_Comm_free

- MPI_Comm_remote_group

- MPI_Comm_remote_size