---
name: "MPI_Intercomm_create.openmpi"
section: "3"
platform: "linux"
package: "openmpi-doc"
version: "5.0.7-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Intercomm_create.openmpi"
---

# MPI\_Intercomm\_create.openmpi(3)

*MPI\_Intercomm\_create* — Creates an intercommunicator from two intracommunicators.

## SYNTAX

### 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)
```

### 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
```

### 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
```

## 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).

## OUTPUT PARAMETERS

- •

  **newintercomm**: Created intercommunicator (handle).

- •

  **ierror**: Fortran only: Error status (integer).

## 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).

## 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.

## 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*

## COPYRIGHT

2003-2025, The Open MPI Community