# MPI_Intercomm_create

## NAME

MPI_Intercomm_create − Creates an intercommuncator 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

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

## C++ Syntax

#include <mpi.h>
Intercomm Intracomm::Create_intercomm(int local_leader, const

| Comm& peer_comm, int remote_leader, int tag) const

## 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 repective 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 value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI:Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

## SEE ALSO

MPI_Intercomm_merge
MPI_Comm_free
MPI_Comm_remote_group
MPI_Comm_remote_size

MPI API Index