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.TH "MPI_INTERCOMM_CREATE" "3" "May 30, 2025" "" "Open MPI"
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\fI\%MPI_Intercomm_create\fP — Creates an intercommunicator from two
intracommunicators.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#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)
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq
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
.ft P
.fi
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.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
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.nf
.ft C
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
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBlocal_comm\fP: The communicator containing the process that initiates the inter\-communication (handle).
.IP \(bu 2
\fBlocal_leader\fP: Rank of local group leader in local_comm (integer).
.IP \(bu 2
\fBpeer_comm\fP: “Peer” communicator; significant only at the local_leader (handle).
.IP \(bu 2
\fBremote_leader\fP: Rank of remote group leader in peer_comm; significant only at the local_leader (integer).
.IP \(bu 2
\fBtag\fP: Message tag used to identify new intercommunicator (integer).
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBnewintercomm\fP: Created intercommunicator (handle).
.IP \(bu 2
\fBierror\fP: Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
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.
.sp
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.
.sp
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).
.SH NOTES
.sp
We recommend using a dedicated peer communicator, such as a duplicate of
MPI_COMM_WORLD, to avoid trouble with peer communicators.
.sp
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 \fBreason\fP given for this in the standard is
\fBnot\fP the reason for this choice; rather, the \fBother\fP operations on
intercommunicators (like \fI\%MPI_Intercomm_merge\fP ) do not make sense if
the groups are not disjoint.
.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
.INDENT 0.0
.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
.UNINDENT
.sp
MPI applications can also implement their own error handlers by calling:
.INDENT 0.0
.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
.UNINDENT
.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.sp
\fBSEE ALSO:\fP
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.INDENT 3.5
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.IP \(bu 2
\fI\%MPI_Intercomm_merge\fP
.IP \(bu 2
\fI\%MPI_Comm_free\fP
.IP \(bu 2
\fI\%MPI_Comm_remote_group\fP
.IP \(bu 2
\fI\%MPI_Comm_remote_size\fP
.UNINDENT
.UNINDENT
.UNINDENT
.SH COPYRIGHT
2003-2025, The Open MPI Community
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