MPI_INTERCOMM_CREATE(3)				     Open MPI				   MPI_INTERCOMM_CREATE(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 <#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>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#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	<#
       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>/MPI_Comm_spawn_multiple	<#
       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 <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#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	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

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

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • MPI_Intercomm_merge <#mpi-intercomm-merge>

	  • MPI_Comm_free <#mpi-comm-free>

	  • MPI_Comm_remote_group <#mpi-comm-remote-group>

	  • MPI_Comm_remote_size <#mpi-comm-remote-size>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				   MPI_INTERCOMM_CREATE(3)
