MPI_INTERCOMM_MERGE(3)				     Open MPI				    MPI_INTERCOMM_MERGE(3)

MPI_Intercomm_merge — Creates an intracommunicator from an intercommunicator.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Intercomm_merge(MPI_Comm intercomm, int high,
	       MPI_Comm *newintracomm)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_INTERCOMM_MERGE(INTERCOMM, HIGH, NEWINTRACOMM, IERROR)
	       INTEGER INTERCOMM, NEWINTRACOMM, IERROR
	       LOGICAL HIGH

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Intercomm_merge(intercomm, high, newintracomm, ierror)
	       TYPE(MPI_Comm), INTENT(IN) :: intercomm
	       LOGICAL, INTENT(IN) :: high
	       TYPE(MPI_Comm), INTENT(OUT) :: newintracomm
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • intercomm: Intercommunicator (type indicator).

       • high:	Used  to  order	 the  groups  of  the  two  intracommunicators	within	comm when creating the new
	 communicator (type indicator).

OUTPUT PARAMETERS
       • newintracomm: Created intracommunicator (type indicator).

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

DESCRIPTION
       This function creates an intracommunicator from the union of  the  two  groups  that  are  associated  with
       intercomm.  All processes should provide the same high value within each of the two groups. If processes in
       one group provide the value high = false and processes in the other group provide the value  high  =  true,
       then  the  union	 orders	 the  “low”  group before the “high” group. If all processes provide the same high
       argument, then the order of the union is arbitrary. This call is blocking and collective within	the  union
       of the two groups.

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_create <#mpi-intercomm-create>

	  • MPI_Comm_free <#mpi-comm-free>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				    MPI_INTERCOMM_MERGE(3)
