MPI_COMM_CREATE_GROUP(3)			     Open MPI				  MPI_COMM_CREATE_GROUP(3)

MPI_Comm_create_group — Creates a new communicator.

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Comm_create_group(MPI_Comm comm, MPI_Group group, int tag, MPI_Comm *newcomm)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'

	  MPI_COMM_CREATE_GROUP(COMM, GROUP, TAG, NEWCOMM, IERROR)
	      INTEGER COMM, GROUP, TAG, NEWCOMM, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08

	  MPI_Comm_create_group(comm, group, tag, newcomm, ierror)
	      TYPE(MPI_Comm), INTENT(IN) :: comm
	      TYPE(MPI_Group), INTENT(IN) :: group
	      INTEGER, INTENT(IN) :: tag
	      TYPE(MPI_Comm), INTENT(OUT) :: newcomm
	      INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • comm : Communicator (handle).

       • group : Group, which is a subset of the group of comm (handle).

       • tag : Tag (integer).

OUTPUT PARAMETERS
       • newcomm : New communicator (handle).

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

DESCRIPTION
       MPI_Comm_create_group  is  similar  to  MPI_Comm_create	<#mpi-comm-create>;  however,  MPI_Comm_create	<#
       mpi-comm-create> must be called by all processes in the group of comm, whereas  MPI_Comm_create_group  must
       be  called  by  all  processes  in  group,  which  is  a	 subgroup  of  the  group  of  comm.  In addition,
       MPI_Comm_create_group requires that comm is an  intracommunicator.   MPI_Comm_create_group  returns  a  new
       intracommunicator,  newcomm,  for  which	 the  group  argument  defines	the communication group. No cached
       information propagates from comm to newcomm. Each process must provide a group argument that is a  subgroup
       of  the	group associated with comm; this could be MPI_GROUP_EMPTY. If a non-empty group is specified, then
       all processes in that group must call the function, and each of	these  processes  must	provide	 the  same
       arguments,  including  a group that contains the same members with the same ordering. Otherwise the call is
       erroneous. If the calling process is a member of the group given as the group argument, then newcomm  is	 a
       communicator  with  group  as  its associated group. If the calling process is not a member of group, e.g.,
       group is MPI_GROUP_EMPTY, then the call is a local operation and MPI_COMM_NULL is returned as newcomm.

NOTES
       MPI_Comm_create_group provides a means of making a subset of processes for the  purpose	of  separate  MIMD
       computation,  with separate communication space. newcomm, which is created by MPI_Comm_create_group, can be
       used in subsequent calls to MPI_Comm_create_group (or other communicator constructors) to further subdivide
       a computation into parallel sub-computations. A more general  service  is  provided  by	MPI_Comm_split	<#
       mpi-comm-split>.

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_Comm_create <#mpi-comm-create>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				  MPI_COMM_CREATE_GROUP(3)
