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;  however,  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.

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_thread,  after
       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 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_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 was called on that communicator. If called on a
         window or file, acts as if 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 then MPI_Comm_set_errhandler

       • MPI_File_create_errhandler then MPI_File_set_errhandler

       • MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

       • MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

       See the MPI man page for a full list of MPI error codes.

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

       SEE ALSO:

          • MPI_Comm_create

COPYRIGHT

       2003-2025, The Open MPI Community

                                                  Feb 17, 2025                          MPI_COMM_CREATE_GROUP(3)