SYNTAX

   C Syntax
          #include <mpi.h>

          int MPI_Cart_sub(MPI_Comm comm, const int remain_dims[], MPI_Comm *comm_new)

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

          MPI_CART_SUB(COMM, REMAIN_DIMS, COMM_NEW, IERROR)
              INTEGER COMM, COMM_NEW, IERROR
              LOGICAL REMAIN_DIMS(*)

   Fortran 2008 Syntax
          USE mpi_f08

          MPI_Cart_sub(comm, remain_dims, newcomm, ierror)
              TYPE(MPI_Comm), INTENT(IN) :: comm
              LOGICAL, INTENT(IN) :: remain_dims(*)
              TYPE(MPI_Comm), INTENT(OUT) :: newcomm
              INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

       • comm : Communicator with Cartesian structure (handle).

       •

         remain_dims (The ith entry of remain_dims specifies whether the ith)
                dimension is kept in the subgrid (true) or is dropped (false) (logical vector).

OUTPUT PARAMETERS

       •

         comm_new (Communicator containing the subgrid that includes the)
                calling process (handle).

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

DESCRIPTION

       If  a  Cartesian topology has been created with MPI_Cart_create, the function MPI_Cart_sub can be used to
       partition the communicator group into subgroups that form lower-dimensional Cartesian  subgrids,  and  to
       build  for each subgroup a communicator with the associated subgrid Cartesian topology. (This function is
       closely related to MPI_Comm_split.)

       Example: Assume that MPI_Cart_create( ..., comm) has defined a (2 x 3 x 4) grid. Let remain_dims = (true,
       false, true). Then a call to

          MPI_Cart_sub(comm, remain_dims, comm_new);

       will create three communicators, each with eight processes in a 2 x 4 Cartesian topology. If  remain_dims
       =  (false,  false,  true)  then  the  call  to  MPI_Cart_sub(comm, remain_dims, comm_new) will create six
       nonoverlapping communicators, each with four processes, in a one-dimensional Cartesian topology.

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_Cart_create

COPYRIGHT

       2003-2025, The Open MPI Community

                                                  Jun 07, 2025                                   MPI_CART_SUB(3)