MPI_Cart_sub(3)

NAME
MPI_Cart_sub - Partitions a communicator into subgroups, which form lower-dimensional Cartesian subgrids.

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 value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO

MPI_Cart_create
MPI_Comm_split