17.2.40. MPI_Cart_sub

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

! or the older form: INCLUDE 'mpif.h'

    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. 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