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MPI_Cart_shift.3.rst

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MPI_Cart_shift

:ref:`MPI_Cart_shift` - Returns the shifted source and destination ranks, given a shift direction and amount.

SYNTAX

C Syntax

#include <mpi.h>

int MPI_Cart_shift(MPI_Comm comm, int direction, int disp,
    int *rank_source, int *rank_dest)

Fortran Syntax

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

MPI_CART_SHIFT(COMM, DIRECTION, DISP, RANK_SOURCE,
        RANK_DEST, IERROR)
    INTEGER COMM, DIRECTION, DISP, RANK_SOURCE
    INTEGER RANK_DEST, IERROR

Fortran 2008 Syntax

USE mpi_f08

MPI_Cart_shift(comm, direction, disp, rank_source, rank_dest, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, INTENT(IN) :: direction, disp
    INTEGER, INTENT(OUT) :: rank_source, rank_dest
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS

  • comm : Communicator with Cartesian structure (handle).
  • direction : Coordinate dimension of shift (integer).
  • disp : Displacement ( > 0: upward shift, < 0: downward shift)
    (integer).

OUTPUT PARAMETERS

  • rank_source : Rank of source process (integer).
  • rank_dest : Rank of destination process (integer).
  • ierror : Fortran only: Error status (integer).

DESCRIPTION

If the process topology is a Cartesian structure, an :ref:`MPI_Sendrecv` operation is likely to be used along a coordinate direction to perform a shift of data. As input, :ref:`MPI_Sendrecv` takes the rank of a source process for the receive, and the rank of a destination process for the send. If the function :ref:`MPI_Cart_shift` is called for a Cartesian process group, it provides the calling process with the above identifiers, which then can be passed to :ref:`MPI_Sendrecv`. The user specifies the coordinate direction and the size of the step (positive or negative). The function is local.

The direction argument indicates the dimension of the shift, i.e., the coordinate whose value is modified by the shift. The coordinates are numbered from 0 to ndims-1, where ndims is the number of dimensions.

Note: The direction argument is in the range [0, n-1] for an n-dimensional Cartesian mesh.

Depending on the periodicity of the Cartesian group in the specified coordinate direction, :ref:`MPI_Cart_shift` provides the identifiers for a circular or an end-off shift. In the case of an end-off shift, the value MPI_PROC_NULL may be returned in rank_source or rank_dest, indicating that the source or the destination for the shift is out of range.

Example: The communicator, comm, has a two-dimensional, periodic, Cartesian topology associated with it. A two-dimensional array of REALs is stored one element per process, in variable A. One wishes to skew this array, by shifting column i (vertically, i.e., along the column) by i steps.

! find process rank
    CALL MPI_COMM_RANK(comm, rank, ierr)
! find Cartesian coordinates
    CALL MPI_CART_COORDS(comm, rank, maxdims, coords, ierr)
! compute shift source and destination
    CALL MPI_CART_SHIFT(comm, 0, coords(2), source, dest, ierr)
! skew array
    CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, dest, 0, source, 0, comm, status,
                              ierr)

NOTE

In Fortran, the dimension indicated by DIRECTION = i has DIMS(i+1) nodes, where DIMS is the array that was used to create the grid. In C, the dimension indicated by direction = i is the dimension specified by dims[i].

ERRORS