MPI_Cart_shift(3)

NAME
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)

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 MPI_Sendrecv operation is likely to be used along a coordinate direction to perform a shift of data. As input, 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 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 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, 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.

  ....
  C find process rank
        CALL MPI_COMM_RANK(comm, rank, ierr)
  C find Cartesian coordinates
        CALL MPI_CART_COORDS(comm, rank, maxdims, coords,
                             ierr)
  C compute shift source and destination
        CALL MPI_CART_SHIFT(comm, 0, coords(2), source,
                            dest, ierr)
  C 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
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.