MPI_Cart_shift(3)                  Open MPI                  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)


Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_CART_SHIFT(COMM, DIRECTION, DISP, RANK_SOURCE,
                 RANK_DEST, IERROR)
            INTEGER   COMM, DIRECTION, DISP, RANK_SOURCE
            INTEGER   RANK_DEST, IERROR


C++ Syntax
       #include <mpi.h>
       void Cartcomm::Shift(int direction, int disp, int& rank_source,
            int& rank_dest) const


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.




1.4.1                            Jan 14, 2010                MPI_Cart_shift(3)
