MPI_Cart_shift(3OpenMPI) MPI_Cart_shift(3OpenMPI)
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 direc-
tion 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 mod-ified modified
ified 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.
Open MPI 1.2 September 2006 MPI_Cart_shift(3OpenMPI)
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