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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
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 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.
! 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
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.