plucky (3) MPI_Cart_shift.openmpi.3.gz
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 return result of the function and
Fortran routines in the last argument.
Before the error value is returned, the current MPI error handler associated with the communication
object (e.g., communicator, window, file) is called. If no communication object is associated with the
MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error
handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after
MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler.
The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using
the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/‐
MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all
other MPI functions.
Open MPI includes three predefined error handlers that can be used:
• MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.
• MPI_ERRORS_ABORT An error handler that can be invoked on a communicator, window, file, or session. When
called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a
window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in
the corresponding window or file. If called on a session, aborts only the local process.
• MPI_ERRORS_RETURN Returns an error code to the application.
MPI applications can also implement their own error handlers by calling:
• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
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2003-2025, The Open MPI Community
Feb 17, 2025 MPI_CART_SHIFT(3)