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.TH "MPI_CART_SHIFT" "3" "May 30, 2025" "" "Open MPI"
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.sp
\fI\%MPI_Cart_shift\fP — Returns the shifted source and destination ranks,
given a shift direction and amount.
.SH SYNTAX
.SS C Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
#include <mpi.h>

int MPI_Cart_shift(MPI_Comm comm, int direction, int disp,
    int *rank_source, int *rank_dest)
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
USE MPI
! or the older form: INCLUDE \(aqmpif.h\(aq

MPI_CART_SHIFT(COMM, DIRECTION, DISP, RANK_SOURCE,
        RANK_DEST, IERROR)
    INTEGER COMM, DIRECTION, DISP, RANK_SOURCE
    INTEGER RANK_DEST, IERROR
.ft P
.fi
.UNINDENT
.UNINDENT
.SS Fortran 2008 Syntax
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
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
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBcomm\fP : Communicator with Cartesian structure (handle).
.IP \(bu 2
\fBdirection\fP : Coordinate dimension of shift (integer).
.IP \(bu 2
.INDENT 2.0
.TP
.B \fBdisp\fP
Displacement ( > 0: upward shift, < 0: downward shift)
(integer).
.UNINDENT
.UNINDENT
.SH OUTPUT PARAMETERS
.INDENT 0.0
.IP \(bu 2
\fBrank_source\fP : Rank of source process (integer).
.IP \(bu 2
\fBrank_dest\fP : Rank of destination process (integer).
.IP \(bu 2
\fBierror\fP : Fortran only: Error status (integer).
.UNINDENT
.SH DESCRIPTION
.sp
If the process topology is a Cartesian structure, an \fI\%MPI_Sendrecv\fP
operation is likely to be used along a coordinate \fBdirection\fP to
perform a shift of data. As input, \fI\%MPI_Sendrecv\fP takes the rank of a
source process for the receive, and the rank of a destination process
for the send. If the function \fI\%MPI_Cart_shift\fP is called for a
Cartesian process group, it provides the calling process with the above
identifiers, which then can be passed to \fI\%MPI_Sendrecv\fP\&. The user
specifies the coordinate \fBdirection\fP and the size of the step
(positive or negative). The function is local.
.sp
The \fBdirection\fP 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.
.sp
Note: The \fBdirection\fP argument is in the range [0, n\-1] for an
n\-dimensional Cartesian mesh.
.sp
Depending on the periodicity of the Cartesian group in the specified
coordinate \fBdirection\fP, \fI\%MPI_Cart_shift\fP provides the identifiers
for a circular or an end\-off shift. In the case of an end\-off shift, the
value \fBMPI_PROC_NULL\fP may be returned in \fBrank_source\fP or
\fBrank_dest\fP, indicating that the source or the destination for the
shift is out of range.
.sp
Example: The \fBcomm\fPunicator, \fBcomm\fP, 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.
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
! 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)
.ft P
.fi
.UNINDENT
.UNINDENT
.SH NOTE
.sp
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].
.SH ERRORS
.sp
Almost all MPI routines return an error value; C routines as the return result
of the function and Fortran routines in the last argument.
.sp
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
\fI\%MPI_Init\fP/\fI\%MPI_Init_thread\fP, after \fI\%MPI_Finalize\fP, or when using the Sessions
Model exclusively) the error raises the initial error handler. The initial
error handler can be changed by calling \fI\%MPI_Comm_set_errhandler\fP on
MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI
argument to mpiexec or info key to \fI\%MPI_Comm_spawn\fP/\fI\%MPI_Comm_spawn_multiple\fP\&.
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.
.sp
Open MPI includes three predefined error handlers that can be used:
.INDENT 0.0
.IP \(bu 2
\fBMPI_ERRORS_ARE_FATAL\fP
Causes the program to abort all connected MPI processes.
.IP \(bu 2
\fBMPI_ERRORS_ABORT\fP
An error handler that can be invoked on a communicator,
window, file, or session. When called on a communicator, it
acts as if \fI\%MPI_Abort\fP was called on that communicator. If
called on a window or file, acts as if \fI\%MPI_Abort\fP 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.
.IP \(bu 2
\fBMPI_ERRORS_RETURN\fP
Returns an error code to the application.
.UNINDENT
.sp
MPI applications can also implement their own error handlers by calling:
.INDENT 0.0
.IP \(bu 2
\fI\%MPI_Comm_create_errhandler\fP then \fI\%MPI_Comm_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_File_create_errhandler\fP then \fI\%MPI_File_set_errhandler\fP
.IP \(bu 2
\fI\%MPI_Session_create_errhandler\fP then \fI\%MPI_Session_set_errhandler\fP or at \fI\%MPI_Session_init\fP
.IP \(bu 2
\fI\%MPI_Win_create_errhandler\fP then \fI\%MPI_Win_set_errhandler\fP
.UNINDENT
.sp
Note that MPI does not guarantee that an MPI program can continue past
an error.
.sp
See the \fI\%MPI man page\fP for a full list of \fI\%MPI error codes\fP\&.
.sp
See the Error Handling section of the MPI\-3.1 standard for
more information.
.SH COPYRIGHT
2003-2025, The Open MPI Community
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