# 18.2.39. MPI_Cart_shift

MPI_Cart_shift — Returns the shifted source and destination ranks, given a shift direction and amount.

## 18.2.39.1. SYNTAX

### 18.2.39.1.1. C Syntax

    #include <mpi.h>

    int MPI_Cart_shift(MPI_Comm comm, int direction, int disp,
        int *rank_source, int *rank_dest)

### 18.2.39.1.2. 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

### 18.2.39.1.3. 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

## 18.2.39.2. INPUT PARAMETERS

- `comm` : Communicator with Cartesian structure (handle).

- `direction` : Coordinate dimension of shift (integer).

- `disp` Displacement ( > 0: upward shift, < 0: downward shift)

    (integer).

## 18.2.39.3. OUTPUT PARAMETERS

- `rank_source` : Rank of source process (integer).

- `rank_dest` : Rank of destination process (integer).

- `ierror` : Fortran only: Error status (integer).

## 18.2.39.4. 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 `comm`unicator, `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)

## 18.2.39.5. 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].

## 18.2.39.6. 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.