--- name: "MPI_Cart_shift.openmpi" section: "3" platform: "linux" package: "openmpi-doc" version: "5.0.7-1" release: "Debian 13 (Trixie)" platforms: ["linux"] url: "https://new.carta.tech/MPI_Cart_shift.openmpi" --- # MPI\_Cart\_shift.openmpi(3) *MPI\_Cart\_shift* — Returns the shifted source and destination ranks, given a shift direction and amount. ## SYNTAX ### C Syntax ``` #include 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 **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) ``` ## 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. ## COPYRIGHT 2003-2025, The Open MPI Community