The Great and Terrible implementation of MPI-2

function index


Makes a new communicator to which topology information has been attached
int MPI_Cart_create(
  MPI_Comm comm_old,
  int ndims,
  int *dims,
  int *periods,
  int reorder,
  MPI_Comm *comm_cart


[in] input communicator (handle)
[in] number of dimensions of cartesian grid (integer)
[in] integer array of size ndims specifying the number of processes in each dimension
[in] logical array of size ndims specifying whether the grid is periodic (true) or not (false) in each dimension
[in] ranking may be reordered (true) or not (false) (logical)
[out] communicator with new cartesian topology (handle)


We ignore reorder info currently.


MPI_CART_CREATE returns a handle to a new communicator to which the cartesian topology information is attached. If reorder = false then the rank of each process in the new group is identical to its rank in the old group. Otherwise, the function may reorder the processes (possibly so as to choose a good embedding of the virtual topology onto the physical machine). If the total size of the cartesian grid is smaller than the size of the group of comm, then some processes are returned MPI_COMM_NULL, in analogy to MPI_COMM_SPLIT. The call is erroneous if it specifies a grid that is larger than the group size.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

Notes for Fortran

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.


All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

No error; MPI routine completed successfully.
Invalid topology. Either there is no topology associated with this communicator, or it is not the correct type (e.g., MPI_CART when expecting MPI_GRAPH).
Invalid dimension argument. A dimension argument is null or its length is less than or equal to zero.
Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Cart_create.

/* A two-dimensional torus of 12 processes in a 4x3 grid */
int main(int argc, char *argv[])
    int rank, size;
    MPI_Comm comm;
    int dim[2], period[2], reorder;
    int coord[2], id;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if (size != 12)
        printf("Please run with 12 processes.\n");fflush(stdout);
        MPI_Abort(MPI_COMM_WORLD, 1);

    dim[0]=4; dim[1]=3;
    period[0]=1; period[1]=0;
    MPI_Cart_create(MPI_COMM_WORLD, 2, dim, period, reorder, &comm);
    if (rank == 5)
        MPI_Cart_coords(comm, rank, 2, coord);
        printf("Rank %d coordinates are %d %d\n", rank, coord[0], coord[1]);fflush(stdout);
        coord[0]=3; coord[1]=1;
        MPI_Cart_rank(comm, coord, &id);
        printf("The processor at position (%d, %d) has rank %d\n", coord[0], coord[1], id);fflush(stdout);
return 0;