The Great and Terrible implementation of MPI-2

function index


Partitions a communicator into subgroups which form lower-dimensional cartesian subgrids
int MPI_Cart_sub(
  MPI_Comm comm,
  int *remain_dims,
  MPI_Comm *newcomm


[in] communicator with cartesian structure (handle)
[in] the ith entry of remain_dims specifies whether the ith dimension is kept in the subgrid (true) or is dropped (false) (logical vector)
[out] communicator containing the subgrid that includes the calling process (handle)


If a cartesian topology has been created with MPI_CART_CREATE, the function
MPI_CART_SUB can be used to partition the communicator group into subgroups that form lower-dimensional cartesian subgrids, and to build for each subgroup a communicator with the associated subgrid cartesian topology. (This function is closely related to MPI_COMM_SPLIT.)


Assume that MPI_CART_CREATE(..., comm) has defined a grid. Let remain_dims = (true, false, true). Then a call to,

MPI_CART_SUB(comm, remain_dims, comm_new), 
will create three communicators each with eight processes in a cartesian topology. If remain_dims = (false, false, true) then the call to MPI_CART_SUB(comm, remain_dims, comm_new) will create six non-overlapping communicators, each with four processes, in a one-dimensional cartesian topology.

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 communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).
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_sub.

#include "mpi.h"
#include <stdio.h>

int main( int argc, char *argv[] )
    int errs = 0;
int size, dims[2], periods[2], remain[2];
int result;
    MPI_Comm comm, newcomm;

    MPI_Init( &argc, &argv );

    /* First, create a 1-dim cartesian communicator */
periods[0] = 0;
    MPI_Comm_size( MPI_COMM_WORLD, &size );
    dims[0] = size;
    MPI_Cart_create( MPI_COMM_WORLD, 1, dims, periods, 0, &comm );

/* Now, extract a communicator with no dimensions */
remain[0] = 0;
    MPI_Cart_sub( comm, remain, &newcomm );

/* This should be congruent to MPI_COMM_SELF */
    MPI_Comm_compare( MPI_COMM_SELF, newcomm, &result );
    if (result != MPI_CONGRUENT) {
        printf( "cart sub to size 0 did not give self\n" );fflush(stdout);

/* Free the new communicator */
MPI_Comm_free( &newcomm );
    MPI_Comm_free( &comm );

return 0;