The Great and Terrible implementation of MPI-2

function index


Create a datatype for a subarray of a regular, multidimensional array
int MPI_Type_create_subarray(
  int ndims,
  int array_of_sizes[],
  int array_of_subsizes[],
  int array_of_starts[],
  int order,
  MPI_Datatype oldtype,
  MPI_Datatype *newtype


[in] number of array dimensions (positive integer)
[in] number of elements of type oldtype in each dimension of the full array (array of positive integers)
[in] number of elements of type oldtype in each dimension of the subarray (array of positive integers)
[in] starting coordinates of the subarray in each dimension (array of nonnegative integers)
[in] array storage order flag (state)
[in] array element datatype (handle)
[out] new datatype (handle)


The subarray type constructor creates an MPI datatype describing an n-dimensional subarray of an n-dimensional array. The subarray may be situated anywhere within the full array, and may be of any nonzero size up to the size of the larger array as long as it is confined within this array. This type constructor facilitates creating filetypes to access arrays distributed in blocks among processes to a single file that contains the global array.

This type constructor can handle arrays with an arbitrary number of dimensions and works for both C and Fortran ordered matrices (i.e., row-major or column-major). Note that a C program may use Fortran order and a Fortran program may use C order.

The ndims parameter specifies the number of dimensions in the full data array and gives the number of elements in array_of_sizes, array_of_subsizes, and array_of_starts.

The number of elements of type oldtype in each dimension of the n-dimensional array and the requested subarray are specified by array_of_sizes and array_of_subsizes, respectively. For any dimension i, it is erroneous to specify array_of_subsizes[i] < 1 or array_of_subsizes[i] > array_of_sizes[i].

The array_of_starts contains the starting coordinates of each dimension of the subarray. Arrays are assumed to be indexed starting from zero. For any dimension i, it is erroneous to specify array_of_starts[i] < 0 or array_of_starts[i] > ( array_of_sizes[i] - array_of_subsizes[i]).

Advice to users.

In a Fortran program with arrays indexed starting from 1, if the starting coordinate of a particular dimension of the subarray is n, then the entry in array_of_starts for that dimension is n-1. ( End of advice to users.)
The order argument specifies the storage order for the subarray as well as the full array. It must be set to one of the following:

The ordering used by C arrays, (i.e., row-major order)
The ordering used by Fortran arrays, (i.e., column-major order)

A ndims-dimensional subarray ( newtype) with no extra padding can be defined by the function Subarray() as follows:

Let the typemap of oldtype have the form: {(type0,disp0),(type1,disp1),...,(typen-1,dispn-1)} where typei is a predefined MPI datatype, and let ex be the extent of oldtype. Then we define the Subarray() function recursively using the following three equations. Equation 1 defines the base step. Equation 1 defines the recursion step when order = MPI_ORDER_FORTRAN, and Equation 1 defines the recursion step when order = MPI_ORDER_C.


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 datatype argument. May be an uncommitted MPI_Datatype (see MPI_Type_commit).
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_Type_create_subarray.

#include "mpi.h"
#include <stdio.h>
int main(int argc, char *argv[])
    int myrank;
    MPI_Status status;
    MPI_Datatype subarray;
    int array[9] = { -1, 1, 2, 3, -2, -3, -4, -5, -6 };
    int array_size[] = {9};
    int array_subsize[] = {3};
    int array_start[] = {1};
    int i;
    MPI_Init(&argc, &argv);
    /* Create a subarray datatype */
    MPI_Type_create_subarray(1, array_size, array_subsize, array_start, MPI_ORDER_C, MPI_INT, &subarray);
    MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
    if (myrank == 0)
        MPI_Send(array, 1, subarray, 1, 123, MPI_COMM_WORLD);
    else if (myrank == 1)
for (i=0; i<9; i++)
            array[i] = 0;
        MPI_Recv(array, 1, subarray, 0, 123, MPI_COMM_WORLD, &status);
        for (i=0; i<9; i++)
            printf("array[%d] = %d\n", i, array[i]);
    return 0;