The Great and Terrible implementation of MPI-2

function index


Creates an indexed datatype
int MPI_Type_indexed(
  int count,
  int blocklens[],
  int indices[],
  MPI_Datatype old_type,
  MPI_Datatype *newtype


[in] number of blocks -- also number of entries in indices and blocklens
[in] number of elements in each block (array of nonnegative integers)
[in] displacement of each block in multiples of old_type (array of integers)
[in] old datatype (handle)
[out] new datatype (handle)


The function MPI_TYPE_INDEXED allows replication of an old datatype into a sequence of blocks (each block is a concatenation of the old datatype), where each block can contain a different number of copies and have a different displacement. All block displacements are multiples of the old type extent.


Let oldtype have type map with extent 16. Let B = (3, 1) and let D = (4, 0). A call to MPI_TYPE_INDEXED(2, B, D, oldtype, newtype) returns a datatype with type map,


That is, three copies of the old type starting at displacement 64, and one copy starting at displacement 0.

In general, assume that oldtype has type map,

with extent ex. Let B be the array_of_blocklength argument and D be the
array_of_displacements argument. The newly created datatype has entries:






A call to MPI_TYPE_VECTOR(count, blocklength, stride, oldtype, newtype) is equivalent to a call to MPI_TYPE_INDEXED(count, B, D, oldtype, newtype) where


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.

The indices are displacements, and are based on a zero origin. A common error is to do something like to following

    integer a(100)
    integer blens(10), indices(10)
    do i=1,10
         blens(i)   = 1
10       indices(i) = 1 + (i-1)*10
    call MPI_TYPE_INDEXED(10,blens,indices,MPI_INTEGER,newtype,ierr)
    call MPI_TYPE_COMMIT(newtype,ierr)
    call MPI_SEND(a,1,newtype,...)
expecting this to send a(1),a(11),... because the indices have values 1,11,.... Because these are displacements from the beginning of a, it actually sends a(1+1),a(1+11),....

If you wish to consider the displacements as indices into a Fortran array, consider declaring the Fortran array with a zero origin

    integer a(0:99)


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.

Invalid count argument. Count arguments must be non-negative; a count of zero is often valid.
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).
This error is returned when some part of the MPICH implementation is unable to acquire memory.

Example Code

The following sample code illustrates MPI_Type_indexed.

#include "mpi.h"
#include <stdio.h>

int main(int argc, char *argv[])
    int rank, size, i;
    MPI_Datatype type, type2;
    int blocklen[3] = { 2, 3, 1 };
    int displacement[3] = { 0, 3, 8 };
    int buffer[27];
    MPI_Status status;

    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if (size < 2)
        printf("Please run with 2 processes.\n");
        return 1;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    MPI_Type_contiguous(3, MPI_INT, &type2);
    MPI_Type_indexed(3, blocklen, displacement, type2, &type);

    if (rank == 0)
        for (i=0; i<27; i++)
            buffer[i] = i;
        MPI_Send(buffer, 1, type, 1, 123, MPI_COMM_WORLD);

    if (rank == 1)
        for (i=0; i<27; i++)
            buffer[i] = -1;
        MPI_Recv(buffer, 1, type, 0, 123, MPI_COMM_WORLD, &status);
        for (i=0; i<27; i++)
            printf("buffer[%d] = %d\n", i, buffer[i]);

    return 0;