User datatypes

C | Fortran-2008 | Fortran-90

MPI_Type_create_hindexed_block

Definition

MPI_Type_create_hindexed_block is the version of MPI_Type_create_indexed_block where displacements are expressed in bytes, not in elements. Other ways to create a datatype are MPI_Type_contiguous, MPI_Type_vector, MPI_Type_create_hvector, MPI_Type_indexed, MPI_Type_create_hindexed, MPI_Type_create_indexed_block, MPI_Type_create_struct, MPI_Type_create_subarray, MPI_Type_create_darray.

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int MPI_Type_create_hindexed_block(int block_count,
                                   int block_length,
                                   MPI_Aint* displacements,
                                   MPI_Datatype old_datatype,
                                   MPI_Datatype* new_datatype);

Parameters

block_count

The number of blocks to create.

block_length

The length of each block, which therefore all have the same length (this is the difference with MPI_Type_indexed where the length of each block is specified separately).

displacements

Array containing the displacement for each block, expressed in bytes. The displacement is the distance between the start of the MPI datatype created and the start of the block. It is different from the stride used in MPI_Type_vector for instance, where it expresses the distance between the start of a block to the next.

old_datatype

The datatype making the element used as unit.

new_datatype

The variable in which store the datatype created.

Return value

The error code returned from the MPI heterogeneous indexed block datatype creation.

Example

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

/**
 * @brief Illustrates how to create an homogeneous indexed block MPI datatype.
 * @details This program is meant to be run with 2 processes: a sender and a
 * receiver. These two MPI processes will exchange a message made of four
 * integers. On the sender, that message is in fact made of the four corners of
 * an array it holds, which will be represented by an MPI indexed type.
 *
 *
 *     Full array          What we want
 *                            to send
 * +-----+-----+-----+  +-----+-----+-----+
 * |  0  |  1  |  2  |  |  0  |  -  |  2  |
 * +-----+-----+-----+  +-----+-----+-----+
 * |  3  |  4  |  5  |  |  -  |  -  |  -  |
 * +-----+-----+-----+  +-----+-----+-----+
 * |  6  |  7  |  8  |  |  6  |  -  |  8  |
 * +-----+-----+-----+  +-----+-----+-----+
 *
 * How to extract these elements with an homogeneous indexed block type:
 *    
 *            +---------------------------------------- displacement for
 *            |                                      block 3: 8 sizeof(int)
 *            |                                               |
 *            +---------------------------- displacement for  |
 *            |                        block 2: 6 sizeof(int) |
 *            |                                   |           |
 *            +---- displacement for              |           |
 *            |  block 2: 2 sizeof(int)           |           |
 *            |           |                       |           |
 *      displacement for  |                       |           |
 * block 1: 0 sizeof(int) |                       |           |
 *            |           |                       |           |
 *            V           V                       V           |
 *            +-----+-----+-----+-----+-----+-----+-----+-----+-----+
 *            |  0  |  -  |  2  |  -  |  -  |  -  |  6  |  -  |  8  |
 *            +-----+-----+-----+-----+-----+-----+-----+-----+-----+
 *             <--->       <--->                   <--->       <--->
 *            block 1     block 2                 block 3     block 4
 *           1 element   1 element               1 element   1 element
 * 
 * Element: MPI_INT
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get the number of processes and check only 2 processes are used
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if(size != 2)
    {
        printf("This application is meant to be run with 2 processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    // Get my rank and do the corresponding job
    enum rank_roles { SENDER, RECEIVER };
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
    switch(my_rank)
    {
        case SENDER:
        {
            // Create the datatype
            MPI_Datatype corner_type;
            MPI_Aint displacements[4] = { 0, 2 * sizeof(int), 6 * sizeof(int), 8 * sizeof(int)};
            MPI_Type_create_hindexed_block(4, 1, displacements, MPI_INT, &corner_type);
            MPI_Type_commit(&corner_type);

            // Send the message
            int buffer[3][3] = { 0, 1, 2, 3, 4, 5, 6, 7, 8 };
            printf("MPI process %d sends values:\n%d %d\n%d %d\n", my_rank, buffer[0][0], buffer[0][2], buffer[2][0], buffer[2][2]);
            MPI_Send(buffer, 1, corner_type, RECEIVER, 0, MPI_COMM_WORLD);
            break;
        }
        case RECEIVER:
        {
            // Receive the message
            int received[4];
            MPI_Recv(&received, 4, MPI_INT, SENDER, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("MPI process %d received values:\n%d %d\n%d %d\n", my_rank, received[0], received[1], received[2], received[3]);
            break;
        }
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}