User datatypes

C | Fortran-2008 | Fortran-90

MPI_Type_vector

Definition

MPI_Type_vector creates an MPI datatype by replicating an existing MPI_Datatype a certain number of times into a block. The MPI_Datatype created will contain a certain number of such blocks separated with the constant displacement specified. Other ways to create a datatype are MPI_Type_contiguous, MPI_Type_create_hvector, MPI_Type_indexed, MPI_Type_create_hindexed, MPI_Type_create_indexed_block, MPI_Type_create_hindexed_block, MPI_Type_create_struct, MPI_Type_create_subarray, MPI_Type_create_darray.

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int MPI_Type_vector(int block_count,
                    int block_length,
                    int stride,
                    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).

stride

Distance between the start of each block, expressed in number of elements.

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 vector datatype creation.

Example

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

/**
 * @brief Illustrates how to create a vector 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 three
 * integers. On the sender, that message is in fact the middle column of an
 * array it holds, which will be represented by an MPI vector.
 *
 *
 *     Full array          What we want
 *                            to send
 * +-----+-----+-----+  +-----+-----+-----+
 * |  0  |  1  |  2  |  |  -  |  1  |  -  |
 * +-----+-----+-----+  +-----+-----+-----+
 * |  3  |  4  |  5  |  |  -  |  4  |  -  |
 * +-----+-----+-----+  +-----+-----+-----+
 * |  6  |  7  |  8  |  |  -  |  7  |  -  |
 * +-----+-----+-----+  +-----+-----+-----+
 *
 * How to extract a column with a vector type:
 *
 *                 distance between the
 *            start of each block: 3 elements
 *          <---------------> <--------------->
 *         |                 |                 |
 *      start of          start of          start of
 *      block 1           block 2           block 3
 *         |                 |                 |
 *         V                 V                 V
 *   +-----+-----+-----+-----+-----+-----+-----+-----+-----+
 *   |  -  |  1  |  -  |  -  |  4  |  -  |  -  |  7  |  -  |
 *   +-----+-----+-----+-----+-----+-----+-----+-----+-----+
 *          <--->             <--->             <--->
 *         block 1           block 2           block 3
 * 
 * Block length: 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 column_type;
            MPI_Type_vector(3, 1, 3, MPI_INT, &column_type);
            MPI_Type_commit(&column_type);

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

    MPI_Finalize();

    return EXIT_SUCCESS;
}