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MPI_Gatherv

Definition

MPI_Gatherv is a version of MPI_Gather in which the data received from processes can vary in the number of elements, and the location in which store these elements in the root process buffer.

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int MPI_Gatherv(void* buffer_send,
                int count_send,
                MPI_Datatype datatype_send,
                void* buffer_recv,
                const int* count_recv,
                const int* displacements,
                MPI_Datatype datatype_recv,
                int root,
                MPI_Comm communicator);

Parameters

buffer_send
The buffer containing the data to send.
count_send
The number of elements in the send buffer.
datatype_send
The type of one send buffer element.
buffer_recv
The buffer in which store the gathered data for the root process. For other processes, the receiving parameters like this one are ignored.
count_recv
An array containing the number of elements in the message to receive from each process, not the total number of elements to receive from all processes altogether. For non-root processes, the receiving parameters like this one are ignored.
displacements
An array containing the displacement to apply to the message received by each process. For non-root processes, the receiving parameters like this one are ignored.
datatype_recv
The type of one receive buffer element. For non-root processes, the receiving parameters like this one are ignored.
root
The rank of the root process, which will collect the data gathered.
communicator
The communicator in which the gather takes place.

Returned value

MPI_SUCCESS
The routine successfully completed.

Example

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

/**
 * @brief Illustrates how to use the variable version of a gather.
 * @details Every MPI process begins with a value, the MPI process 0 will gather
 * all these values and print them. The example is designed to cover all cases:
 * - Different displacements
 * - Different receive counts
 * It can be visualised as follows:
 * This application is meant to be run with 3 processes.
 *
 * +-----------+ +-----------+ +-------------------+ 
 * | Process 0 | | Process 1 | |     Process 2     |
 * +-+-------+-+ +-+-------+-+ +-+-------+-------+-+
 *   | Value |     | Value |     | Value | Value |
 *   |  100  |     |  101  |     |  102  |  103  |
 *   +-------+     +-------+     +-------+-------+
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *      |                |            |     |
 *   +-----+-----+-----+-----+-----+-----+-----+
 *   | 100 |  0  |  0  | 101 |  0  | 102 | 103 |
 *   +-----+-----+-----+-----+-----+-----+-----+
 *   |                Process 0                |
 *   +-----------------------+-----+-----+-----+
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get number of processes and check only 3 processes are used
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);
    if(size != 3)
    {
        printf("This application is meant to be run with 3 processes.\n");
        MPI_Abort(MPI_COMM_WORLD, EXIT_FAILURE);
    }

    // Get my rank
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    // Determine root's process rank
    int root_rank = 0;

    switch(my_rank)
    {
        case 0:
        {
            // Define my value
            int my_value = 100;

            // Define the receive counts
            int counts[3] = {1, 1, 2};

            // Define the displacements
            int displacements[3] = {0, 3, 5};

            int* buffer = (int*)calloc(7, sizeof(int));
            printf("Process %d, my value = %d.\n", my_rank, my_value);
            MPI_Gatherv(&my_value, 1, MPI_INT, buffer, counts, displacements, MPI_INT, root_rank, MPI_COMM_WORLD);
            printf("Values gathered in the buffer on process %d:", my_rank);
            for(int i = 0; i < 7; i++)
            {
                printf(" %d", buffer[i]);
            }
            printf("\n");
            free(buffer);
            break;
        }
        case 1:
        {
            // Define my value
            int my_value = 101;

            printf("Process %d, my value = %d.\n", my_rank, my_value);
            MPI_Gatherv(&my_value, 1, MPI_INT, NULL, NULL, NULL, MPI_INT, root_rank, MPI_COMM_WORLD);
            break;
        }
        case 2:
        {
            // Define my values
            int my_values[2] = {102, 103};

            printf("Process %d, my values = %d %d.\n", my_rank, my_values[0], my_values[1]);
            MPI_Gatherv(my_values, 2, MPI_INT, NULL, NULL, NULL, MPI_INT, root_rank, MPI_COMM_WORLD);
            break;
        }
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}