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MPI_Ibsend

Definition

MPI_Ibsend stands for MPI Buffered Send with Immediate return; sending a message in a non-blocking asynchronous fashion. The routine returns immediately, before completion. It implies that the buffer provided may not be copied yet so it is not safe to modify it. Refer to its blocking counterpart, MPI_Bsend, to understand when the completion is reached. Other non-blocking sends are MPI_Isend, MPI_Issend and MPI_Irsend.

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int MPI_Ibsend(const void* buffer,
               int count,
               MPI_Datatype datatype,
               int recipient,
               int tag,
               MPI_Comm communicator,
               MPI_Request* request);

Parameters

buffer
The buffer to send.
count
The number of elements to send.
datatype
The type of one buffer element.
recipient
The rank of the recipient MPI process.
tag
The tag to apply to the message.
communicator
The communicator in which the communication takes place.
request
The request handle on the non-blocking communication taking 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 send a message in a non-blocking asynchronous
 * fashion.
 * @details This program is meant to be run with 2 processes: a sender and a
 * receiver.
 **/
int main(int argc, char* argv[])
{
    MPI_Init(&argc, &argv);

    // Get the number of processes and check 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 role_ranks { SENDER, RECEIVER };
    int my_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
    switch(my_rank)
    {
        case SENDER:
        {
            // Declare the buffer and attach it
            int buffer_attached_size = MPI_BSEND_OVERHEAD + sizeof(int);
            char* buffer_attached = malloc(buffer_attached_size);
            MPI_Buffer_attach(buffer_attached, buffer_attached_size);

            // Issue the MPI_Ibsend
            int buffer_sent = 12345;
            MPI_Request request;
            printf("MPI process %d sends value %d.\n", my_rank, buffer_sent);
            MPI_Ibsend(&buffer_sent, 1, MPI_INT, 1, 0, MPI_COMM_WORLD, &request);
            
            // Let's wait for the MPI_Ibsend to complete before progressing further.
            MPI_Wait(&request, MPI_STATUS_IGNORE);

            // Detach the buffer. It blocks until all messages stored are sent.
            MPI_Buffer_detach(&buffer_attached, &buffer_attached_size);
            free(buffer_attached);
            break;
        }
        case RECEIVER:
        {
            int received;
            MPI_Recv(&received, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("MPI process %d received value: %d.\n", my_rank, received);
            break;
        }
    }

    MPI_Finalize();

    return EXIT_SUCCESS;
}