MPI_Allreduce(MPI_IN_PLACE, pData, (int) nData, GetDataType(pData), MPI_SUM, Communicator())

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

int main(int argc, char** argv) {
    // Initialize the MPI environment
    MPI_Init(NULL, NULL);
    // Find out rank, size
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    // We are assuming at least 2 processes for this task
    if (world_size < 2) {
        fprintf(stderr, "World size must be greater than 1 for %sn", argv[0]);
        MPI_Abort(MPI_COMM_WORLD, 1);
    }

    int number;
    if (world_rank == 0) {
        // If we are rank 0, set the number to -1 and send it to process 1
        number = -1;
        MPI_Send(&number, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);
    } else if (world_rank == 1) {
        MPI_Recv(&number, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
        printf("Process 1 received number %d from process 0n", number);
    }
    MPI_Finalize();
}

//MPI_Request send_req_, recv_req_;
//MPI_Status send_status_, recv_status_;
void NnetMpiSync::SyncTest() {
    int s = 1, r = 2;
    MPI_Isend(&s, 1, MPI_INT, peer_, 2, MPI_COMM_WORLD, &send_req_);
    MPI_Irecv(&r, 1, MPI_INT, peer_, 2, MPI_COMM_WORLD, &recv_req_);
    MPI_Wait(&send_req_, &send_status_);
    MPI_Wait(&recv_req_, &recv_status_);
    std::cout << "rank " << rank_ << " " << s << " " << r 
    << std::endl << std::flush;
}