MPI_Send/Recv failed sending dynamically allocated int array

Question

I am trying to understand how sending an array works. I wrote a simple program but with not good results, as it usually crashes on segmentation fault. There are arguments for my program:

mpirun -np 2 sendTest

Here is the code:

#include <mpi.h>
#include <iostream> 

using namespace std;

int main(int argc, char *argv[])
{
    int cpuNum;
    int myId;
    MPI_Status mpiStatus;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&cpuNum);
    MPI_Comm_rank(MPI_COMM_WORLD,&myId);

    int *myNumbs=(int*)malloc(2*(sizeof(int)));
    int *neighNumbs=(int*)malloc(2*(sizeof(int)));

    if(myId==0){
        myNumbs[0]=0;
        myNumbs[1]=0;

        MPI_Send(&myNumbs, 2, MPI_INT, 1, 0, MPI_COMM_WORLD);
        MPI_Recv(&neighNumbs, 2, MPI_INT, 1, 0, MPI_COMM_WORLD, &mpiStatus);

        cout<<"this is cpu 0, neigh myNumbs=";
        for (int i = 0; i < 2; ++i){
            printf("%d,",neighNumbs[i]);
        }
        cout<<endl;
    }
    else{
        myNumbs[0]=1;
        myNumbs[1]=1;

        MPI_Recv(&neighNumbs, 2, MPI_INT, 0, 0, MPI_COMM_WORLD, &mpiStatus);
        MPI_Send(&myNumbs, 2, MPI_INT, 0, 0, MPI_COMM_WORLD);

        cout<<"this is cpu 1, neigh myNumbs=";
        for (int i = 0; i < 2; ++i){
            printf("%d,",neighNumbs[i]);
        }
        cout<<endl;
    }

    MPI_Finalize();
    return 0;
}

As I said it usually crashes with error like this: *** Process received signal *** Signal: Segmentation fault (11) Signal code: Address not mapped (1) . But sometimes one or the other process receives the data.

Answer 1

You must pass the address of the allocated arrays, not the address of the pointers who points to the allocated arrays.

Remove the two & before the pointer variables: try

        MPI_Send(myNumbs, 2, MPI_INT, 1, 0, MPI_COMM_WORLD);
        MPI_Recv(neighNumbs, 2, MPI_INT, 1, 0, MPI_COMM_WORLD, &mpiStatus);

instead of

        MPI_Send(&myNumbs, 2, MPI_INT, 1, 0, MPI_COMM_WORLD);
        MPI_Recv(&neighNumbs, 2, MPI_INT, 1, 0, MPI_COMM_WORLD, &mpiStatus);

and

        MPI_Recv(neighNumbs, 2, MPI_INT, 0, 0, MPI_COMM_WORLD, &mpiStatus);
        MPI_Send(myNumbs, 2, MPI_INT, 0, 0, MPI_COMM_WORLD);

instead of

        MPI_Recv(&neighNumbs, 2, MPI_INT, 0, 0, MPI_COMM_WORLD, &mpiStatus);
        MPI_Send(&myNumbs, 2, MPI_INT, 0, 0, MPI_COMM_WORLD);