Question

I am starting out with MPI and have written a quick demo program:

int main(int argc, char** argv)
{
    MPI_Init(&argc, &argv);
    int myRank = MPI_Comm_rank(MPI_COMM_WORLD, &myRank);
    if (myRank) {
    cout << "slave" << endl;
    }
    else {
    cout << "master" << endl;
    }
    MPI_Finalize();
    return 0;
}

I run it with the following command:

aprun -n 4 test

My output is

master
master
master
master

I was expecting something like

slave
master
slave
slave

Why is this happening? Why are all my threads masters?

Answer 1

The problem is in this line:

int myRank = MPI_Comm_rank(MPI_COMM_WORLD, &myRank);

You should not assign myRank to the result of the call to MPI_Comm_rank. Just do:

int myRank;
MPI_Comm_rank(MPI_COMM_WORLD, &myRank);

and it will work.