title: MPI code on Ubuntu have only one processor used
body: <p>I'm trying to learn parallel computing with Open MPI. I use a Ubuntu 16 boot on a MacBook Pro.</p>
  
  <p>I've installed OpenMP and try to run an <code>hello_world</code> to test it.</p>
  
  <pre><code>#include &lt;mpi.h&gt;
  #include &lt;stdio.h&gt;
  
  int main(int argc, char** argv) {
  // Initialize the MPI environment
  MPI_Init(NULL, NULL);
  
  // Get the number of processes
  int world_size;
  MPI_Comm_size(MPI_COMM_WORLD, &amp;world_size);
  
  // Get the rank of the process
  int world_rank;
  MPI_Comm_rank(MPI_COMM_WORLD, &amp;world_rank);
  
  // Get the name of the processor
  char processor_name[MPI_MAX_PROCESSOR_NAME];
  int name_len;
  MPI_Get_processor_name(processor_name, &amp;name_len);
  
  // Print off a hello world message
  printf("Hello world from processor %s, rank %d"
         " out of %d processors\n",
         processor_name, world_rank, world_size);
  
  // Finalize the MPI environment.
  MPI_Finalize();
  }
  </code></pre>
  
  <p>I had no problem to compile it with <code>mpicc</code> but when I try to launch it, I've got the same result for <code>./hello_world -n 4</code>, <code>./hello_world -n 2</code>, <code>./hello_world -np 4</code> and so on.</p>
  
  <p>It always writes:</p>
  
  <blockquote>
    <p>Hello world from processor ubuntu-mac , rank 0 out of 1 processor</p>
  </blockquote>
  
  <p>I don't understand why can't it run on several processor... Do I launch it incorrectly or is it my config or anything else?</p>