Estou tentando rodar este exemplo de programa hello world com o openmpi e o mpirun no debian 7.

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

int main (int argc, char **argv) {
   int nProcId, nProcNo;

   int nNameLen;
   char szMachineName[MPI_MAX_PROCESSOR_NAME];

   MPI_Init (&argc, &argv); // Start up MPI

   MPI_Comm_size (MPI_COMM_WORLD,&nProcNo); // Find out number of processes
   MPI_Comm_rank (MPI_COMM_WORLD, &nProcId); // Find out process rank
   MPI_Get_processor_name (szMachineName, &nNameLen); // Get machine name

   printf ("Hello World from process %d on %s\r\n", nProcId, szMachineName);

   if (nProcId == 0)
      printf ("Number of Processes: %d\r\n", nProcNo);

   MPI_Finalize (); // Shut down MPI

   return 0;
}

Meu problema éMPI_Comm_Rank retorna 0 para todas as cópias do processo. Quando executo este comando no shell:

mpirun -np 4  helloWorld

Produz esta saída:

Hello World from process 0 on debian
Number of Processes: 1
Hello World from process 0 on debian
Number of Processes: 1
Hello World from process 0 on debian
Number of Processes: 1
Hello World from process 0 on debian
Number of Processes: 1

Por que o número de processos ainda é 1?