Openmpi mpmd get communication size

Question

I have two openmpi programs which I start like this

mpirun -n 4 ./prog1 : -n 2 ./prog2

Now how do I use MPI_Comm_size(MPI_COMM_WORLD,

Answer 1

Since you are using Open MPI, there is a very simple OMPI-specific solution:

#include 

MPI_Comm appcomm;
int app_id = atoi(getenv("OMPI_MCA_orte_app_num"));
MPI_Comm_split(MPI_COMM_WORLD, app_id, 0, &appcomm);

There will be now as many different appcomm communicators as there are application contexts.