[SOLVED] c++: MPI communicator as global variable

c++: MPI communicator as global variable

I need the MPI world communicator to be accessible in functions/class member functions. But by design/convention, MPI environments and communicators are always defined and initialized at the beginning of int main().

The only, simple solution I can think of using a global pointer to the communicator.

Does anybody know of a better way? Is it dangerous to use the global pointer solution?

This problem applies equally well to bare-bones MPI, and to Boost::MPI (which I use below)

Example of my proposed solution (untested):

//globals.h
extern   boost::mpi::communicator  * my_MPI_world_ptr;

and

//main.cpp
...
int main(int argc, char* argv[])
{        
    boost::mpi::environment my_boost_mpi_env(argc, argv);
    boost::mpi::communicator my_MPI_world; 
    my_MPI_world_ptr = &my_MPI_world;       

    my_MPI_rank = my_MPI_world_ptr->rank();
    size_MPI_WORLD = my_MPI_world_ptr->size();    

    my_class an_Object;
    an_Object.member_function_that_uses__MPI_world();
   ...
}

Solution

Do you mean the actual MPI MPI_COMM_WORLD communicator (or the Boost wrapper of it)? That is already global. If you are using a different communicator to separate communication from a library that you are writing, it would be better to avoid using a global variable for it at all. In that case, you might want to just pass it (or a pointer to it) around and store it in the classes that need it.