c++: MPI communicator as global variable

Question

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

Answer 1

1. For boost mpi, the default constructed (i.e., empty initializer) communicator corresponds to MPI_COMM_WORLD, so you could simply define another

   mpi::communicator world;
   

   inside your function use it as if it is the one you defined outside.

2. MPI_INIT is called when constructing mpi::environment. So as long as this is placed in the beginning of the of your main program, you are free to define a global mpi::communicator somewhere else. It is not necessary to use a pointer. (Actually you can even place MPI_INIT somewhere else as well, see below).

3. For bare-bones MPI, I have tested that calling MPI_INIT elsewhere besides the main is also allowed. For example, you can define the following wrapper for a global worker in a header file,

       class MpiWorker_t
       {
       public:
         int  NumberOfWorkers, WorkerId, NameLen;
         char HostName[MPI_MAX_PROCESSOR_NAME];
         MpiWorker_t()
         {
               MPI_Init(NULL, NULL);
               MPI_Comm_size(MPI_COMM_WORLD,&NumberOfWorkers);
               MPI_Comm_rank(MPI_COMM_WORLD,&WorkerId);
               MPI_Get_processor_name(HostName, &NameLen);
         }
         ~MpiWorker_t()
         {
               MPI_Finalize();
         }
       }
   
      extern MpiWorker_t GlobalWorker;
   

   and in the source file, define a global instance at any place outside main():

     MpiWorker_t GlobalWorker;
   

   The construction and destruction of the global variable should be able to take care of MPI initialization and finalization, before and after any MPI function call.