#Fortran

Copy this into the file hello_processes.f

      program functions

      include "mpif.h"

      integer nprocs
      integer rank
      integer err

c     Initialise MPI
      call MPI_INIT(err)

c     Get the number of cores in the MPI cluster
      call MPI_Comm_size( MPI_COMM_WORLD, nprocs, err )

c     Get the ID of this core in the MPI cluster
      call MPI_Comm_rank( MPI_COMM_WORLD, rank, err )

      print *,"I am process ",rank,". The number of processes is ",
     .        nprocs

c     Shut down MPI
      call MPI_Finalize(err)

      end 

This example calls two MPI functions;

• MPI_Comm_size(MPI_COMM_WORLD, integer size, integer err) : Places the number of processes in the team into the variable size, with any errors reported in the integer err
• MPI_Comm_rank(MPI_COMM_WORLD, integer rank, integer err) : Places the rank of the calling process in the team into the variable rank, with any errors reported in the integer err

Note that using these functions requires you to include the mpif.h header file.

Note also that you have to pass MPI_COMM_WORLD as the first argument. This is a handle to the global communicator that is used to coordinate communication between all processes in the MPI process team. It is possible to create and use different MPI communicators in a program, e.g. by calling MPI_Comm_create, and passing the resulting handle to the new communicator in place of MPI_COMM_WORLD. Creating and managing different communicators is beyond the scope of this simple introduction, so all examples in this course will always use MPI_COMM_WORLD.

You can compile this program as you did for the hello_mpi.f in the last section, e.g.

mpif77 hello_processes.f -o hello_processes

This will produce the executable, hello_processes.

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