#C

Copy this into the file hello_processes.c

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

int main(int argc, char **argv)
{
    int nprocs;    //holder for the number of cores
    int rank;   //holder for the ID number of this core

    // Initialise MPI
    MPI_Init(&argc, &argv);

    // Get the number of cores in the MPI cluster
    MPI_Comm_size( MPI_COMM_WORLD, &nprocs );

    // Get the ID number of this core in the MPI cluster
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );

    printf("I am process %d. The number of processes is %d.\n", rank, nprocs);

    // Shut down MPI
    MPI_Finalize();

    return 0;
} 

This example calls two MPI functions;

• MPI_Comm_size(MPI_COMM_WORLD, int *size) : Places the number of processes in the team into the variable pointed to by size
• MPI_Comm_rank(MPI_COMM_WORLD, int *rank) : Places the rank of the calling process in the team into the variable pointed to by rank.

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

Note also that you have to pass MPI_COMM_WORLD as the first argument. This is a pointer 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 pointer 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.c in the last section, e.g.

mpicc hello_processes.c -o hello_processes

This will produce the executable, hello_processes.

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