/* file: reduce2.c
   compile: cc reduce2.c -lmpi
   run: mpirun -np 4 ./a.out
*/

#include "mpi.h" 

main( argc, argv ) 
int argc; 
char **argv; 
{
    int myrank; 
    int p;
    int sum, v;

    MPI_Init( &argc, &argv ); 
    MPI_Comm_rank( MPI_COMM_WORLD, &myrank ); 
    MPI_Comm_size( MPI_COMM_WORLD, &p ); 

    v = myrank;

    MPI_Reduce(&v, &sum, 1, MPI_INT,
        MPI_SUM, 0, MPI_COMM_WORLD);

    if (myrank == 0){
      printf("sum of all ranks = %d\n", sum);
    }
    MPI_Finalize(); 
  }