RowMatrixVectorMultiply(int n, double *a, double *b, double *x,MPI_Comm comm) 
{ 
  int i, j; 
  int nlocal;        /* Number of locally stored rows of A */ 
  double *fb;        /* Will point to a buffer that stores the entire vector b */ 
  int npes, myrank; 
  MPI_Status status; 

  /* Get information about the communicator */ 
  MPI_Comm_size(comm, &npes); 
  MPI_Comm_rank(comm, &myrank); 

  /* Allocate the memory that will store the entire vector b */ 
  fb = (double *)malloc(n*sizeof(double)); 

  nlocal = n/npes; 

  /* Gather the entire vector b on each processor using MPI's ALLGATHER operation */ 
  MPI_Allgather(b, nlocal, MPI_DOUBLE, fb, nlocal, MPI_DOUBLE,comm); 

  /* Perform the matrix-vector multiplication involving the locally stored submatrix */ 
  for (i=0; i<nlocal; i++) { 
    x[i] = 0.0; 
    for (j=0; j<n; j++) 
      x[i] += a[i*n+j]*fb[j]; 
  } 

  free(fb); 
}