279. Subarray Filetype Constructor

Assume we are writing out a 100x100 2D array of double precision floating point numbers that is distributed among 4 processes such that each process has a block of 25 columns (e.g., process 0 has columns 0-24, process 1 has columns 25-49, etc.; see Figure 28 ). To create the filetypes for each process one could use the following C program (see Section Subarray Datatype Constructor on page Subarray Datatype Constructor ):

   double subarray[100][25]; 
   MPI_Datatype filetype; 
   int sizes[2], subsizes[2], starts[2]; 
   int rank; 
 
   MPI_Comm_rank(MPI_COMM_WORLD, &rank); 
   sizes[0]=100; sizes[1]=100; 
   subsizes[0]=100; subsizes[1]=25; 
   starts[0]=0; starts[1]=rank*subsizes[1]; 
 
   MPI_Type_create_subarray(2, sizes, subsizes, starts, MPI_ORDER_C, 
                            MPI_DOUBLE, &filetype); 

       double precision subarray(100,25) 
       integer filetype, rank, ierror 
       integer sizes(2), subsizes(2), starts(2) 
 
       call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierror) 
       sizes(1)=100 
       sizes(2)=100 
       subsizes(1)=100 
       subsizes(2)=25 
       starts(1)=0 
       starts(2)=rank*subsizes(2) 
 
       call MPI_TYPE_CREATE_SUBARRAY(2, sizes, subsizes, starts, & 
                  MPI_ORDER_FORTRAN, MPI_DOUBLE_PRECISION,       & 
                  filetype, ierror)