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 31 ). To create the filetypes for each process one could use the following C program (see Section 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);Or, equivalently in Fortran:
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)The generated filetype will then describe the portion of the file contained within the process's subarray with holes for the space taken by the other processes. Figure 32 shows the filetype created for process 1.