Writing by Pattern

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This is another example of writing data into disconnected locations in a file. Each process writes data from the contiguous buffer into regularly scattered locations in the file.

Each process defines a hyperslab in the file as described below and writes data to it. The C and Fortran 90 examples below result in the same data layout in the file.

Figure a C Example	Figure b Fortran Example

The C and Fortran 90 examples use four processes to write the pattern shown above. Each process defines a hyperslab by:

• Specifying a stride of 2 for each dimension, which indicates that you wish to write to every other position along a dimension.
• Specifying a different offset for each process:

  C	Process 0	Process 1	Process 2	Process 3
  	offset[0] = 0	offset[0] = 1	offset[0] = 0	offset[0] = 1
  	offset[1] = 0	offset[1] = 0	offset[1] = 1	offset[1] = 1
  Fortran	Process 0	Process 1	Process 2	Process 3
  	offset(1) = 0	offset(1) = 0	offset(1) = 1	offset(1) = 1
  	offset(2) = 0	offset(2) = 1	offset(2) = 0	offset(2) = 1

• Specifying the size of the slab to write. The count is the number of positions along a dimension to write to. If writing a 4 x 2 slab, then the count would be:

  C	Fortran
  count[0] = 4	count(1) = 2
  count[1] = 2	count(2) = 4

For example, the offset, count, and stride parameters for Process 2 would look like:

Figure a C Example	Figure b Fortran Example

Below are example programs for writing hyperslabs by pattern in Parallel HDF5:

hyperslab_by_pattern.c

hyperslab_by_pattern.F90

The following is the output from h5dump for the HDF5 file created in this example:

HDF5 "SDS_pat.h5" {
GROUP "/" {
   DATASET "IntArray" {
      DATATYPE  H5T_STD_I32BE  
      DATASPACE  SIMPLE { ( 8, 4 ) / ( 8, 4 ) } 
      DATA {
         1, 3, 1, 3,
         2, 4, 2, 4,
         1, 3, 1, 3,
         2, 4, 2, 4,
         1, 3, 1, 3,
         2, 4, 2, 4,
         1, 3, 1, 3,
         2, 4, 2, 4
      } 
   } 
} 
} 

The h5dump utility is written in C so the output is in C order.

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