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| HDF5 Last Updated on 2025-11-11 The HDF5 Field Guide

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Writing by Chunk

Navigate back: Main / Getting Started with HDF5 / A Brief Introduction to Parallel HDF5

In this example each process writes a "chunk" of data to a dataset. The C and Fortran 90 examples result in the same data layout in the file.

Figure a C Example | Figure b Fortran Example
--- | ---

For this example, four processes are used, and a 4 x 2 chunk is written to the dataset by each process.

To do this, you would:

- Use the block parameter to specify a chunk of size 4 x 2 (or 2 x 4 for Fortran).
- Use a different offset (start) for each process, based on the chunk size:

  C | Process 0 | Process 1 | Process 2 | Process 3
  --- | --- | --- | --- | ---
  offset[0] = 0 | offset[0] = 0 | offset[0] = 4 | offset[0] = 4
  offset[1] = 0 | offset[1] = 2 | offset[1] = 0 | offset[1] = 2
  Fortran | Process 0 | Process 1 | Process 2 | Process 3
  offset(1) = 0 | offset(1) = 2 | offset(1) = 0 | offset(1) = 2
  offset(2) = 0 | offset(2) = 0 | offset(2) = 4 | offset(2) = 4

For example, the offset and block 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_chunk.c
hyperslab_by_chunk.F90

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

HDF5 "SDS_chnk.h5" {
GROUP "/" {
DATASET "IntArray" {
DATATYPE H5T_STD_I32BE
DATASPACE SIMPLE { ( 8, 4 ) / ( 8, 4 ) }
DATA {
1, 1, 2, 2,
1, 1, 2, 2,
1, 1, 2, 2,
1, 1, 2, 2,
3, 3, 4, 4,
3, 3, 4, 4,
3, 3, 4, 4,
3, 3, 4, 4
}
}
}
}
H5T_STD_I32BE
#define H5T_STD_I32BE
Definition H5Tpublic.h:401

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

Navigate back: Main / Getting Started with HDF5 / A Brief Introduction to Parallel HDF5