Writing Grids to HDF5

One of the most popular file formats for writing numerical data is the HDF5 format. This format is widely supported by visualisation tools and can be read by many other software including Matlab and others. Schnek supports writing data in HDF5 using parallel file output. This means that each process in a parallel execution environment writes its own piece of data into a single file. Creating parallel HDF5 diagnostic in Schnek is relatively simple. First of all you need to create a class that inherits from HDFGridDiagnostic and implements two methods.

typedef Field<double, 2> SimField;

Array<int,2> lo(-N/2,-N/2);
Array<int,2> hi( N/2, N/2);

class Diagnostic : public HDFGridDiagnostic<SimField, SimField*, DeltaTimeDiagnostic>
    Array<int,2> getGlobalMin() {
      return lo;

    Array<int,2> getGlobalMax() {
      return hi;

The HDFGridDiagnostic class takes three arguments. The first argument is the Field or Grid type that should be written out. Note that specifying a Field type will automatically take care of the ghost cells of the field. HDFGridDiagnostic obtains the grid for output through the Block‘s retrieveData() function (see the section on sharing data between blocks). The second argument is the pointer type of the field that should be retrieved from the Block. The third template argument specifies whether the diagnostic should use the physical time or the integer time step to determine when data should be written. The choices here are DeltaTimeDiagnostic and IntervalDiagnostic.

Two functions need to be implemented. The getGlobalMin() and getGlobalMax() function should return the global extent of the grid across all processes. This range should not include any ghost cells.

Once the class has been defined it can be included in the simulation block hierarchy for the setup file parser.



During the initialisation of the blocks, the field that should be available to the HDF5 diagnostic must be added to the Block shared data. Note that the HDFGridDiagnostic class sets up its internal parameters for writing the HDF5 files during the init() phase of block initialisation. This means that the grid or field must have been added and properly resized before the init() function is called. It is advisable to do this during the preInit() phase (see the section on block initialisation).

Field<double, 2> field(localLo, localHi, localDomain, stagger, 1);

addData("field", field);

Here localLo and localHi defines the local extent of the grid or field.

Finally we need to call the DiagnosticManager to write out the fields when needed.


while (time <= simulationEndTime) {
  time += dt;

See the previous section on the DiagnosticManager for details.

Now the diagnostic output can be controlled through the setup file. A typical entry in the setup file for writing a field would be the following.

Diagnostic Field {
  file = "Field_#t.h5";
  field = "field";
  deltaTime = 0.2;

First of all, note the #t token in the file name. This character sequence #t will be replaced with the count of the diagnostic output. This means that the code will end up generating output files named


The field = "field"; line specifies the field that should be written. The string on the right hand side corresponds to the string passed to the addData() function in the code. Usually there are multiple data field in the code, such as the following.

addData("Ex", Ex);
addData("Ey", Ey);
addData("Ez", Ez);

To generate HDF5 output for each field you would add separate diagnostic blocks in the setup file.

Diagnostic Ex {
  file = "Ex_#t.h5";
  field = "Ex";
  deltaTime = 0.2;
Diagnostic Ey {
  file = "Ey_#t.h5";
  field = "Ey";
  deltaTime = 0.2;
Diagnostic Ez {
  file = "Ez_#t.h5";
  field = "Ez";
  deltaTime = 0.2;

Finally deltaTime specifies the physical time interval after which an output should be generated. For IntervalDiagnostic the number of tie steps is specified using interval which is illustrated in the following snippet.

IntervalDiagnostic Field {
  file = "Field_#t.h5";
  field = "field";
  interval = 100;

A minimal example demonstrating the HDFGridDiagnostic can be downloaded here. The setup file can be found here