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****
Null
****
The ``Null`` Engine performs no internal work and no I/O. It was created for testing applications that have ADIOS2 output in it by turning off the I/O easily. The runtime difference between a run with the Null engine and another engine tells us the IO overhead of that particular output with that particular engine.
.. code-block:: c++
adios2::IO io = adios.DeclareIO("Output");
io.SetEngine("Null");
or using the XML config file:
.. code-block:: xml
<adios-config>
<io name="Output">
<engine type="Null">
</engine>
</io>
</adios-config>
Although there is a reading engine as well, which will not fail, any variable/attribute inquiry returns `nullptr` and any subsequent Get() calls will throw an exception in C++/Python or return an error in C/Fortran.
Note that there is also a `Null transport` that can be used by a BP engine instead of the default `File transport`. In that case, the BP engine will perform all internal work including buffering and aggregation but no data will be output at all. A run like this can be used to assess the overhead of the internal work of the BP engine.
.. code-block:: c++
adios2::IO io = adios.DeclareIO("Output");
io.SetEngine("BP5");
io.AddTransport("Null", {});
or using the XML config file
.. code-block:: xml
<adios-config>
<io name="Output">
<engine type="BP5">
</engine>
<transport type="Null">
</transport>
</io>
</adios-config>
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