Commit 3afef05f authored by Dmitry Romanov's avatar Dmitry Romanov
Browse files

Merge branch 'drateots-master-patch-35728' into 'master'

Update docs/howto/full_simulation.rst

See merge request !6
parents 330333ca fb6d3177
......@@ -103,4 +103,36 @@ One can use dd_web_display to actually just save root geometry
.. code:: bash
dd_web_display --export athena.xml # will create a .root file with the geometry
\ No newline at end of file
dd_web_display --export athena.xml # will create a .root file with the geometry
FAQ
---
> How does the XML file in the compact directory know which c++ file to load when we include the respective XML file in the main athena.xml file?
1. Xml compact files has `<detector ...></detector>` tag which has `type` attribute that tells which C++ type to use:
.. code:: xml
<detector id="ForwardRICH_ID" name="DRICH" type="athena_DRICH" ... >
2. C++ file usually has `createDetector(...)` function and `DECLARE_DETELEMENT` macro which binds the function to the name used in xml `<detector>` type attribute. C++ code looks like this:
.. code:: c++
static Ref_t createDetector(Detector& desc, xml::Handle_t handle, SensitiveDetector sens) {
// ...
}
DECLARE_DETELEMENT(athena_DRICH, createDetector)
3. How DD4Hep finds and loads compiled components?
> This is going into technical details which users usually don't need. Installation paths and environment variables are set by container/spack and should work out of the box.
DD4Hep uses modular plugin mechanism to load C++ code. When athena C++ is compiled, two files are created:
- `athena.components` - a text file stating what components one can find in athena.so. From our example, there will be a record like `v2:libathena.so:athena_DRICH` among other records.
- `libathena.so` - compiled C++ library with all detectors from athena repo
So when the type of the detector is given, like `athena_DRICH`. DD4Hep uses `LD_LIBRARY_PATH` to look through .components files and then figures out what .so file to load to get the correct C++ code executed.
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