From fb6d3177b5da2afe3d84ed262a7f106dd83d4f65 Mon Sep 17 00:00:00 2001
From: Dmitry Romanov <romanov@jlab.org>
Date: Thu, 10 Mar 2022 05:35:41 +0000
Subject: [PATCH] Update docs/howto/full_simulation.rst
---
docs/howto/full_simulation.rst | 34 +++++++++++++++++++++++++++++++++-
1 file changed, 33 insertions(+), 1 deletion(-)
diff --git a/docs/howto/full_simulation.rst b/docs/howto/full_simulation.rst
index 2cee93a..a2ede96 100644
--- a/docs/howto/full_simulation.rst
+++ b/docs/howto/full_simulation.rst
@@ -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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