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A tool to convert Geant4 geometries into shared libraries

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PyPI version

This is g42so, a simple Python script that compiles Geant4 detector-construction and primary-generator-action classes, wraps them in C-style (unmangled) adapter functions and produces a shared library. The library can then be loaded via dlopen by third-party code, allowing instantiation of a Geant4 detector construction or primary generator action even if the name of the specific class is not known in advance.

Installing g42so

The recommended installation procedure for g42so is to use pip:

$ pip install g42so

Quick tutorial

Compilation is delegated to g++ by default (no, I did not reimplement gcc from scratch), but this can be configured using the -c option. Note however that g42so must know what compilation flags to use for your compiler; at the moment, it only recognizes g++ and clang++.

Running g42so -h gives a list of available options. You will typically want to run something along the lines of the following:

$ g42so -I /path/to/include/ \
   /path/to/src/ /path/to/src/


  • Geant4 must be installed and the geant4-config script must be in the $PATH, or its location can be specified with the --geant4-config option.

  • g42so tries to automatically detect the names of the classes you want to wrap. If it fails or is unsure, it will ask you to explicitly specify your class names.

  • If your class constructors require arguments, you will need to customise the C wrappers generated by the code. First run

    $ g42so --dump-detector-wrapper >

    to dump the relevant wrapper. Modify it to your needs and pass it to g42so along with the --custom-detector-wrapper option, which tells g42so not to include its default wrapper. For instance:

    $ g42so --custom-detector-wrapper -I /path/to/include/ \

    The --dump-pga-wrapper and --custom-pga-wrapper do the same job for primary-generator-action classes.

  • Note that dlopening shared libraries that link to Geant4 is likely to fail if Geant4 was compiled is multi-threaded mode (the GEANT4_BUILD_MULTITHREADED CMake flag is set to ON) with the default value for the GEANT4_BUILD_TLS_MODEL (the default is initial-exec). If you want to dlopen the libraries created by g42so (which is the whole point of this tool, really!), then you should make sure that either Geant4 was compiled in single-threaded mode, or GEANT4_BUILD_TLS_MODEL was set to global-dynamic.

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