Calling G4 actions in Julia

This is a very simple example of calling user actions in Julia from a C++ Geant4 application. We define the user actions in Julia language in the file MyCode.jl and call them from the C++ application. The name and signatures of the functions are important since the C++ will associate them in the corresponding inherited classes.

The C++ code is a single file G4example.cpp that defines the Geant4 the minimal set of classes to run a simulation.

• The main program is responsible of initializing Julia by calling julia_init and loading the Julia code executing.

  jl_init()
  jl_eval_string("include(\"MyCode.jl\")");

• Each constructor of a user action class needs to initialize a C++ function pointer to the corresponding Julia function. This is done in the constructor to avoid any dynamic dispatch at runtime. For example, for the EventAction class:

  typedef  void (*eventaction_f)(const G4Event*);
  class EventAction : public G4UserEventAction {
    public:
      EventAction() {
          beginevent_jl = (eventaction_f)(jl_unbox_voidpointer(jl_eval_string("@cfunction(begin_of_event_action, Nothing, (CxxPtr{G4Event},))")));
          endevent_jl = (eventaction_f)(jl_unbox_voidpointer(jl_eval_string("@cfunction(end_of_event_action, Nothing, (CxxPtr{G4Event},))")));
      }
      ...
    private:
      eventaction_f beginevent_jl;
      eventaction_f endevent_jl;
  };

• Finally the actions are called in the corresponding Geant4 classes. For example in the EventAction class:

  void EventAction::BeginOfEventAction(const G4Event* event) {
      beginevent_jl(event);
  }
  ...

Table of contents

• Loading the necessary Julia modules
• Building G4Example Application
• Run the application
• Display the results

Loading the necessary Julia modules

using Geant4_jll   # Needed to locate the Geant4 installation directory

Building G4Example Application

The custom library is defined in the C++ file G4example.cpp. It is a single file to facilitate the building of the executable.

The attribute Geant4_jll.artifact_dir provides the path to the Geant4 installation directory. Sources are in the same location as this script.

cd(@__DIR__)
g4prefix = Geant4_jll.artifact_dir
jlprefix = dirname(Sys.BINDIR);

We use the executables geant4-config and julia-config.jl to get the needed libraries and compiler/linker flags.

g4libs = read(`$g4prefix/bin/geant4-config --libs`, String) |> split
filter!(x -> x != "-lG4gdml", g4libs)
jllibs = read(`$jlprefix/share/julia/julia-config.jl --ldlibs`, String) |> split
append!(jllibs, ["-L$jlprefix/lib"])
cflags = read(`$g4prefix/bin/geant4-config --cflags`, String) |> split
ldflags = ["-Wl,-rpath,$g4prefix/lib", "-Wl,-rpath,$jlprefix/lib"];
Sys.KERNEL == :Linux  && append!(ldflags, ["-Wl,--no-as-needed"]);

Run the compilation and link command

Base.run(`c++ -O2 -fPIC $cflags -I$jlprefix/include/julia $ldflags $g4libs $jllibs
          -o G4example.exe $(@__DIR__)/G4example.cpp`).exitcode == 0 || error("Compilation failed");

Run the application

We need to set the variable JULIA_PROJECT pointing to correctly setup Julia environment.

withenv("JULIA_PROJECT" => "@.", "G4NUMTHREADS" => "4") do
   Base.run(`./G4example.exe`).exitcode == 0 || error("Execution failed")
end

true

Display the results

println("=====> The file edepHist.png should have been saved")

=====> The file edepHist.png should have been saved

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