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);
}
...
Note that

You can also download this example as a Jupyter notebook and a plain Julia source file.

The C++ code is available as a source file and the Julia code is available as a source file.

Table of contents

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");
In file included from /opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia.h:73,
                 from /home/runner/work/Geant4.jl/Geant4.jl/docs/src/examples/G4example.cpp:18:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/gc-interface.h: In function ‘void jl_gc_wb_fresh(const void*, const void*)’:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/gc-interface.h:243:47: warning: unused parameter ‘parent’ [-Wunused-parameter]
  243 | STATIC_INLINE void jl_gc_wb_fresh(const void *parent, const void *ptr) JL_NOTSAFEPOINT {}
      |                                   ~~~~~~~~~~~~^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/gc-interface.h:243:67: warning: unused parameter ‘ptr’ [-Wunused-parameter]
  243 | STATIC_INLINE void jl_gc_wb_fresh(const void *parent, const void *ptr) JL_NOTSAFEPOINT {}
      |                                                       ~~~~~~~~~~~~^~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/gc-interface.h: In function ‘void jl_gc_wb_knownold(const void*, const void*)’:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/gc-interface.h:246:50: warning: unused parameter ‘parent’ [-Wunused-parameter]
  246 | STATIC_INLINE void jl_gc_wb_knownold(const void *parent, const void *ptr) JL_NOTSAFEPOINT {}
      |                                      ~~~~~~~~~~~~^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/gc-interface.h:246:70: warning: unused parameter ‘ptr’ [-Wunused-parameter]
  246 | STATIC_INLINE void jl_gc_wb_knownold(const void *parent, const void *ptr) JL_NOTSAFEPOINT {}
      |                                                          ~~~~~~~~~~~~^~~
In file included from /opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia.h:2446:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h: In constructor ‘jl_unique_gcsafe_lock::jl_unique_gcsafe_lock(std::mutex&)’:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h:115:48: warning: declaration of ‘native’ shadows a member of ‘jl_unique_gcsafe_lock’ [-Wshadow]
  115 |     explicit jl_unique_gcsafe_lock(std::mutex &native) JL_NOTSAFEPOINT_ENTER
      |                                    ~~~~~~~~~~~~^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h:114:34: note: shadowed declaration is here
  114 |     std::unique_lock<std::mutex> native;
      |                                  ^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h: In constructor ‘jl_unique_gcsafe_lock::jl_unique_gcsafe_lock(std::mutex&)’:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h:115:48: warning: declaration of ‘native’ shadows a member of ‘jl_unique_gcsafe_lock’ [-Wshadow]
  115 |     explicit jl_unique_gcsafe_lock(std::mutex &native) JL_NOTSAFEPOINT_ENTER
      |                                    ~~~~~~~~~~~~^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h:114:34: note: shadowed declaration is here
  114 |     std::unique_lock<std::mutex> native;
      |                                  ^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h: In constructor ‘jl_unique_gcsafe_lock::jl_unique_gcsafe_lock(std::mutex&)’:
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h:115:48: warning: declaration of ‘native’ shadows a member of ‘jl_unique_gcsafe_lock’ [-Wshadow]
  115 |     explicit jl_unique_gcsafe_lock(std::mutex &native) JL_NOTSAFEPOINT_ENTER
      |                                    ~~~~~~~~~~~~^~~~~~
/opt/hostedtoolcache/julia/1.12.2/x64/include/julia/julia_locks.h:114:34: note: shadowed declaration is here
  114 |     std::unique_lock<std::mutex> native;
      |                                  ^~~~~~

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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