test_python_callback.cc

// Author: Pranav Sharma
#include <pybind11/pybind11.h>
#include <pybind11/functional.h>
#include <thread>
#include <iostream>
#include <chrono>

#define STRINGIFY(x) #x
#define MACRO_STRINGIFY(x) STRINGIFY(x)

namespace py = pybind11;

#include <pybind11/pybind11.h>

using PyCallback = std::function<void()>;

using CppCallback = std::function<void(void*)>;

struct UserData {
  PyCallback py_callback;
};

void Callback(void* user_data) {
  std::cout << "Inside CPP callback\n";
  UserData* user_data_holder = reinterpret_cast<UserData*>(user_data);
  PyGILState_STATE gstate;
  gstate = PyGILState_Ensure();  
  user_data_holder->py_callback();
  PyGILState_Release(gstate);
}
  
  void RunAsync(CppCallback cpp_callback, void* user_data) {
    std::thread([=]()
    {
      std::cout << "Inside thread..sleeping\n";
      std::this_thread::sleep_for(std::chrono::milliseconds(5000));
      std::cout << "Inside thread...done sleeping, calling cpp callback\n";
      if (cpp_callback) {
	cpp_callback(user_data);
      }
    }).join();
  }

PYBIND11_MODULE(test_python_callback, m) {
  m.def("run_async", [](PyCallback py_callback)->void{
    auto user_data_holder = new UserData();
    user_data_holder->py_callback = py_callback;
    {
      py::gil_scoped_release release;
      RunAsync(Callback, user_data_holder);
    }
  });

#ifdef VERSION_INFO
  m.attr("__version__") = MACRO_STRINGIFY(VERSION_INFO);
#else
  m.attr("__version__") = "dev";
#endif
}