python -> C++ callback

pycallback

#this is the variable that will hold a reference to the python function
PyObject *py_callback;

#the following function will invoked from python to populate the call back reference
PyObject *set_py_callback(PyObject *callable)
{
    py_callback = callable;       /* Remember new callback */
    return Py_None;
}
...
#Initialize and acquire the global interpreter lock
PyEval_InitThreads();

#Ensure that the current thread is ready to call the Python C API 
PyGILState_STATE state = PyGILState_Ensure();

#invoke the python function
boost::python::call<void>(py_callback);

#release the global interpreter lock so other threads can resume execution
PyGILState_Release(state);