[Swig-user] Fw: Using swig and C++ callbacks into Python

[Filipe C. <fi...@gm...>]

Hi all!

I'm using a C++ library with Python bindings (lldb), and I'm having some trouble with callbacks.

I implemented typemaps and a helper function, in order to be able to use a Python callable object as a callback in the C++ library. My problem is when Python code calls C++ code that then calls a Python callback.

For reference, I have these type maps:
-------------------------------------------------

// For SBDebugger::DispatchInput (the function that will call the callback function)
%typemap(in) (const void *data, size_t data_len) {
if (PyString_Check($input)) {
$1 = static_cast<void *>(PyString_AsString($input));
$2 = PyString_Size($input);
}
else if(PyByteArray_Check($input)) {
$1 = static_cast<void *>(PyByteArray_AsString($input));
$2 = PyByteArray_Size($input);
}
else {
PyErr_SetString(PyExc_ValueError, "Expecting a string or byte array");
return NULL;
}
}

// For lldb::SBInputReader::Callback
lldb::SBInputReader::Callback LLDBSwigPythonCallSBInputReaderCallback;
%typemap(in) (lldb::SBInputReader::Callback callback, void *callback_baton) {
if (!PyCallable_Check(reinterpret_cast<PyObject*>($input))) {
PyErr_SetString(PyExc_TypeError, "Need a callable object!");
return NULL;
}
$1 = LLDBSwigPythonCallSBInputReaderCallback;
$2 = $input;
}

%typemap(typecheck) (lldb::SBInputReader::Callback callback, void *baton) {
if (!PyCallable_Check(reinterpret_cast<PyObject*>($input))) {
$1 = 0;
} else {
$1 = 1;
}
}

-------------------------------------------------

And this helper function:
-------------------------------------------------

// For the InputReader Callback functions
%{
size_t LLDBSwigPythonCallSBInputReaderCallback(void *baton,
lldb::SBInputReader *reader,
lldb::InputReaderAction notification,
const char*bytes,
size_t bytes_len) {
SWIG_PYTHON_THREAD_BEGIN_BLOCK;




PyErr_Clear();


PyObject *res = PyObject_CallFunction(reinterpret_cast<PyObject*>(baton),
// s# converts to char * + Py_ssize_t
const_cast<char*>("Ois#"),
SWIG_NewPointerObj(reader, SWIGTYPE_p_lldb__SBInputReader, false),
notification,
bytes,
bytes_len);
PyObject *exc = PyErr_Occurred();
if (exc) {
::puts("Erroring out at LLDBSwigPythonCallSBInputReaderCallback\n");
return 0;
}
size_t result = static_cast<size_t>(PyInt_AsSsize_t(res));
SWIG_PYTHON_THREAD_END_BLOCK;
return result;
}
%}

-------------------------------------------------



I even tried adding an exception block, but that was a no-op (the cpp file generated by swig didn't change):
-------------------------------------------------

%exception lldb::SBDebugger::DispatchInput(const void *data, size_t data_len) {
$action
if (PyErr_Occurred())
return NULL;
}


-------------------------------------------------



Am I doing anything wrong? What can I do to try and signal an error when returning from DispatchInput, if an error was signaled in the callback?

Thanks in advance,

Filipe