Swig pass argument to python callback function

Question

So I'm almost done. Now I have a working code that calls the python callback function.

Now all I have to do is pass the argument to the python callback function.

My callback.c:

#include <stdio.h>

typedef void (*CALLBACK)(void);
CALLBACK my_callback = 0;

void set_callback(CALLBACK c);
void test(void);

void set_callback(CALLBACK c) {
  my_callback = c;
}

void test(void) {
  printf("Testing the callback function\n");
  if (my_callback) (*my_callback)();
  else printf("No callback registered\n");
}

My callback.i:

// An entirely different mechanism for handling a callback

%module callback
%{
typedef void (*CALLBACK)(void);
extern CALLBACK my_callback;

extern void set_callback(CALLBACK c);
extern void my_set_callback(PyObject *PyFunc);

extern void test(void);
%}

extern CALLBACK my_callback;

extern void set_callback(CALLBACK c);
extern void my_set_callback(PyObject *PyFunc);

extern void test(void);

%{
static PyObject *my_pycallback = NULL;
static void PythonCallBack(void)
{
   PyObject *func, *arglist;
   PyObject *result;

   func = my_pycallback;     /* This is the function .... */
   arglist = Py_BuildValue("()");  /* No arguments needed */
   result =  PyEval_CallObject(func, arglist);
   Py_DECREF(arglist);
   Py_XDECREF(result);
   return /*void*/;
}

void my_set_callback(PyObject *PyFunc)
{
    Py_XDECREF(my_pycallback);          /* Dispose of previous callback */
    Py_XINCREF(PyFunc);         /* Add a reference to new callback */
    my_pycallback = PyFunc;         /* Remember new callback */
    set_callback(PythonCallBack);
}

%}

%typemap(python, in) PyObject *PyFunc {
  if (!PyCallable_Check($input)) {
      PyErr_SetString(PyExc_TypeError, "Need a callable object!");
      return NULL;
  }
  $1 = $input;
}

It works well. What should I do, can I pass an argument my_callback? Any help would be greatly appreciated!

Answer 1

Callback arguments are the second argument PyEval_CallObject(). Right now you are creating an empty tuple, which means "no arguments". So change that. Where are you doing now:

arglist = Py_BuildValue("()");  /* No arguments needed */

Py_BuildValue , Python. , , Python, -, :

arglist = Py_BuildValue("(isO)", the_int, the_str, the_pyobject);