Python ctypes and function pointers

Question

This is related to my other question, but I felt like I should ask it in a new question.

Basically FLAC uses function pointers for callbacks, and to implement callbacks with ctypes, you use CFUNCTYPE to prototype them, and then you use the prototype() function to create them.

The problem I have with this is that I figured that I would create my callback function as such (I am not showing the structures that I have recreated, FLAC__Frame is a Structure):

write_callback_prototype = CFUNCTYPE(c_int, c_void_p, 
                                     POINTER(FLAC__Frame), 
                                     POINTER(c_int32), v_void_p)

The problem that I have is the implementation. FLAC__Frame is never instantiated by the programmer, it's only called from from the initialization function, and the processing functions.I have to write the callback function myself, but he problem is that I don't know how I would do this, so if anyone knows how I should do this, then some help would be greatly appreciated.

Answer 1

According to the ctypes callback docs you can define python function

def my_callback(a, p, frame, p1, p2)
    pass

and then create a pointer to a C callable function like this:

callback = write_callback_prototype(my_callback)

This function pointer can then be passed into FLAC

Answer 2

The problem that I have is the implementation. FLAC__Frame is never instantiated by the programmer, it's only called from from the initialization function, and the processing functions.I have to write the callback function myself, but he problem is that I don't know how I would do this, so if anyone knows how I should do this, then some help would be greatly appreciated.

In that case, just use:

import ctypes

class FLAC__Frame(ctypes.Structure):
    pass

and pretend that it is already defined, and do not care because you only need pointer to it, which is basically position in memory.