Have C library call python function with ctypes

Question

I have a DLL in c and meant for windows platforms which has a structure similar to the following:

C Structure

typedef struct some_struct {
 int (_stdcall *function)(int arg1, int arg2);
 ...
}SOME_STRUCT;

I've defined a python ctypes structure to mimic this with the following

Python Structure

class SOME_STRUCT(Structure):
 _fields_ = [('function', POINTER(CFUNCTYPE(c_int, c_int, c_int))), ...]

The point of this structure in the C code is to register a callback function that gets executed upon certain triggers in its own thread. What I want to be able to do, if possible, is set that callback to be a Python function such that when the function in the C structure gets called from the C code, it is the python function that gets executed.

What I've got in python to try and accomplish this (which doesn't work) is the following:

def func(arg1,arg2):
 print('I was called!')
 return 0
struct = SOME_STRUCT()
prototype = CFUNCTYPE(c_int, c_int, c_int)
struct.function = byref(prototype(func))

The specific error I get (that may not be my only issue) is that it complains that struct.function was expecting a LP_CFunctionType instance but got a CArgObject instance. How can I do what I'm trying to do?

• Paul Cornelius: The example in the ctypes docs docs.python.org/3/library/ctypes.html#callback-functions doesn't use the call to byref as you are doing. Does it work if you remove the byref call and just do struct.function = prototype(func)?
• zephyr: @PaulCornelius I tried doing that but the function never seems to get called then. I never see an output from it. I was trying to follow the example you pointed out, but I believe I need byref or something similar since I'm trying to assign a pointer to a function rather than a function itself (which the example is doing).
• Paul Cornelius: In that case I have one more thought. If CFUNCTYPE returns an object that represents a pointer to a function, maybe you don't want the POINTER() function call in your structure definition.

Answer 1

Here's a working example and test DLL source. Oddly, I couldn't get it to work when the callback was the only member of the struct (crash). It seemed like a bug, because a callback without a struct wrapper or adding a second member to the struct made it work.

Things to note:

• Use WINFUNCTYPE with __stdcall. CFUNCTYPE is for __cdecl.
• You don't need POINTER or byref to make it work.
• The @CALLBACK decorator is equivalent to func = CALLBACK(func).

test.c

#include <stdio.h>
typedef int (__stdcall *CALLBACK)(int arg1, int arg2);
typedef struct some_struct {
 CALLBACK function;
 int other;
} SOME_STRUCT;
__declspec(dllexport) int func(SOME_STRUCT* pss)
{
 printf("%d\n",pss->other);
 return pss->function(1,2);
}

test.py

from ctypes import *
CALLBACK = WINFUNCTYPE(c_int,c_int,c_int)
class SOME_STRUCT(Structure):
 _fields_ = [('function', CALLBACK),
 ('other', c_int)]
@CALLBACK
def callback(arg1,arg2):
 return arg1 + arg2
dll = CDLL('test')
dll.argtypes = POINTER(SOME_STRUCT),
dll.restype = c_int
struct = SOME_STRUCT(callback,7)
print(dll.func(byref(struct)))

Output

7
3

• zephyr: Thanks so much. This has been very useful. One minor thing though, I had to do dll.func(byref(struct)) (last line of test.py) to make the call work.
• Mark Tolonen: @zephyr It worked without it for me but that is more technically correct. Did you have the final comma in argtypes? It must be a sequence and that makes it a tuple.
• zephyr: Yes, I did. In fact I copied and pasted your two programs and they worked just fine with the exception of the need for the byref. Without it, I get an OSError saying an access violation when writing to some random memory address.
• Mark Tolonen: @zephyr 32-bit or 64-bit Python? I'm just wondering why it worked for me and looking for the difference. I was using 64-bit and am guessing the marshaling of a struct is always by pointer in that ABI. It might also explain why I crashed when there was only one structure member. I'll have to go back and try that fix.
• zephyr: I'm using 32-bit.