In Python, how to pass a callback array as arguments to a C function using ctypes?

Question

This is my problem, I have a legacy library (.so) written in C with APIs like this:

typedef void (*CALLBACK)( void);
typedef CALLBACK CALLBACK_TBL[ 5 ];
void init(CALLBACK_TBL callbackTbl)
{
        T_MYCALLBACK *myCallback1 = (T_MYCALLBACK *)(callbackTbl[0]);
        if (myCallback1 )
        {
            myCallback1(2,3);
        }
}

Of course, because it is a legacy library, I cannot change the API signature.

Now From Python, I am trying to call init with callback defined into python:

CallbackType1 = ctypes.CFUNCTYPE(None, c_ulong, c_ulong)
CallbackType2 = ctypes.CFUNCTYPE(None, c_ubyte, c_ubyte)
...
CallbackType5 = ctypes.CFUNCTYPE(None, c_int32, c_int32)


def callback1(long1, long2):
    print("callback1")

def callback2(bool1, bool2):
    print("callback2")
...
def callback5(int1, int2):
    print("callback5")

But I am not able to understand how am I supposed to make such an array of callbacks:

_callback1 = CallbackType1(callback1)
_callback2 = CallbackType1(callback2)
...
_callback5 = CallbackType1(callback5)

lib = CDLL("lib.so")
lib.init(....) ?????

Does somebody have an idea ?

Answer 1

A working minimal example would be nice. I've created one below but if it doesn't work for you update your question with a similar DLL example that matches your situation:

test.cpp

typedef void (*CALLBACK)(); // generic
typedef void (*CALLBACK1)(long,long);
typedef void (*CALLBACK2)(bool,bool);
typedef void (*CALLBACK3)(int,int,int);

typedef CALLBACK CALLBACK_TBL[3];

extern "C" __declspec(dllexport)
void init(CALLBACK_TBL callbackTbl)
{
    ((CALLBACK1)callbackTbl[0])(1L,2L);
    ((CALLBACK2)callbackTbl[1])(true,false);
    ((CALLBACK3)callbackTbl[2])(1,2,3);
}

test.py

from ctypes import *

CALLBACK1 = CFUNCTYPE(None,c_long,c_long)
CALLBACK2 = CFUNCTYPE(None,c_bool,c_bool)
CALLBACK3 = CFUNCTYPE(None,c_int,c_int,c_int)

class CALLBACK_TBL(Structure):
    _fields_ = [('cb1',CALLBACK1),
                ('cb2',CALLBACK2),
                ('cb3',CALLBACK3)]

@CALLBACK1
def callback1(a,b):
    print('callback1',a,b)

@CALLBACK2
def callback2(a,b):
    print('callback2',a,b)

@CALLBACK3
def callback3(a,b,c):
    print('callback3',a,b,c)

cbt = CALLBACK_TBL(callback1,callback2,callback3)

dll = CDLL('./test')
dll.init.argtypes = CALLBACK_TBL,
dll.init.restype = None

dll.init(cbt)

Output:

callback1 1 2
callback2 True False
callback3 1 2 3

Answer 2

Listing [Python 3.Docs]: ctypes - A foreign function library for Python .

The simplest thing is to take the C code as it is (without thinking too much about it) and convert it to Python :

 >>> import ctypes as ct >>> >>> Callback = ct.CFUNCTYPE(None) # Generic callback type - might be a ct.c_void_p as well >>> CallbackArray = Callback * 5 >>> >>> Callback1 = ct.CFUNCTYPE(None, ct.c_ulong, ct.c_ulong) >>> def func1(long1, long2): pass ... >>> callback1 = Callback1(func1) >>> >>> # The rest of Callback# func# and callback# >>> >>> callback_array = CallbackArray() >>> callback_array <__main__.CFunctionType_Array_5 object at 0x000001517253CA48> >>> callback_array[0] <CFunctionType object at 0x000001517240BBA8> >>> >>> callback_array[0] = ct.cast(callback1, Callback) # !!! Conversion needed !!! >>> callback_array[0] <CFunctionType object at 0x000001517240BE18> >>> # Same thing for callback_array[1] .. callback_array[4]

And the rest of the code (I didn't paste it in the console as it wouldn't work):

lib = ct.CDLL("./lib.so")

lib.init.argtypes = [CallbackArray]  # Check the URL at the end

lib.init(callback_array)

Always define argtypes (and restype ) for functions imported from .dll s. Check [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for more details.