Callback with Custom Types in Python ctypes

Question

I'm trying to wrap a C library in Python using ctypes. A function call requires a callback function which I implemented using the documentation. The problem is that the callback function is expecting custom objects from the library. Here is the code in C I'm trying to copy in python

void outputCallback(const A* a, void* b) {
    //
}

a = function1(0, 0, outputCallback, 0, 0)

The structure definition for A in the header file is:

typedef struct A
{
     const unsigned char* a1;
     unsigned int a2;
} A;

and my attempt at the Python equivalent.

class A(Structure):
    _fields_ = [
       ("a1", ?, ?),
       ("a2", c_int, 16)]

class Callback():
    def outputCallback(self, a):
        print a.a2
        return 1

cb = Callback() 

CMPFUNC = CFUNCTYPE(c_int, POINTER(A))

cb.cmp_func = CMPFUNC(cb.outputCallback)

cdll.LoadLibrary("library.so")
libc = CDLL("library.so")

a = libc.function1(0, 0, cb.cmp_func, 0, 0)

The reason I wrapped the callback in a class is because of this post. Basically it was my attempt at keeping this callback from being garbage collected.

Thanks for any help you can offer.

score 4 · Accepted Answer · 2013-03-27 20:27:14Z

4

There are a few errors there, and some are basic Python errors:

from ctypes import *

class A(Structure):
    _fields_ = [
       ("a1", c_char_p),
       ("a2", c_int)]


class Callback(object):
    def outputCallback(self, a, b):         # outputCallback(): ?
        print a.contents.a1, a.contents.a2
                                            # The prototype of the `outputCallback`
                                            # tells that it returns nothing, `void`
cb = Callback()
CMPFUNC     = CFUNCTYPE(None, POINTER(A), c_void_p) # so `restype` shoud be
                                                    # None, void

cb.cmp_func = CMPFUNC(cb.outputCallback)

libc = CDLL('library.so')
libc.function1(0, 0, cb.cmp_func, 0, 0)

The testing DLL:

#include <stdio.h>

#ifdef _WIN32
#define DLLEXPORT __declspec(dllexport) 
#else
#define DLLEXPORT
#endif

#ifdef __cplusplus
extern "C" {
#endif

typedef struct A
{
     const unsigned char* a1;
     unsigned int a2;
} A;

int DLLEXPORT function1(int a,
                        int b,
                        void (*outputCallback)(const A* a, void* b),
                        int c,
                        int d)
{
    A obj;

    obj.a1 = "Hello";
    obj.a2 = 5;

    outputCallback(&obj, NULL);
    return 0;
}

#ifdef __cplusplus
};
#endif

Test:

>gcc library.c -o library.so -shared

>python py.py
Hello 5

>

edited Mar 27, 2013 at 20:27

answered Mar 27, 2013 at 19:45

user1129665

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1 Comment

dvreed77 Over a year ago

Thanks, but I'm not sure this is exactly what I wanted. I updated the post with a Struct definition for A I found in the header file. I apologize if I'm not being very clear.

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