Assigning Python function to ctypes pointer variable

Question

I have the following C source that I compile into a DLL:

int (*pfuncExtB)(int a, int b);

int funcB(int a, int b)
{
    return funcExtB(a, b);
}

int funcExtB(int a, int b)
{
    return pfuncExtB(a, b);
}

What I want to do is to make pfuncExtB "point" to a Python function, so this is what I do in Python:

from ctypes import *

def add(a, b):
    return a + b

mutdll = cdll.LoadLibrary("my.dll")

pfuncExtB = (POINTER(CFUNCTYPE(c_int, c_int, c_int))).in_dll(mutdll, 'pfuncExtB')

funcB = mutdll.funcB
funcB.argtypes = [c_int, c_int]
funcB.restype = c_int

pfuncExtB.contents = CFUNCTYPE(c_int, c_int, c_int)(add)

print funcB(3 , 4)

After this I expect that the following call to return 7

print funcB(3, 4)

But I get:

Traceback (most recent call last):
..................
print funcB(3, 4)
WindowsError: exception: access violation reading 0x00000001

So what am I doing wrong here? Is it possible to have a Python function "assigned" to a ctypes pointer-to-function variable?

Edit: After seeing Mark Tolonen's workaround (a set function for the pointer to function variable written in C), I found why it didn't work for me when I tried it.

This does not work:

set(CFUNCTYPE(c_int,c_int,c_int)(add))
print funcB(2, 3)

While this works:

callback = CFUNCTYPE(c_int,c_int,c_int)(add)
set(callback)
print funcB(2, 3)

Where set is a C function that assigns the pointer-to-function argument to the global like in Mark's answer. As he pointed out the answer is lays in the docs:

Important note for callback functions: Make sure you keep references to CFUNCTYPE() objects as long as they are used from C code. ctypes doesn’t, and if you don’t, they may be garbage collected, crashing your program when a callback is made.

possible duplicate of Python ctypes and function pointers

Martijn Pieters
– Martijn Pieters

2013-03-01 14:32:14 +00:00
Commented Mar 1, 2013 at 14:32 — Martijn Pieters
– Martijn Pieters, Commented Mar 1, 2013 at 14:32

Mark Tolonen · Accepted Answer · 2013-03-02 05:56:47Z

1

The correct type of the global variable in Python is CFUNCTYPE(c_int,c_int,c_int) (no POINTER()), but I don't see a method to change the value of the variable once you have it. If you can add a set function it can work:

C

typedef int (*FUNC)(int,int);

__declspec(dllexport) FUNC pfuncExtB;

__declspec(dllexport) void set(FUNC f)
{
    pfuncExtB = f;
}

int funcExtB(int a, int b)
{
    return pfuncExtB(a, b);
}

__declspec(dllexport) int funcB(int a, int b)
{
    return funcExtB(a, b);
}

Python

from ctypes import *

FUNC = CFUNCTYPE(c_int,c_int,c_int)

@FUNC
def add(a, b):
    return a + b

mutdll = cdll.LoadLibrary('x')

mutdll.set.argtypes = [FUNC]
mutdll.set.restype = None

mutdll.set(add) # set the global variable

pfuncExtB = FUNC.in_dll(mutdll,'pfuncExtB')
print(pfuncExtB(1,2)) # -> 3

funcB = mutdll.funcB
funcB.argtypes = [c_int, c_int]
funcB.restype = c_int

print(funcB(3 , 4)) # -> 7

Note that this does not work:

pfuncExtB = POINTER(FUNC).in_dll(mutdll,'pfuncExtB')
pfuncExtB.contents(1,2) # exception!

edited Mar 2, 2013 at 5:56

answered Mar 1, 2013 at 16:57

Mark Tolonen

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2 Comments

vanem Over a year ago

Thanks for your input. As a matter of fact I considered your workaround myself, but I didn't worked for me! I tried again after seeing your answer and found the problem with my "set" function, see my edit

Mark Tolonen Over a year ago

Your original set version didn't work because the CFUNCTYPE object passed to it has no reference after the set call completes, so it is destroyed. Assigning it to callback in your second version keeps a reference to it. See the last paragraph of section 16.17.1.17 in the docs.

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