Re: [ctypes-users] weird "bytes missing" error for a callback definition

[Mark T. <met...@gm...>]

"Patricio Stegmann" <kp...@ho...> wrote in message 
news:SNT...@ph...l...

> Hello to all,
>
> I am doing a little wrapper for a library that performs some live image 
> acquisition.
> This library has a callback mechanism which is called when there is an 
> available live image.
>
> The headers are quite simple:
> # ------------------------
>
> DLLIMPORT SetImageCallBack(void (*ptr_ImageCallback)(int, int, char * ) );
>
> This got translated to:
> # ------------------------
>
> STRING = c_char_p
> SetImageCallBack = _stdcall_libraries['mylib.dll'].SetImageCallBack
> SetImageCallBack.restype = c_int
> SetImageCallBack.argtypes = [CFUNCTYPE(None, c_int, c_int, STRING)]
>
> However I am getting this error:
> # ------------------------
>
> ValueError: Procedure probably called with not enough arguments (4 bytes 
> missing)
>
> This is the simplified code that makes this error happen:
> # ------------------------
>
> # initialize the callbacks dictionnary
> callbacks = {}
>
> # define python function to be called by the callback
> def ViewFinderCallback(int_1, int_2, charp_1):
>         print 'viewfindercallback'
>         return 0 # return None does the same thing
>
> # declaration
> SetImageCallBack = _stdcall_libraries['mylib.dll'].SetImageCallBack
> SetImageCallBack.restype = c_int
> CallbackFunctionImageCallback = CFUNCTYPE(None, c_int, c_int, STRING)
> SetImageCallBack.argtypes = [CallbackFunctionImageCallback]
> callbacks['ViewFinderCallback'] = 
> CallbackFunctionImageCallback(self.ViewFinderCallback)
>
> # then registry of the callback
> SetImageCallBack(callbacks['ViewFinderCallback'])
>
> This gives me the error ... I dont know what is wrong here.
>
> I hope someone can help me please,

Your code is not a functional example, but I suspect from 
"self.ViewFinderCallback" that you've made your callback a member of a 
class, and there is an implied "self" parameter that is missing when your 
callback is called.

Here's a rough example.  I'm running Windows XP.

============== DLL code compiled with VS2008 "cl /LD /W4 x.c" ===========

void (*g_func)(int,int,char*) = 0;

__declspec(dllexport) int SetImageCallBack(void (*ptr_ImageCallback)(int, 
int, char * ) )
{
    g_func = ptr_ImageCallback;
 return 1;
}

__declspec(dllexport) void doit()
{
    g_func(1,2,"Hello");
}

==================== Python code ==================================

# DLLIMPORT SetImageCallBack(void (*ptr_ImageCallback)(int, int, char * ) );
import ctypes as c

STRING = c.c_char_p
dll = c.CDLL('x')
SetImageCallBack = dll.SetImageCallBack
SetImageCallBack.restype = c.c_int
CallbackFunctionImageCallback = c.CFUNCTYPE(None, c.c_int, c.c_int, STRING)
SetImageCallBack.argtypes = [CallbackFunctionImageCallback]

# initialize the callbacks dictionnary
callbacks = {}

# define python function to be called by the callback
def ViewFinderCallback(int_1, int_2, charp_1):
    print 'viewfindercallback',int_1,int_2,charp_1
    return 0 # return None does the same thing

# declaration
callbacks['ViewFinderCallback'] = 
CallbackFunctionImageCallback(ViewFinderCallback)

# then registry of the callback
SetImageCallBack(callbacks['ViewFinderCallback'])
dll.doit()

============= OUTPUT ===========
viewfindercallback 1 2 Hello
================================

-Mark