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The problem is in poorly designed callback. A good callback should at least pass object pointer to callback function, or user data associated with it in the best case. In that cases there wouldn't be any problems. This problem is similar to passing a pointer to object method as callback. The only way I see is to use a platform-dependant hack. For 32 bit Windows I can tell you how to do it. It should be easy to tweak it to use it on 32 bit Linux.

----- Original Message ----- From: Jiří Prymus
To: Lua list
Sent: Saturday, October 17, 2009 11:22 PM
Subject: Re: Conversion Lua function to C function

Thanks for answers.

I'm little confused from yours replies but I try to explain my problem clearly.
I need to pass lua function to C++ wrapper function where it will be
used as "update event" of GtkTrackBar widget.
My wrapper is compiled as shared library for lua.

Maybe that sample code make this problem clear.

C++ wrapper function -----------------

static int luacv_cvCreateTrackbar(lua_State *L)
 const char *trackbar_name=luaL_checkstring(L,1);
 const char *window_name=luaL_checkstring(L,2);
 int value=luaL_checkint(L,3);
 int count=luaL_checkint(L,4);
 CvTrackbarCallback on_change=NULL;

 if (lua_isfunction(L,5))
   luaL_error(L,"luacv.cvCreateTrackbar(string trackbar_name,string
window_name,int value,int count, void func(int pos))");


 return 0;

Type CvTrackbarCallback is defined like
typedef void (CV_CDECL *CvTrackbarCallback)(int pos);

As you can see , the "on_change" variable  is pointer to c function
with int as argument.

Lua script may be look like this


function on_trackbar(pos) some stuff here..


So I can get pointer of lua function "on_trackbar" via stack in
wrapper , but when I want to convert it to c function, it doesn't
No segmentation fault, but it does nothing.

I tried functions lua_tolightuserdata, lua_touserdata, lua_tocfunction
but none of them worked.

Thanks for answer and sorry for my english.

With regards,
Jiri Prymus

2009/10/17 Asko Kauppi <>:
What he needs to do is store the Lua function entry as such (make a copy,
forget about C pointers) in the Lua registry or a userdata's environment.
This is a common need and many source codes will carry samples. Then have a
C side common callback which picks up that stored ref to the Lua function
and executes it. I've made a helper for all this but that code is not

- asko

On 17.10.2009, at 21.03, Wesley Smith <> wrote:

I don't completely follow, but you can store arbitrary function pointers
as lightuserdata.


On Oct 17, 2009, at 1:47 AM, Jiří Prymus <> wrote:


I would like to know if there is any chance to convert lua function to
c function. I program a small opencv wrapper and there is one wrapping
function where I need pointer to function. And that's a problem
because I can get only pointer to Lua function and when I convert it
to C function then it does nothing. I know that lua functions are
stored in tables and that's probably why it doesn't work.

In fact my problem is function gtk_signal_connect(...) where slot
function is my lua function.

Thanks for answer.

With regards,
Jiri Prymus