OpenEuphoria

RE: Callback problem

• Posted by Andy Serpa <ac at onehorseshy.com> Oct 13, 2002
• 461 views

stabmaster_ at hotmail.com wrote:
> >This sounds like the cdecl-stdcall problem.
> >...
> >There probably is a way to fix it by poking assembly.
> >
> >Karl Bochert
> 
> 
> I did just that in Glue, my GLUT wrapper for Euphoria. In glue.e there 
> is a 
> function called cdecl_callback that takes a routine_id and a sequence of 
> 
> parameter sizes (in bytes). E.g. if you have a function foo that takes 3 
> 
> integer parameters, you'd do something like:
> 
> atom my_callback
> my_callback = cdecl_callback(routine_id("foo"),{4,4,4,4})
> 
> Note that i passed 4 4's, this is intentionally. You see, there's a 
> small 
> problem: Since EIP is pushed on the stack after the parameters, you'll 
> get 
> the return address in the first parameter of your function. To get 
> around 
> this you must prepend a dummy argument to your function. I.e. if you 
> had:
> 
> function foo(integer a,integer b,integer c)
> 
> You'd have to change it to:
> 
> function foo(integer dummy,integer a,integer b,integer b)
> 
> 

It works!

Here is the basic assembler your routine generates:

    #B8,#00,#00,#00,#00,    --    0: mov eax,callback (1)
    #FF,#D0,                --    5: call near eax
    #83,#EC,#00,            --    7: sub esp, 0
    #C3                     --    A: ret


So, you replace the "#00,#00,#00,#00" with the 4-byte call_back() 
address of your routine, and then replace the #00 in line 7 with the 
number of args * 4 (dummy argument included).

Works like a charm!