1. Callback problem
- Posted by kbochert at copper.net
Oct 12, 2002
>From: Andy Serpa <renegade at earthling.net>
>Subject: RE: Wrapping C - Callback troubles
Bernie Ryan wrote:
>
> Andy Serpa wrote:
> > Here's another problem. I'm making the Berkeley DB wrapper=
which
allows
> > for certain callback functions to be used. One particular=
one
I'm
> > trying to set up is supposed to be a void function. The=
callback
works
> > (my function is called), but when it returns a value (any=
value)
it
> > crashes. (Callback functions are required to return=
something.)
>But that's not the problem. Setting the callback function is no=
>problem, and my Euphoria function is being called with the=
correct
>args.
>The problem is when my function exits (returns a value) the=
whole
>thing
>crashes with an exception. I was just wondering is that because=
it
>is a
>supposed to be void function? (It seems like I should be able=
to
>return
>anything in that case, but maybe it wants me to return nothing?)=
>Anybody know anything about that?
This sounds like the cdecl-stdcall problem. Many packages that=
come
from the unix world expect the callback function to be cdecl=
rather
than the windows standard stdcall. If the callback is of the=
wrong
variety, the stack gets messed up.
I modified Bach to accept cdecl callbacks -- under Euphoria I=
think
the easiest option is to modify the calling convention in the=
source
and re-compile the package.
There probably is a way to fix it by poking assembly.
Karl Bochert
2. Re: Callback problem
>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)
