Re: Pushing a UserData pointer?

[Chris Marrin <chris@...>]

Ben Sunshine-Hill wrote:
> ...
> \begin{hack}
> If you know that a particular object is being stored in a userdata,
> you CAN refer to it as a userdata. All it takes is some pointer
> arithmetic.
>
> Ya see, a userdata, as allocated on the heap, consists of a struct
> Udata (well, actually a union Udata, but only for alignment purposes;
> treat it as a struct) followed by an arbitrary amount of data. The
> struct portion is a GCable object, which can be set into a stack
> position with setuvalue (a macro defined in lobject.h). This function,
> therefore, should work, though it's untested. Written based on the
> 5.0.2 source. (You will, of course, need to modify the include to
> correctly penetrate into the internals of Lua; lobject.h is not a
> public header.)
>
> #include <lobject.h>
> void pushexistinguserdata(lua_State *L, void *ud)
> {
>   lua_lock(L);
>   u = luaS_newudata(L, size);
>   setuvalue(L->top, ((Udata*)ud)-1);
>   api_incr_top(L);
>   lua_unlock(L);
> }
>
> If you call this with a pointer that is not actually a pointer to a
> preexisting userdata, horrible horrible things will happen within the
> GC. Use with caution and revulsion.

Yes, this is just about the code I came up with (without the erroneous 
newudata), by patterning after lua_newuserdata(). I am not very happy 
about subverting Lua like this either, except that I really need the 
performance boost and I REALLY, REALLY promise that the pointer will 
always be good :-)

This could be made at least conceptually type safe by returning the 
userdata pointer from lua_newuserdata() as an opaque pointer rather than 
 a void*:

    typedef struct lua_UserData lua_UserData;
    lua_UserData* lua_newuserdata(lua_State* L, size_t size);
    void          lua_pushuserdata(lua_State* L, lua_UserData* U);
    lua_UserData* lua_touserdata(lua_State* L);

For now I will hack it in!!!

Thanks...

--
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"As a general rule,don't solve puzzles that open portals to Hell"'