Re: userdata and the gc

[Chris Marrin <chris@...>]

Adrian Sietsma wrote:
> ...
> > Chris :
> > I'm not sure if Roberto intended what you think you read. If and when 
> > Lua gets a compacting garbage collector, a userdata's address would 
> > move around just like any other gc object. Maybe that will never 
> > happen and maybe they have some clever way to guarantee that userdatas 
> > never move. But it seems like a better idea to not rely on this and 
> > use your own data.
>
> I read A to mean that userdata never moves, and will code accordingly.
> It would be nice to have this point clarified in the manual.
>
> ps
> consider - if a userdata can move due to garbage collection, it would 
> have _major_ implications for c routines. There would need to be clear 
> rules on the lifetime of pointers returned from lua_newuserdata() and 
> lua_touserdata(), and which operations may invalidate them. Or, some 
> contract with the gc to say "leave this block alone".

I think it would be a mistake not to write code protecting your C 
functions from disappearing userdata, which by implication can protect 
you from moving userdata as well. I have had places in my code where I 
maintained pointers to userdata and it inevitibly caused problems. Now I 
let Lua maintain the pointers and I simply have references into the Lua 
VM. I either use the registry or similar tables. The __gc metamethod can 
keep you fully informed about the lifetime of each userdata. And Lua has 
a powerful feature for maintining weak pointers to avoid frozen userdata 
pointers. When attempting to get a reference to a weak pointer you will 
get back a nil so you can cleanly handle this case. All these techniques 
keep your C state consistent with your Lua state.

I think one of Lua's most powerful and useful features is its simple yet 
flexible C interface.

--
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