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- Subject: Re: gc problem when using lightuserdata to reference C++ objects
- From: Javier Guerra Giraldez <javier@...>
- Date: Fri, 14 Nov 2008 07:48:32 -0500
On Friday 14 November 2008, Cheng, Long wrote:
> lua VM does not think there are enough "wasted memory" to reclaim. I'm
> just wondering is there any way I can tell the GC algorithm how much
> "external" memory the lightuserdata is referencing? Or is there other
> "proper" ways to handle the objects life cycle? Thanks!
there are two related problems:
- as Matias has noted, lightuserdata doesn't have a __gc. that is because,
being 'light', they're just values, as numbers, or booleans. the 'reference'
part is done by your C(++) code, so it's not a Lua thing.
- even if you use 'full' userdatas with __gc, if all you store there is a
pointer to your (potentially heavy) C++ object, Lua's GC doesn't have a clue
that keeping this tiny 4-byte object around implies keeping a much bigger
object on the C side.
in general, when a not-so-heavy Lua object is backed by a more expensive
resource (might be a memory hog, or an open file, or a DB connection,
whatever), you need some way for your program to explicitly close the object.
Of course, the __gc of an 'open' object should properly close it too.
The Lua object might not be collected immediately; but the C++ side should be.
after that, any access to the Lua object should result in an error.
in short, just add a 'close' method to the Lua object, and call it as soon as
you don't need it.
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