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Ahh. I used to believe __gc works for tables too. Now I checked the manual and learnt it doesn't. Regarding the binding library I was talking about, I read the source code carefully and confirmed that it was binding __gc on a userdata object (a C++ pointer). The lightuserdata was used to find objects in lua, exactly follow what the pil book 28.5 says. 
Anyway, as the userdata is a pointer to a C++ object, the GC problem still exists.

2008/11/15 Ariel Manzur <>
did you modify lua to call a __gc metamethod for tables?

On Fri, Nov 14, 2008 at 1:46 PM, Long Cheng <> wrote:
> I don't have the first problem. Actually our binding library was designed
> and implemented so that for each C++ object exported to lua, it has a
> corresponding lua table, which minimics an "object" in lua, which you can
> manuplate with and call class methods exported from c++. The lightuserdata
> was a entry on this table so that when calling the object methods or access
> the object attributes, our binding library can find the right C++ object to
> work with. Also the "__gc" method is bind to this table, so that whenever
> this table was abondoned, the table is reclaimed. When the __gc metamethod
> is called, the C++ object is also cleaned. Sorry for not explain it clearly
> in the first message.
> Thanks all for your suggestions. It looks now the best solution is to use
> userdata to allocate memory for C++ objects. I'll go back look at our
> binding library source and see if there is any trouble doing so.
> Regards
> Long
> 2008/11/14 Javier Guerra Giraldez <>
>> 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.
>> --
>> Javier