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> Does anyone know -

> Is there a document that describes what the
> Lua garbage collector collects?

I don't know of one. However, a document is probably not necessary. It
collects anything that it can prove is not in use.

> How does it define "in use"?

It is the transitive closure of "references" starting with the globals
table, the stack, the reference table, and perhaps a few other things that
I have forgotten. In other words, if there is a reference to an object, it
is in use and so is everything that it references.

> Is it true that unreferenced userdata that has
> a gc method always called during a collection cycle?

I don't think you can count on that. It might be *logically* unreferenced,
but not actually garbage collectable yet (for example, it might be
referenced by a local on the stack which will never be used again, but the
garbage collector doesn't know that so it won't be garbage collected until
the stack slot is reused or the stack frame is cleared by the function

However, if there is a gc method, it will be called before the object is

> Does lua_close call the gc method for user data
> regardless of whether it is  referenced?

Yes. lua_close systematically deletes every allocated object, calling gc
methods if defined. However, in this case you cannot rely on the order of
collection to guarantee that gc methods will be called in the order you
think is logical. (The order is logical, but it might not accord with your
idea of your data, so it is unwise to count on this.) In particular, there
is no way to guarantee that objects will be garbage collected before
anything they reference has been garbage collected. There is no way to do
this because there might be circular references.