Re: __gc userdata problem

[Kevin Martin <kev82@...>]

On 26 Aug 2013, at 15:02, Dominik Zaczkowski wrote:

> I have problem with userdata and garbage collection, until now I thought that after userdata __gc
> metamethod is called it means that lua cannot touch my data in any way,


It doesn't work the way you want it to. imagine if it did, how is the garbage collector supposed to dispose of cyclic data structures. For example, in the below code, A and B are both unreachable, which gc method should be called first?

local m = {}
function m.__gc(self)
	print(string.format("Garbage Collecting %s, still paired with %s", self.name, self.paired.name)
end

do
	local A = {name = "A"}
	local B = {name = "B"}
	
	setmetatable(A, m)
	setmetatable(B, m)

	A.paired = B
	B.paired = A
end

--A and B both unreachable, but reference each other.

> How can I be sure that my userdata is gone, and I can free its resources?
> Only solution that I see is to mark userdata as free on __gc and then check for that it in every
> function exposed to lua.

Either that or change the userdata's metatable to one where __index and __newindex both raise errors - that way if any code tries to access it, an error is raised.

Thanks,
Kev