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For the sake of the example, try defining your GC function outside and pass it in 

function detectGc()
 print("!!! cleanup")
 ldb();
end

function tryThis(gcFunc)
 myVar = GfcTimer();
 local mt = getmetatable(myVar);
 mt.__gc = gcFunc;
end

then use tryThis(detectGc) instead of just tryThis()

Daniel.


Matthew Armstrong wrote:
I'm only using the __gc method for cleanup detection. If it is creating another reference to the environment, is there another way to detect collection that doesn't maintain a reference?
On Tue, Mar 18, 2008 at 7:02 PM, Daniel Stephens <daniel@danielstephens.com <mailto:daniel@danielstephens.com>> wrote: 

    The __gc closure you create inside tryThis will have inherited the
    global environment from tryThis at the time it was executed, so it's
    going to have a reference to that environment, but I'm not sure that
    would be a reachable reference.

    Daniel.

    Matthew Armstrong wrote:
    > I had already tried the setfenv(tryThis, {}) idea, and it didn't
    > work.  After your mail, I also tried tryThis = nil, which also
    didn't
    > work.  Here is my non-trimmed code:
    >
    > function tryThis()
    >     myVar = GfcTimer()
    >     local mt = getmetatable(myVar)
    >     mt.__gc = function()
    >         print("!!! cleanup")
    >         ldb()
    >     end
    >
    >     --myVar = nil
    > end
    >
    > function yup()
    >     local env = {}
    >     setmetatable(env, { __index = _G })
    >     setfenv(tryThis, env)
    >
    >     tryThis()
    >
    >     setfenv(tryThis, {})
    >     tryThis = nil
    >     env = nil
    >
    >     collectgarbage()
    > end
    >
    > yup()
    >
    > Note:  there is one external function, GfcTimer, which generates a
    > userdata object from our library.
    >
    > If I uncomment the line:
    >
    > -- myVar = nil
    >
    > ... then I get a confirmed call to the __gc method.
    >
    > Can you see anything else which would maintain a reference?
    >
    > On Tue, Mar 18, 2008 at 5:55 PM, Roberto Ierusalimschy
    > <roberto@inf.puc-rio.br <mailto:roberto@inf.puc-rio.br>
    <mailto:roberto@inf.puc-rio.br <mailto:roberto@inf.puc-rio.br>>>
    wrote:
    >
    >     > So let's say I say:
    >     >
    >     > function tryThis()
    >     >     myVar = { foo="baz" }
    >     > end
    >     >
    >     > local env = {}
    >     > setfenv(tryThis, env)
    >     > tryThis()
    >     >
    >     > My question is: when does myVar get collected?  With my
    current
    >     testing, it
    >     > seems that myVar never gets collected.  If so, how do I
    >     "convince" it to be
    >     > collected?
    >
    >     'env' is the environment of 'tryThis' (which is global), so it
    >     cannot be
    >     collected until tryThis is garbage or until you change its
    environment
    >     again. You may try "tryThis = nil" or "setfenv(tryThis, {})".
    >
    >     -- Roberto
    >
    >