Re: Random crashes with ABC enabled
- From: Mike Pall <mike-1403@xxxxxxxxxx>
- To: luajit@xxxxxxxxxxxxx
- Date: Thu, 27 Mar 2014 23:56:54 +0100
Richard Hundt wrote:
> So are you saying that if - using your example - I do:
>
> local x = baz()
>
> that x can get collected out from under me? I don't think that's right.
> I've always relied on cdata being collected when they're not in any GC
> roots.
Your understanding is correct. Lua stack slots are GC anchors and
prevent collection of an object stored to them.
OTOH here's a concise example that demonstrates what NOT to do:
-- Run this again with the next line enabled and it won't complain.
-- collectgarbage("stop")
local ffi = require("ffi")
local a = ffi.new("int *[?]", 10000)
for i=0,9999 do
local p = ffi.new("int[10]")
-- THIS IS WRONG!
-- An array element (or struct field or pointer) does NOT keep
-- the cdata object returned by ffi.new alive. The contents of
-- cdata objects are not traversed by the GC and cannot serve as
-- GC anchors. The stored cdata object will eventually be garbage
-- collected and the memory will likely be reused soon.
a[i] = p
for j=0,i-1 do assert(a[j] ~= p, "found reused memory block") end
end
--Mike
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