ffi_gc_behavior.lua

local ffi = require("ffi")
ffi.cdef("typedef struct { int *a; } foo_t;")

local function force_gc()
	-- silly way to force collection
	for i = 1, 10000 do
		collectgarbage()
	end
end

do -- WRONG
	local s = ffi.new(
        "foo_t", 
        ffi.new("int[4]", 1, 3, 3, 7) -- this temp value can get garbage collected from this point on
	)
	force_gc()
	local buf = {}

	for i = 0, 3 do
		buf[i + 1] = s.a[i]
	end

	print(table.concat(buf) == "1337")
end

do -- WRONG
	local function get_s()
		local a = ffi.new("int[4]", 1, 3, 3, 7)
		local s = ffi.new("foo_t", a) -- 'a' can get garbage collected from this point on
		return s
	end

	local s = get_s()
	force_gc() -- [[ here ]]
	local buf = {}

	for i = 0, 3 do
		buf[i + 1] = s.a[i]
	end

	print(table.concat(buf) == "1337")
end

do -- WRONG
	local function get_s()
		local a = ffi.new("int[4]", 1, 3, 3, 7)
		local s = ffi.new("foo_t", a)
		return a, s
	end

	local a, s = get_s() -- 'a' can get garbage collected from this point on as it's never used
	force_gc()
	local buf = {}

	for i = 0, 3 do
		buf[i + 1] = s.a[i]
	end

	-- 'a' can get garbage collected from this point on
	print(table.concat(buf) == "1337")
end


do -- OK
	local function get_s()
		local a = ffi.new("int[4]", 1, 3, 3, 7)
		local s = ffi.new("foo_t", a)
		return a, s
	end

	local a, s = get_s()
	_G.TEMP_A = a
	force_gc()
	local buf = {}

	for i = 0, 3 do
		buf[i + 1] = s.a[i]
	end
	
	TEMP_A = nil
	-- 'a' can get garbage collected from this point on
	
	print(table.concat(buf) == "1337")
end

do -- OK
	local function get_s()
		local a = ffi.new("int[4]", 1, 3, 3, 7)
		local s = ffi.new("foo_t", a)
		return {__a = a, s = s}
	end

	local s = get_s()
	force_gc()
	local buf = {}

	for i = 0, 3 do
		buf[i + 1] = s.s.a[i]
	end

	-- 's.s.a' can get garbage collected from this point on
	print(table.concat(buf) == "1337")
end