-- 参考：http://ju.outofmemory.cn/entry/97724
local function guid()
        local template ="xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
        local d = io.open("/dev/urandom", "r"):read(4)
        math.randomseed(os.time() + d:byte(1) + (d:byte(2) * 256) + (d:byte(3) * 65536) + (d:byte(4) * 4294967296))
        return string.upper(string.gsub(template, "x", function (c)
                                        local v = (c == "x") and math.random(0, 0xf) or math.random(8, 0xb)
                                                return string.format("%x", v)
                                        end
                        ))
end

local file_id =  guid();