Lua Academy · Lesson

Avoiding Memory Leaks

Common leak patterns in Lua and strategies to prevent them.

Lesson 4 of 413 steps

Avoiding Memory Leaks is a free Lua Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, and your progress syncs across the web and the CoddyKit app. The Lua Academy course includes 4 lessons in total.

What Is a Memory Leak in Lua?

A memory leak occurs when objects accumulate in memory because a reference prevents GC from collecting them, even though the program no longer needs them.

Common Leak: Global Accumulation

Storing objects in a global table without removing them is a classic leak. The global table is always a GC root.

local cache = {}  -- global-like upvalue
function storeResult(key, val)
  cache[key] = val  -- never evicted!
end

Fix: Bounded Cache

Limit the cache size and evict old entries using an LRU strategy or simple size cap.

local MAX = 100
local function boundedStore(cache, key, val)
  if #cache >= MAX then
    table.remove(cache, 1)  -- evict oldest
  end
  cache[key] = val
end

Common Leak: Event Listeners

Registering a closure as an event listener and never deregistering it keeps both the closure and its upvalues alive.

local listeners = {}
function on(event, fn)
  listeners[event] = listeners[event] or {}
  listeners[event][#listeners[event]+1] = fn
end
-- Always provide off() to deregister!

Fix: Weak Listener Tables

Store listeners in a weak-value table so that if the subscriber object is collected, its listener is automatically removed.

local listeners = setmetatable({}, {__mode = "v"})

Common Leak: Closures Capturing Large Tables

A closure that captures a large table keeps it alive as long as the closure lives, even if only a small piece of the table is needed.

local bigData = {-- 100MB of data --}
local fn = function()
  return bigData[1]  -- bigData stays alive!
end

Fix: Extract Needed Values

Extract only the needed values before creating the closure, so the large table can be collected.

local needed = bigData[1]
bigData = nil  -- allow collection
local fn = function() return needed end

Coroutine Leaks

A suspended coroutine is alive and holds all its local variables. Dead coroutines (status "dead") can be collected. Never lose track of suspended coroutines.

String Accumulation

Concatenating strings in a loop with .. creates many intermediate strings. Use table.concat to build large strings efficiently.

local parts = {}
for i = 1, 1000 do parts[i] = tostring(i) end
local result = table.concat(parts, ",")

Debugging Leaks

Use collectgarbage("count") before and after operations to measure memory growth. Log suspicious growth to narrow down the leak source.

local before = collectgarbage("count")
-- ... run operation ...
collectgarbage()
local after = collectgarbage("count")
print(("Delta: %.1f KB"):format(after - before))

Profiling Tools

Tools like luamemprof and custom debug.sethook allocation counters can track which code paths allocate the most memory.

Memory Leak Question

Which pattern is most likely to cause a memory leak?

Answer it in the interactive lesson

Recap: Avoiding Memory Leaks

Leaks in Lua arise from unbounded caches, underegistered listeners, closures over large tables, and forgotten suspended coroutines. Use weak tables, bounded caches, and collectgarbage("count") to detect and prevent leaks.

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Frequently Asked Questions

Is the “Avoiding Memory Leaks” lesson free?

Yes — any 3 lessons of the Lua Academy learning path, including “Avoiding Memory Leaks”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Lua Academy course includes 4 lessons in total.

What will I learn in “Avoiding Memory Leaks”?

Common leak patterns in Lua and strategies to prevent them. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Avoiding Memory Leaks” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Lua Academy lesson?

Yes. Every Lua Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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