Thread locals in Lua

Question

In my application a Lua script can subscribe to certain events from a Client. It is also possible to have one script subscribe to multiple Clients. Currently I am setting th

Answer 1

Lua doesn't have thread local variables built-in, but you could use a separate table for each Lua thread to store thread local variables, and figure out which thread is running using coroutine.running (or lua_pushthread in C). Then make it more convenient with metatables. Something like:

local _G, coroutine = _G, coroutine
local main_thread = coroutine.running() or {} -- returns nil in Lua 5.1
local thread_locals = setmetatable( { [main_thread]=_G }, { __mode="k" } )
local TL_meta = {}

function TL_meta:__index( k )
  local th = coroutine.running() or main_thread
  local t = thread_locals[ th ]
  if t then
    return t[ k ]
  else
    return _G[ k ]
  end
end

function TL_meta:__newindex( k, v )
  local th = coroutine.running() or main_thread
  local t = thread_locals[ th ]
  if not t then
    t = setmetatable( { _G = _G }, { __index = _G } )
    thread_locals[ th ] = t
  end
  t[ k ] = v
end

-- convenient access to thread local variables via the `TL` table:
TL = setmetatable( {}, TL_meta )
-- or make `TL` the default for globals lookup ...
if setfenv then
  setfenv( 1, TL ) -- Lua 5.1
else
  _ENV = TL -- Lua 5.2+
end

Answer 2

Lua threads are child states from a single mother state. All global variables are shared by these Lua threads.

Separate Lua states have separate globals.