Elie Roux wrote:

I tried the following code:

\directlua0{ local function simpleone() return "I'm number 1" end

local function simpletwo() texio.write_nl("I'm number 1") end

callback.register("start_run",simpletwo) }

and got no result neither by replacing simpletwo by simpleone in the callback. Is it normal?

The luatex run has started already, so your callback.register comes too late. You need to define the function and the callback.register in the startup lua script. Best wishes, Taco