Calling a function on completion / exit from a lua script (atexit () / cleanup function)

Question: Is there a way to call a function when a lua script is terminated either by the system or by the program that launched the script (like a C program)? Atexit () / cleanup function for lua.

Situation: an external C program (calling PROG) controls a lua script (calls it SCRIPT) and calls its functions, the lua script uses a shared library (.so, cal it LIB), which reserves resources to be freed when the lua script exits. The Lua script is driven (and thus ends up) with PROG, which I cannot change. The script should notify the LIB when it's finished.

How can I do that? Note: I am pretty new to lua, so please explain your answer, greatly appreciate :)

I am on Linux using Lua 5.3.1

This currently works:

a = {__gc = function() print'exit function from LIB called' end}
setmetatable(a,a)