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Here's what I use for this situation. You create a lua_CFunction as the module unload handler. When the module is collected, either when the lua_State is closed, or when the package.loaded["modulename"] is nilled, this lua_CFunction is called.

Use it right after your luaL_register call in luaopen_x, e.g.:

luaL_register(L, "modulename", module_lib);
gc_sentinel(L, -1, module_unload_function);

	get a callback when the value at stack index idx is collected:
static void gc_sentinel(lua_State * L, int idx, lua_CFunction callback) {

	lua_pushvalue(L, idx); // value @idx
	lua_newuserdata(L, sizeof(void *)); // sentinel userdata
		lua_newtable(L);	// userdata metatable with __gc = callback
		lua_pushcfunction(L, callback);

		lua_setfield(L, -2, "__gc");
		lua_setmetatable(L, -2);

	/* check for (weak-valued) sentinel table; create if needed */
	lua_getfield(L, LUA_REGISTRYINDEX, "gc_sentinels");
	if (lua_isnoneornil(L, -1)) {
		lua_pop(L, 1);

		// make weak-keyed
		lua_pushstring(L, "k");
		lua_setfield(L, -2, "__mode");
		lua_pushvalue(L, -1);
		lua_setfield(L, -2, "__index");
		lua_pushvalue(L, -1);
		lua_setmetatable(L, -2);
		lua_pushvalue(L, -1);
		lua_setfield(L, LUA_REGISTRYINDEX, "gc_sentinels");

	lua_insert(L, -3);
	lua_settable(L, -3); // lua::sentinel[value @idx] = sentinel userdata
	lua_pop(L, 1); // lua::sentinel

On Jan 31, 2009, at 11:31 AM, Luiz Henrique de Figueiredo wrote:

You cannot set __gc for tables. But you can say use a udata as upvalue for all functions registered in the module and then clean up when that udata
is collected.

I have also thought about what to do in this situation and I am still
learning these tricks. Any examples would be very helpful. Thanks.

See loadlib.c, which sets __gc for library handles to tell the OS to
unload libraries from the app when they become garbage in Lua. This is
not the same technique I described above, though.