Garbage collection vs. userdata

["O. Wölfelschneider" <lua@...>]

Hello List!

Say I have a C function like this:

int new_fnord(lua_State *L) {

  struct fnord_t *fnord = alloc_fnord();
  struct fnord_t **fnord_p = lua_newuserdata(L, sizeof(struct fnord_t *));
  *fnord_p = fnord;

  luaL_getmetatable(L, "FNORD_METATABLE");
  lua_setmetatable(L, -2);

  return 1;
}

(Error handling not shown.)
(Metatable contains __gc to clean up at GC time.)

Now imagine that alloc_fnord() grabs a large chunk of memory and is outside of
my control (Lua-unaware external library).

As lua is concerned, it's got only one pointer of a few bytes to worry about,
since it cannot know that there is actually a big memory footprint behind it.

After a lot of calls to new_fnord(), the program has grabbed a huge amount of
memory while lua still doesn't see a reason to garbage collect, since, hey,
it's just a few pointers...

Can I somehow tell the garbage collector that new_fnord() allocates big time?

Currently I'm calling collectgarbage("collect") every now and then, but this
doesn't feel right.

BTW: In this application, lua is running on an embedded ARM with 32MB and a
fnord_t actually caches an unpacked GIF animation. Having these pending GC
eats up the little memory quickly.

Greetings,
Olav Wölfelschneider