What is the second argument of lua_newstate(lua_Alloc f, void *ud) used for?

[孙世龙 sunshilong <sunshilong369@...>]

Hi, list

What is the second argument of lua_newstate(lua_Alloc f, void *ud) used for?

As per the manual of
Lua(https://www.lua.org/manual/5.3/manual.html#lua_Alloc), which says
that:
The second argument, ud, is an opaque pointer that Lua passes to the
allocator in every call.

The second argument is NULL when lua_newsate() is called by
luaL_newstate() which is invoked by main() function.
For your reference, here is the related code snippet:
LUALIB_API lua_State *luaL_newstate (void) {
  lua_State *L = lua_newstate(l_alloc, NULL);
  if (L) {
    int *warnstate;  /* space for warning state */
    lua_atpanic(L, &panic);
    warnstate = (int *)lua_newuserdatauv(L, sizeof(int), 0);
    luaL_ref(L, LUA_REGISTRYINDEX);  /* make sure it won't be collected */
    *warnstate = 0;  /* default is warnings off */
    lua_setwarnf(L, warnf, warnstate);
  }
  return L;
}


I am a little confused that when should I pass a valid pointer to
lua_newstate() as a second argument?
What is it used for?

I would be grateful to have some help with this question.
It would better if you can give me some simple examples.

Best regards
Sunshilong