Function luaL_ref

3rd/lua-5.4.3/src/lauxlib.c:660–685

** The previously freed references form a linked list: ** t[freelist] is the index of a first free index, or zero if list is ** empty; t[t[freelist]] is the index of the second element; etc. */

** empty; t[t[freelist]] is the index of the second element; etc.
*/
LUALIB_API int luaL_ref (lua_State *L, int t) {
  int ref;
  if (lua_isnil(L, -1)) {
    lua_pop(L, 1);  /* remove from stack */
    return LUA_REFNIL;  /* 'nil' has a unique fixed reference */
  }
  t = lua_absindex(L, t);
  if (lua_rawgeti(L, t, freelist) == LUA_TNIL) {  /* first access? */
    ref = 0;  /* list is empty */
    lua_pushinteger(L, 0);  /* initialize as an empty list */
    lua_rawseti(L, t, freelist);  /* ref = t[freelist] = 0 */
  }
  else {  /* already initialized */
    lua_assert(lua_isinteger(L, -1));
    ref = (int)lua_tointeger(L, -1);  /* ref = t[freelist] */
  }
  lua_pop(L, 1);  /* remove element from stack */
  if (ref != 0) {  /* any free element? */
    lua_rawgeti(L, t, ref);  /* remove it from list */
    lua_rawseti(L, t, freelist);  /* (t[freelist] = t[ref]) */
  }
  else  /* no free elements */
    ref = (int)lua_rawlen(L, t) + 1;  /* get a new reference */
  lua_rawseti(L, t, ref);
  return ref;
}


LUALIB_API void luaL_unref (lua_State *L, int t, int ref) {