Purpose of lua_lock and lua_unlock?

What is the point of lua_lock and lua_unlock? The following implies it's important: LUA_API void lua_gettable (lua_State *L, int idx) { StkId t; lua_lock(L); t = index2adr(L, idx); api_checkvalidindex(L, t); luaV_gettable(L, t, L->top - 1, L->top - 1); lua_unlock(L); } LUA_API void lua_getfield (lua_State *L, int idx, const char *k) { StkId t; TValue key; lua_lock(L); t = index2adr(L, idx); api_ch

[masu_mi]

上書きの為に空にしてるのか…((void)0)って条件コンパイルの定石なのかな…