As already noted, lua_Objects are volatile.
If the C code needs to keep a lua_Object outside block boundaries,
it has to lock the object.
The routines to manipulate locking are the following:
int lua_lock (void);
lua_Object lua_getlocked (int ref);
void lua_pushlocked (int ref);
void lua_unlock (int ref);
The function lua_lock locks the object
which are on the top of the stack,
and returns a reference to it.
Whenever the locked object is needed,
a call to lua_getlocked
returns a handle to it,
while lua_pushlocked pushes the handle on the stack.
When a locked object is no longer needed,
it can be unlocked with a call to lua_unlock.
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