C and Lua - Pass native object instance to native Lua function - c

Question

C functions that are passed to Lua to so that Lua can call native functions are static functions and thus not related to an object instance.
In my application, I have multiple sessions. Each session runs on its own thread, has its own data and its own scripts, and a script of the session N must be able to access the data of that session N.
The problem is that when you register a native C function to be callable from Lua, I can't pass the instance of the session object to make it available in the static Lua function.
One possibility could to be store the instance of the session into a static variable that can be called from the static C function (called from Lua), but this looks dirty and would require synchronization that could cause some scripts to hang.
A second possibility might be to create an Lua object that represents the session and call member methods on it so that, in my C function, I would have a way to access the Lua object representing the session ("this") and retreive the actual native session instance represented by this object. But I have no idea how to do that.
Is there a way to create Lua objects representing native object instances in Lua so that native Lua functions would have access to that native object instance ?

Answer 1

When you register a C function with Lua, you can store additional values as so-called "upvalues". The C function can then retrieve those values when it's called:
// ...
// push session instance pointer
lua_pushlightuserdata( L, (void*)get_current_session() );
// create a closure with one upvalue
lua_pushcclosure( L, my_static_C_function, 1 );
// ... (the resulting Lua function is at the stack top now)
In your static C function you access the instance data like this:
static int my_static_C_function( lua_State* L ) {
session_t* instance = (session_t*)lua_touserdata( L, lua_upvalueindex( 1 ) );
instance->do_something();
return 0;
}

Answer 2

Since each session has its own thread, why don't you link them together?
Whenever your function is called, get the session from the current thread.
You might want to take a look at the manuals about userdata.

Answer 3

Since "Each session runs on its own thread" you could simply save session data as static thread-local.
Better would be to transport session data by userdata, with a metatable bound for associated functions, and don't rely on static stuff.
As last alternative, let's combine: save session data as userdata/pointer Lua-state-local.
In all cases you have to care about class instances destruction because userdata are freed in C-style, same as thread data.