Save reference to Lua's userdata

#1 Lua:

local test = Test();

#2 C:

//creating "lua's test"
luaL_newmetatable(L, "someTable");
lua_userdata *userData = (lua_userdata *)lua_newuserdata(L, sizeof(lua_userdata));

luaL_getmetatable(L, "someTable");
lua_setmetatable(L, -2);

#3 Lua:

function test.newMethod()
end

#4 C:

//this part is not executed from Lua
//what i have to have here from #2 to call "test.newMethod" and how to call it?
//if userdata would be on stack i guess i could:
luaL_getmetafield (L, 1, "newMethod");
lua_call(L, 0, 0);
//but because this part is not executed by Lua call its not on stack.

Edited:

will try to explain simpler in pseudo code:

Lua:

local test = Object(); 

C:

int Object(){ 
    ... 
    somePointer = luaL_newmetatable(...); //how to get this "somePointer"? maybe luaL_ref?
    push ... 
} 

Lua: makes new method

function test.newMethod() 
    ... 
end 

In C some event (lets say timer) triggers C method

void triggeredCMethod(){ 
    //here i need to call test.newMethod 
    //if i would have here 'somePointer' and could push it to Lua stack i could find and call newMethod 
}

so question is: how in C store pointer to some Lua object (hope i need that), get Lua object by that pointer and call method in it

I'm assuming you want to be able to call dynamically added functions. This code should explain it relatively simply. Note I don't do much error checking and make a few assumptions, don't copy paste this as a solution.

typedef struct
{
    int number;
    int reference;
    lua_State *L;
} TestUserdata;

static int m_newindex( lua_State *L )
{
    /* This is passed three values, first ( at -3 ) is the object, bring this to the front */
    lua_getfenv( L, -3 );
    /* Now bring the second arg forward, the key */
    lua_pushvalue( L, -3 );
    /* And the third arg, the value */
    lua_pushvalue( L, -3 );
    /* And we're done */
    lua_rawset( L, -3 );

    return 0;
}

static int m_tostring( lua_State *L )
{
    lua_pushstring( L, "TestUserdata" );
    return 1;
}

static int callobject( lua_State *L )
{
    /* Grab the object passed, check it's the right type */
    TestUserdata *data = luaL_checkudata( L, 1, "TestUserdata" );

    /* Grab the function environment we gave it in createobject, and look in there for newmethod */
    lua_getfenv( L, -1 );
    lua_pushstring( L, "newmethod" );
    lua_rawget( L, -2 );

    /* Call the function */
    lua_pushinteger( L, data->number );
    lua_call( L, 1, 0 );

    return 0;
}

static const struct luaL_reg userdata_m[] = {
    { "__newindex", m_newindex },
    { "__tostring", m_tostring },
    { NULL, NULL }
};

int main (int argc, char *argv[])
{
    lua_State *L = luaL_newstate();
    luaL_openlibs( L );

    /* Let's create us a userdatum metatable, and fill it up with goodies */
    luaL_newmetatable( L, "TestUserdata" );
    /* Use luaL_register to fill up the metatable for us */
    luaL_register( L, NULL, userdata_m );
    lua_pop( L, 1 ); /* Clean up the stack, we won't need the metatable left here */

    TestUserdata *data = lua_newuserdata( L, sizeof( TestUserdata ) );
    lua_pushvalue( L, -1 ); /* Copy for luaL_ref */
    int ref = luaL_ref( L, LUA_REGISTRYINDEX );
    data->reference = ref;
    data->number = 42;
    data->L = L;

    /* Load the metatable from before and 'give' it to this userdatum */
    luaL_getmetatable( L, "TestUserdata" );
    lua_setmetatable( L, -2 );

    /* Give this object an empty function environment */
    lua_newtable( L );
    lua_setfenv( L, -2 );

    lua_setglobal( L, "test" );

    luaL_dostring( L, "function test.newmethod( num ) print( num ) end" );

    /* Now provided we have the object, we can call any method defined anywhere */
    lua_rawgeti( data->L, LUA_REGISTRYINDEX, data->reference );
    lua_getfenv( data->L, -1 );
    lua_pushstring( data->L, "newmethod" );
    lua_rawget( data->L, -2 );
    lua_remove( data->L, -2 );

    if( lua_isfunction( data->L, -1 ) == 1 )
    {
        lua_pushinteger( data->L, data->number );

        lua_pcall( data->L, 1, 0, 0 );
    }

    lua_close( L );

    return 0;
}

Check that, I think that's what you're after.