Passing userdata to and from stack

[Bob Smith <bob@...>]

Hi,

I have been reading the docs and going through the mailing list archive 
but have yet found the answer to my question.

Let's say that in my C program, I have the following
lua_pushlightuserdata(L, someVoidPtr);

This I assume pushes someVoidPtr onto the stack of L

Now I want to call the script using lua_dofile(L, "script.lua");
- Do I need some other lines of code between lua_pushlightuserdata and 
lua_dofile?

In my script my first task is to obtain this value from the stack
local tmpUserData = ???

- How do I get this value from the stack? There's lots of documentation 
how to manipulate the stack on the C side but I can't seem to find 
any on how to manipulate the stack (except using a return statement to 
push values on the stack) inside a Lua script

Thanks,
Bob