Re: Eval a Lua expression

[Eugen-Andrei Gavriloaie <shiretu@...>]

On Apr 3, 2008, at 1:09 AM, Javier Guerra wrote:

> On Wed, Apr 2, 2008 at 4:50 PM, Eugen-Andrei Gavriloaie
> <shiretu@gmail.com> wrote:
> > On Apr 3, 2008, at 12:27 AM, Javier Guerra wrote:
> > > no, each call to lua_evaluate_expression() would compile and  
> > > execute a
> > > new script in the same context. it won't 'accumulate' a long script.
> > Hmmm, that is even worse unfortunately because I don't want to  
> > recompile
> > the script over and over again. Some Lua variables may contain  
> > valuable
> > informations and on the first call
>
> it's not _re_compiling.
>
> first you compile and run your config, that sets one or more 'global'
> vars with your data.  then you compile and run several *very* short
> scripts, each one returns one or a few datums to the stack, where your
> C code can easily fetch it/them.
>
>
> --
> Javier

Well,  I need to do more reading on this matter. It's obvious that I  
don't fully understand the concept of loading,compiling and executing  
Lua scripts.

Here is what I know so far:

1. Instantiate a Lua VM
_pLuaState = lua_newstate(_lua_alloc, NULL);

2. Adding some more standard functionality to the newly created VM
luaL_openlibs(_pLuaState);

3. Adding custom made functionality represented by c++ functions
lua_pushcfunction(pLuaState, w_cfunc_call);
lua_setglobal(pLuaState, "cfunc");
......

At this stage we have a fully working Lua VM with some API provided by  
the standard libraries and custom made c++ functions. We will continue  
by loading a script into our VM

4. Load a Lua script from disk. This will result in adding 0 or more  
APIs (functions written in Lua) and 0 or more already initialized  
variables (like my config file who is a bunch of variables).
I think (not 100% sure) that this is only transforming Lua script into  
bytecode and loading it into VM

luaL_loadfile(pLuaState, STR(fileName));

Next step (who seems that I don't fully understand) is start the VM  
(give it a kick)

5. lua_pcall(pLuaState, 0, 0, 0)

At this stage, the entire VM is performing it's first step from the  
rest of it's life cycle. This is represented by initializing the  
global variables from the script (like my config file) and doing some  
functions calls (I don't have any in a config file)

6. Now, the cloudy part for me is what is happening when I do a  
luaL_loadstring(...). From the docs I understood that the string is  
compiled and transformed into byte code and than added to the VM. But  
this "adding" process is playing tricks with my mind. What is really  
happening there? Is permanently/temporarily added? Is only executed  
and after that discarded?

In my point of view (hopefully correct :) ) loading a script means  
compiling it into a byte code form and after that make it available to  
the VM. What would happen if we do a second luaL_loadfile(...) on the  
same lua_State? replacing the previous one or appending? Same question  
with the luaL_loadstring(...)


Thank you very much!