Re: The purpose of LUA_ENVIRONINDEX pseudo index

[David Jones <drj@...>]

On 29 Aug 2006, at 07:57, Jose Luis Hidalgo Valiño wrote:

> El 29/08/2006, a las 2:58, Mildred escribió:
>
> > > For example,
> > > could lua_environindex be used to hold data locally( or private ) to
> > > a C function that can be accessed in successive calls?
> >
> > I don't know about LUA_ENVIRONINDEX but if you want to keep some
> > variables between the function call (like static variables in C) you
> > may find closures useful.
>
> I'm not sure if closures are mutable or not (but it's name suggest  
> they are not mutable), if you want a mutable variable, that can be  
> accessed by a function, you should use one of these pseudo indices:
>     - LUA_REGISTRY --> accessible by C code only
>     - LUA_GLOBALSINDEX ---> the thread environment, for global  
> variables.
>     - LUA_ENVIRONINDEX --> that's new to lua 5.1 and I'm not sure  
> how to use it.
>
> LUA_ENVIRONINDEX seems a way to hold data locally to a set of  
> functions ( or only one ) but without loosing the possibility of  
> access the full global environment (the thread environment) through  
> LUA_GLOBALSINDEX.
>
> In lua 5.1 lua_setfenv "changes" the behavior of LUA_ENVIRONINDEX  
> or LUA_GLOBALSINDEX ? I'm not sure.

It depends whether you apply lua_setfenv to a Lua function or a C  
function.  It has completely different meanings.

applied to a Lua function it changes where that Lua function finds  
global variables;

applied to a C function it changes its LUA_ENVIRONINDEX table.  As if  
lua_replace(L, LUA_ENVIRONINDEX) was called.  This has, of course,  
got nothing to do with global variables (or environment in the usual  
sense of the world).

Look at the source code for lua_setfenv, and lua_replace in lapi.c;  
it's instructive.

drj