RE: Matching multibyte alphabetical characters with LPeG

["Thijs Schreijer" <thijs@...>]

> -----Original Message-----
> From: lua-l-bounces@lists.lua.org [mailto:lua-l-bounces@lists.lua.org]
> On Behalf Of William Ahern
> Sent: donderdag 21 juni 2012 4:25
> To: Lua mailing list
> Subject: Re: Matching multibyte alphabetical characters with LPeG
> 
> On Wed, Jun 20, 2012 at 04:49:34PM +0200, Thijs Schreijer wrote:
> >
> > > > and why cast it (incorrectly) as a lua_CFunction? Why not a boxed
> > > userdata?
> > >
> > > It is not incorrect. ANSI C ensures that all pointers to functions
> > > are compatible. That is, if A and B are pointers to functions, we
> > > can convert from A to B and back to A without loss. Now that Lua
> 5.2
> > > has light C functions, this looks like an interesting technique to
> > > store pointers to functions in Lua (as long as Lua code has no
> > > access to them).
> > >
> > > -- Roberto
> >
> > You refer specifically to 5.2. Does this mean it cannot be done
> > reliably in
> > 5.1 using lightuserdata for example?
> >
> 
> Standard C doesn't allow conversion between between object pointers and
> function pointers. This code is undefined in C:
> 
> void *foo(void) { return (void *)&foo; }
> 
> See 6.3.2.3p1, 6.3.2.3p7, and 6.3.2.3p8 of C99. Those are the sections
> which would define the behavior, but they don't; and no other section
> does.
> 
> In practice most platforms allow this, and POSIX actually requires it:
> 
> 	2.12.3 Pointer Types
> 
> 	All function pointer types shall have the same representation as
> the
> 	type pointer to void. Conversion of a function pointer to void *
> 	shall not alter the representation. A void * value resulting from
> 	such a conversion can be converted back to the original function
> 	pointer type, using an explicit cast, without loss of
> information.
> 
> Lua, OTOH, strives to be a strict adherent to the church of ISO C. She
> may fantasize about--and ocassionally indulge--inappropriate relations
> between object and void pointers, but she knows it's wrong. Still, if
> your platform allows these conversions, then Lua can't prevent it.
> 

Thx! Will read up on that.