Re: Fwd: error in variable dereferencing

[Andrew Haley <aph at gcc dot gnu dot org>]

Thibaud GUERIN writes:
 > On 4/20/06, Andrew Haley <aph@redhat.com> wrote:
 > > Thibaud GUERIN writes:
 > >  >
 > >  >
 > >  > Not clear in the first message, (and maybe not in this one too..), sorry
 > >  >
 > >  >
 > >  > This asm inline was something like a "test/patch code".
 > >  >
 > >  > I try to have a simple :
 > >  >
 > >  > char        **ap = (char **)(&fmt);
 > >
 > > I don't think that's legal.  (Actually, I'm not perfectly sure it's
 > > not legal, but I think not.)  Also, I have no idea why you're trying
 > > to do such a thing.
 > 
 > as i'm compiling with :
 > -Wall -Werror -nostdinc -Wstrict-aliasing=2
 > it will not compile if it wasn't (i think, not sure too...)

That is not true.  We don't gurantee to generate an error for all
invalid source.

 > >  > working, It didn't :
 > >  > ap was equal to &fmt BUT *ap wasn't equal to fmt (don't kown why...)
 > >
 > > I'm sure that's impossible.  :-)
 > 
 > it wasn't belive me... and it's driving me crasy since days....
 > 
 > >  > so i try by my self... to do :
 > >  >
 > >  > ap = &fmt;
 > >  > *ap = fmt;
 > >  >
 > >  > in asm inline.... (dirty i know...)
 > >  >
 > >  > problem is :
 > >  > With this asm code in the binary *s is equal to fmt
 > >  > Without this asm code in the binary *s isn't equal to fmt
 > >  >
 > >  >
 > >  > All the 's' variable stuffs are from my debug...
 > >  > Again :
 > >  > My only aim is to have an 'ap = &fmt' valid (->ap = &fmt AND *ap = fmt)
 > >
 > > So why not do the obvious
 > >
 > >   const char **ap = &fmt ;
 > >
 > > ?
 > 
 > because i need to do some :
 > ap++;
 > to get the next args in stack, as my end aim is to do re-write a printf...

So why not use va_list?  That's what va_list is for.

 > >  > The resulting asm was here to help you to understand wath's wrong ....
 > >  > I'm looking for some days now without answer...
 > >
 > > You're still not explaining yourself.  You have a const char* arg that
 > > you are trying to alter, but instead of doing it the obvious way with
 > > an assignment, you're taking the address of the arg, casting the
 > > resulting pointer to a different pointer type, and then overwriting
 > > the arg through the resulting pointer.
 > >
 > > What's the point of all this?
 > 
 > i'm not trying to alter an (const char*) but to get the args in my
 > stack by getting some pointers on it, as in all va_args fonctions....

Trying to do all this stuff behind the compiler's back is likely to
break things.  Use va_list.

Andrew.