Re: Warning for different pointer signedness

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Joe Buck <Joe.Buck@synopsys.com> writes:

| On Mon, Dec 20, 2004 at 10:21:38PM +0100, Gabriel Dos Reis wrote:
| > Pointers-to-object comparaison make sense (from the relevant standards
| > point of view) only if they
| > 
| >   (i)   point into the same object; or
| >   (ii)  are (or one of them is) one-past-the-end of the same object; or
| >   (iii) are (or one of them is) null.
| > 
| > You then realize that if the pointers are not of the same type or cannot be
| > implicitly converted to each other, then the comparison become
| > invalid.
| 
| But there is the interesting fact that pointers that differ only in
| the signed-ness of the pointed-to object can alias each other.  That
| seems to imply that testing for equality makes sense, even without casting.

Are you advocating that we should be accepting this?

   typedef struct S S;

   int cmp(S* p, double* q)  { return p == q; }



   struct S {
      double m;
   };

If not why?  The provision that supports the aliasing rule you're
alluding to is also the rule that would support the above.

I think the bottom line is that aliasing is very different from pointer
comparison, and aliasing is better tested when both pointers are
casted either to void* or char*.

-- Gaby