Re: is this a valid approach to aliasing?

[Andrew Haley <aph at redhat dot com>]

Robert William Fuller wrote:
> Andrew Haley wrote:
> > Robert William Fuller wrote:
> > > Naturally, when I started trying to call this function, I started 
> > > getting the type-punned pointer aliasing warnings.  The code in 
> > > question looked roughly like this:
> > >
> > > static opt_param_t *shrtOpts, *longOpts;
> > >
> > > rc = reallocItems((void **) &shrtOpts, sizeof(opt_param_t), 
> > > &shrtOptAlloc, numShrtOpts, newShrtOpts, shrtOptHint);
> > >
> > > Obviously, this breaks the aliasing rules.  I read that I could work 
> > > around this by casting through a union.  I settled on this approach, 
> > > but I'm not sure if it is valid, or if I'm merely masking the problem:
> > >
> > > typedef union _opt_param_alias_t {
> > >
> > >     opt_param_t *o;
> > >     void *v;
> > >
> > > } opt_param_alias_t;
> > >
> > > rc = reallocItems(&((opt_param_alias_t *) &shrtOpts)->v, 
> > > sizeof(opt_param_t), &shrtOptAlloc, numShrtOpts, newShrtOpts, 
> > > shrtOptHint)
> >
> > It really isn't necessary to do this.  The rule is that you mustn't use
> > a pointer cast to create an lvalue of incompatible type.  So, do this:
> >
> > static opt_param_t *shrtOpts, *longOpts;
> >
> > void *p1;
> >
> > rc = reallocItems(&p1, sizeof(opt_param_t),
> >    &shrtOptAlloc, numShrtOpts, newShrtOpts, shrtOptHint);
> > shrtOpts = p1;
>
> I suspected that might work.  I asked so I would not have to look at 
> gcc's output.  I have not looked at (dis)assembly since I worked on 
> Windows NT in its early days when things were even more closed than they 
> are now.  I am now a totally reformed free software user and programmer. 
>  (Ask yourself if there is a causal relation there.)
>
> I want to make sure I am very clear on this.  Are you saying the 
> following is okay?
>
> p1 = shrtOpts;
> rc = reallocItems(&p1, ...)
> shrtOpts = p1;

Yes.  But you shouldn't believe me!  Read the C Standard.
The section you need to understand is 6.3.2.3.

http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf

Andrew.


http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf