is this a valid approach to aliasing?

[Robert William Fuller]

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;

Thank you.

Regards,

Rob