severity 561691 wishlist
tag 561691 + wontfix

On Sat, Dec 19, 2009 at 06:03:48PM +0100, Juliusz Chroboczek wrote:
> Package: libc6-dev
> Version: 2.10.2-2
> Type punning between pointers to struct sockaddr, struct sockaddr_storage
> and sockaddr_in(6) is explicitly required by the sockets interface.  For
> example, RFC 3493 Section 3.8 gives the following example:
>       if (bind(s, (struct sockaddr *) &sin6, sizeof(sin6)) == -1)
> Unfortunately, gcc-4.4 with -fstrict-aliasing=2 complains on this kind
> of code.  This may potentially break in code optimised with -O2 or greater
> (it probably doesn't, though, since most of the code that triggers this
> warning doesn't actually violate the C99 aliasing restrictions).

gcc 4.4 with -Wstrict-aliasing=3 does not emit a warning on that. 
Moreover quoting the gcc manual:

| Level 2: Aggressive, quick, not too precise.  May still have many false
| positives (not as many as level 1 though), and few false negatives (but
| possibly more than level 1).  Unlike level 1, it only warns when an
| address is taken.  Warns about incomplete types.  Runs in the frontend
| only.
| Level 3 (default for -Wstrict-aliasing): Should have very few false
| positives and few false negatives.  Slightly slower than levels 1 or 2
| when optimization is enabled.  Takes care of the common punn+dereference
| pattern in the frontend: "*(int*)&some_float".  If optimization is
| enabled, it also runs in the backend, where it deals with multiple
| statement cases using flow-sensitive points-to information.  Only warns
| when the converted pointer is dereferenced.  Does not warn about
| incomplete types.

This is clearly what happens here, gcc emits a false positive. You 
should use -Wstrict-aliasing=3, which is actually the default value.
There is no risk of breakage in optimised code with -O2 or more.

> I suggest that the definitions of struct sockaddr and struct sockaddr_storage
> should be equipped with
>   __attribute__ ((may_alias))
>                                         Juliusz
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Aurelien Jarno                          GPG: 1024D/F1BCDB73       

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