Date: Sun, 12 Nov 2006 16:11:51 +0100 From: Giorgos Keramidas <keramida@freebsd.org> To: Ruslan Ermilov <ru@freebsd.org> Cc: arm@freebsd.org, current@freebsd.org Subject: Re: [head tinderbox] failure on arm/arm Message-ID: <20061112151150.GA2988@kobe.laptop> In-Reply-To: <20061112145151.GC49703@rambler-co.ru> References: <20061112133929.9194773068@freebsd-current.sentex.ca> <20061112140010.GA47660@rambler-co.ru> <20061112142710.GE91556@wombat.fafoe.narf.at> <20061112133929.9194773068@freebsd-current.sentex.ca> <20061112140010.GA47660@rambler-co.ru> <20061112144230.GC2331@kobe.laptop> <20061112145151.GC49703@rambler-co.ru>
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On 2006-11-12 17:51, Ruslan Ermilov <ru@FreeBSD.org> wrote:
> On Sun, Nov 12, 2006 at 03:42:30PM +0100, Giorgos Keramidas wrote:
> > On 2006-11-12 17:00, Ruslan Ermilov <ru@freebsd.org> wrote:
> > > %%%
> > > $ cat a.c
> > > struct foo {
> > > char x;
> > > };
> > >
> > > struct foo *
> > > bubu(char *s)
> > > {
> > >
> > > return (struct foo *)s;
> > > }
> > > $ cc -c -Wcast-align a.c
> > > a.c: In function `bubu':
> > > a.c:9: warning: cast increases required alignment of target type
> > > %%%
> > >
> > > (None of other supported architecutes see the issue here.)
> >
> > You can't cast any random (char *) pointer to a pointer of a type which
> > is (potentially) larger than 1 byte. It's the same sort of warning you
> > will get if you try to:
> >
> > char ch[] = "\x00\x00\x00\x00";
> > char *p = &(ch[0]);
> > unsigned long *lptr = (unsigned long *)p;
> >
> > You cannot guarantee that `ch' is stored in an address that is properly
> > aligned for (unsigned long), and this is what GCC warns about here.
>
> No, your example I perfectly understand but it is completely different.
> Note that the first (and only) member in my structure is "char", so it
> doesn't need to be more than sizeof(char) aligned.
Ah, but the tricky part is that inside bubu() there is no knowledge that
`s' may be properly aligned for a (struct foo *) pointer. All the
compiler knows is that it is a (char *), possibly misaligned for any
pointer whose object has a size > 1.
> > On 2006-11-12 15:27, Stefan Farfeleder <stefan@fafoe.narf.at> wrote:
> > > What is sizeof(struct foo)? If it's > 1 it makes sense.
> >
> > Exactly :)
>
> Still doesn't make much sense to me. If all structure members are chars
> (like is the case with "struct ar_hdr" from <ar.h> which GCC complains
> about, and in my example, the required alignment shouldn't be more than
> sizeof(char). What am I missing?
You are missing that inside bubu() the compiler 'believes' that:
* The `s' pointer is (char *)-aligned.
* The sizeof(struct foo) is >1.
* You are trying to assign `s' (with it's possibly misaligned
value) to the `return value' place, whose type is (at
least, as far as the compiler knows) is (struct foo *).
That may break alignment assumptions the compiler is making inside bubu(),
especially about the `s' pointer, hence the warning.
- Giorgos
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