netdev - Re: [Patch net-next] net: clean up skb headers code

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Message-ID: <20130530114556.GC2475@ritirata.org>
Date:	Thu, 30 May 2013 13:45:56 +0200
From:	Antonio Quartulli <ordex@...istici.org>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>, amwang@...hat.com,
	netdev@...r.kernel.org, horms@...ge.net.au
Subject: Re: [Patch net-next] net: clean up skb headers code

On Thu, May 30, 2013 at 12:41:03PM +0100, David Laight wrote:
> > > > > I want to use something that will either break the build or
> > > > > automatically work if the type changes again.
> > > > >
> > > > > So something like "X = (typeof(X)) ~0U;".
> > > >
> > > > I think you mean ~(typeof(X))0.
> > >
> > > Am I wrong or you should cast the value once again, like this:
> > >
> > > ((typeof(X))~(typeof(X))0)
> > >
> > > because the ~ operator will implicitly cast the argument to int (if I remember
> > > correctly).
> > 
> > It will promote to at least int, but that still results in the right
> > value and doesn't provoke a warning.  Try this test (with -Wall
> > -Wextra):
> > 
> > #include <stdio.h>
> > 
> > #define ALL_ONES(v) (~(typeof(v))0)
> > /* #define ALL_ONES(v) (typeof(v))~0U) */
> > 
> > int main(void)
> > {
> >     char a = ALL_ONES(a);
> >     unsigned char b = ALL_ONES(b);
> >     short c = ALL_ONES(c);
> >     unsigned short d = ALL_ONES(d);
> >     int e = ALL_ONES(e);
> >     unsigned int f = ALL_ONES(f);
> >     long g = ALL_ONES(g);
> >     unsigned long h = ALL_ONES(h);
> >     long long i = ALL_ONES(i);
> >     unsigned long long j = ALL_ONES(j);
> > 
> >     printf("%hhx %hhx %hx %hx %x %x %lx %lx %llx %llx\n",
> > 	   a, b, c, d, e, f, g, h, i, j);
> >     return 0;
> > }
> 
> The printf format is masking the high bits for you.
> If you change to:
> 	printf("%x %x %x %x %x %x %lx %lx %llx %llx\n", ...)
> you get different values.
> To get the 'expected' values you need casts both sides of the ~

I tried with

19     printf("%x %x %x %x %x %x %lx %lx %llx %llx\n",

but I still see all FF.

Somewhere else we have some hidden conversion?

Cheers,


-- 
Antonio Quartulli

..each of us alone is worth nothing..
Ernesto "Che" Guevara

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