From: Antonio Quartulli <ordex@autistici•org>
To: David Laight <David.Laight@ACULAB•COM>
Cc: Ben Hutchings <bhutchings@solarflare•com>,
David Miller <davem@davemloft•net>,
amwang@redhat•com, netdev@vger•kernel.org, horms@verge•net.au
Subject: Re: [Patch net-next] net: clean up skb headers code
Date: Thu, 30 May 2013 14:23:00 +0200 [thread overview]
Message-ID: <20130530122300.GD2475@ritirata.org> (raw)
In-Reply-To: <AE90C24D6B3A694183C094C60CF0A2F6026B725A@saturn3.aculab.com>
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On Thu, May 30, 2013 at 01:12:15PM +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?
>
> Of course - we need to look at the value that ALL_ONES()
> generates in an expression. So the test print needs to be:
> printf("%hhx %hhx %hx %hx %x %x %lx %lx %llx %llx\n",
> ALL_ONES(a), ALL_ONES(b), ALL_ONES(c), ALL_ONES(d),
> ALL_ONES(e), ALL_ONES(f), ALL_ONES(g), ALL_ONES(h),
> ALL_ONES(i), ALL_ONES(j));
> which then gives 0xffffffff for the first 4 entries.
It gives all FF for every argument. So, I'm not understanding the problem..this
is what Ben wanted to achieve, no?
Cheers,
--
Antonio Quartulli
..each of us alone is worth nothing..
Ernesto "Che" Guevara
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next prev parent reply other threads:[~2013-05-30 12:24 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-05-29 6:09 [Patch net-next] net: clean up skb headers code Cong Wang
2013-05-29 6:36 ` David Miller
2013-05-29 6:48 ` Cong Wang
2013-05-29 6:49 ` David Miller
2013-05-29 19:34 ` Ben Hutchings
2013-05-29 19:44 ` Antonio Quartulli
2013-05-29 21:02 ` Ben Hutchings
2013-05-30 8:59 ` Antonio Quartulli
2013-05-30 11:41 ` David Laight
2013-05-30 11:45 ` Antonio Quartulli
2013-05-30 12:12 ` David Laight
2013-05-30 12:22 ` David Laight
2013-05-30 12:24 ` Antonio Quartulli
2013-05-30 12:40 ` David Laight
2013-05-30 12:23 ` Antonio Quartulli [this message]
2013-05-30 1:59 ` Cong Wang
2013-05-30 12:30 ` Sergei Shtylyov
2013-05-30 10:40 ` David Miller
2013-05-30 14:11 ` Ben Hutchings
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