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

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B7259@saturn3.aculab.com>
Date:	Thu, 30 May 2013 12:41:03 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Ben Hutchings" <bhutchings@...arflare.com>,
	"Antonio Quartulli" <ordex@...istici.org>
Cc:	"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

> > > > 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 ~

	David