Re: ntohs/ntohl and bitops

[Arjan van de Ven <arjan@infradead.org>]

On Wed, 2006-01-11 at 00:00 -0800, David S. Miller wrote:
> From: Ulrich Drepper <drepper@redhat.com>
> Date: Tue, 10 Jan 2006 14:02:52 -0800
> 
> > I just saw this in a patch:
> > 
> > +               if (ntohs(ih->frag_off) & IP_OFFSET)
> > +                       return EBT_NOMATCH;
> > 
> > This isn't optimal, it requires a byte switch little endian machines.
> > The compiler isn't smart enough.  It would be better to use
> > 
> >      if (ih->frag_off & ntohs(IP_OFFSET))
> > 
> > where the byte-swap can be done at compile time.  This is kind of ugly,
> > I guess, so maybe a dedicate macro
> > 
> >     net_host_bit_p(ih->frag_off, IP_OFFSET)
> 
> The first suggestion isn't considered ugly, and the best form is:
> 
> 	if (ih->frag_off & __constant_htons(IP_OFFSET))

why this __constant_htons and not just plain htons ??
htons() gets auto-remapped to that anyway via the builtin "is this a
constant" thing...... and to be honest htons() is more readable.


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