netdev - Re: sfc: an enumeration is not a bitmask

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Message-ID: <1305660147.2848.37.camel@bwh-desktop>
Date:	Tue, 17 May 2011 20:22:27 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Michał Mirosław <mirqus@...il.com>
Cc:	Jeff Garzik <jgarzik@...ox.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: sfc: an enumeration is not a bitmask

On Tue, 2011-05-17 at 21:09 +0200, Michał Mirosław wrote:
> 2011/5/17 Jeff Garzik <jgarzik@...ox.com>:
> > 2011/5/17 David Miller <davem@...emloft.net>:
> >> An enumeration is not a bitmask, instead it means one out of the set
> >> of enumerated values will be used.
> >
> > It's a decade-old kernel practice to use 'enum' to define typed
> > constants, preferred over  macros that convey no type information and
> > disappear after cpp phase.
> >
> > So your assertion about enumerations is demonstrably not true, as it
> > is often used in the kernel.  Call it enum abuse if you want, but it
> > is consistent with code all over the kernel.
> >
> > That said, I agree that warnings should of course be addressed in some manner.
> 
> Old age of the mistake doesn't make it correct.
> 
> Disappearance of the #defines can be resolved by using enum of bit
> positions (and maybe field lengths) and #define of (1 << bit_position)
> if it is useful for something to remain after preprocessing.

The point is that there is no specific type information for macros,
whether they are simple literal numbers or left-shift expressions.

The type of an enumerator in C is actually that of the underlying
integer type, not the enumeration type as in C++.  However, a compiler
or static analysis tool (such as sparse) may keep track of both the
language-specified type and some other associated type of an expression
in order to diagnose possible type errors.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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