linux-kernel - Re: [RFC] [PATCH] v4l2: use unsigned rather than enums in ioctl() structs

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Date:	Thu, 12 Apr 2012 13:22:45 -0400
From:	Nick Bowler <nbowler@...iptictech.com>
To:	Rémi Denis-Courmont <remi@...lab.net>
Cc:	Mauro Carvalho Chehab <mchehab@...hat.com>, mchehab@...radead.org,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Hans Verkuil <hverkuil@...all.nl>
Subject: Re: [RFC] [PATCH] v4l2: use unsigned rather than enums in ioctl()
 structs

On 2012-04-11 23:32 +0300, Rémi Denis-Courmont wrote:
> From the perspective of the compiler, this is a feature not a bug. In
> C and C++, loading or storing a value in an enumerated variable
> whereby the value is not a member of the enumeration is undefined.

I'm afraid that this is not the case in C, although it may be in C++
(enums are very different in C++ than they are in C).  In C, enum types
are required to be compatible with some integer type capable of storing
the values of all the enum members (see C11§6.7.2.2#4).  Compatibility
is a very strong condition, and implies that the two types are
interchangable without affecting the meaning of the program (see
C11§6.2.7).  Integer types have a number of specific requirements, one
thing that's relevant here is that they do not have "holes" in their
representable values: there is a minimum and maximum representable
value, and all integers between them are representable (C11§6.2.6.2#1).

Thus, while the choice of integer type used may depend on the values of
the corresponding enum constants, storing any value (regardless of
whether or not its a member of the enumeration) is subject to the same
rules as the implementation-defined compatbile integer type.  This is
always well-defined for values within the range of the type.
(C11§6.3.1.3#1 and C11§6.3.1.4#1).

> In other words, the compiler can assume that this does not happen, and
> optimize it away.

No, a conforming C compiler cannot assume such assignments do not
happen, for the reasons outlined above.

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

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