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Re: [XEN PATCH v2] xen/include: avoid undefined behavior.
- To: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 21 Jun 2023 10:48:39 +0200
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- Delivery-date: Wed, 21 Jun 2023 08:49:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.06.2023 09:58, Nicola Vetrini wrote:
> Redefine BUILD_BUG_ON_ZERO to fully comply with C99 avoiding
> undefined behavior 58 ("A structure or union is defined as
> containing no named members (6.7.2.1)."
Here and in the title I'm not happy about you referencing undefined
behavior. What we do here is use a well-known compiler extension (and I'm
sure you're aware we do so elsewhere, where it's actually going to remain
as is from all I can tell right now).
> --- a/xen/include/xen/lib.h
> +++ b/xen/include/xen/lib.h
> @@ -51,9 +51,10 @@
> e.g. in a structure initializer (or where-ever else comma expressions
> aren't permitted). */
> #define BUILD_BUG_ON_ZERO(cond) \
> - sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
> + (sizeof(struct { char c; _Static_assert(!(cond), "!(" #cond ")"); }) -
> 1U)
To be compatible with whatever odd ABIs new ports may have, maybe better to
AND or multiply with 0? (I also don't think a U suffix is warranted here.)
> #else
> -#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
> +#define BUILD_BUG_ON_ZERO(cond) \
> + (sizeof(struct { unsigned u : (cond) ? -1 : sizeof(unsigned) * 8; }) -
> sizeof(unsigned))
What's wrong with just giving the bitfield a name:
#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int _:-!!(cond); })
Jan
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