Re: [Qemu-devel] [PATCH v3 3/4] compiler: expression version of QEMU_BUILD_BUG_ON

[Eric Blake <eblake@redhat.com>]

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On 01/19/2017 03:07 PM, Michael S. Tsirkin wrote:
> QEMU_BUILD_BUG_ON uses a typedef in order to be safe
> to use outside functions, but sometimes it's useful
> to have a version that can be used within an expression.
> Following what Linux does, introduce QEMU_BUILD_BUG_ON_ZERO
> that return zero after checking condition at build time.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/qemu/compiler.h | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/include/qemu/compiler.h b/include/qemu/compiler.h
> index c6f673e..8271eab 100644
> --- a/include/qemu/compiler.h
> +++ b/include/qemu/compiler.h
> @@ -92,6 +92,9 @@
>  #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
>      glue(qemu_build_bug_on__, __LINE__) __attribute__((unused))
>  
> +#define QEMU_BUILD_BUG_ON_ZERO(x) (sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)) - \
> +                                   sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)))

Although this should be equally effective, with less typing:

#define QEMU_BUILD_BUG_ON_ZERO(x) (!sizeof(QEMU_BUILD_BUG_ON_STRUCT(x)))

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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