Re: [PATCH] build_bug.h: add wrapper for _Static_assert
From: Masahiro Yamada
Date: Tue Feb 05 2019 - 03:06:14 EST
On Mon, Feb 4, 2019 at 4:24 AM Rasmus Villemoes
> BUILD_BUG_ON() is a little annoying, since it cannot be used outside
> function scope. So one cannot put assertions about the sizeof() a
> struct next to the struct definition, but has to hide that in some
> more or less arbitrary function.
> Since gcc 4.6 (which is now also the required minimum), there is
> support for the C11 _Static_assert in all C modes, including gnu89. So
> add a simple wrapper for that.
> _Static_assert() requires a message argument, which is usually quite
> redundant (and I believe that bug got fixed at least in newer C++
> standards), but we can easily work around that with a little macro
> magic, making it optional.
> For example, adding
> static_assert(sizeof(struct printf_spec) == 8);
> in vsprintf.c and modifying that struct to violate it, one gets
> ./include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(struct printf_spec) == 8"
> #define __static_assert(expr, msg, ...) _Static_assert(expr, "" msg "")
> godbolt.org suggests that _Static_assert() has been support by clang
> since at least 3.0.0.
> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> include/linux/build_bug.h | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
> diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
> index faeec7433aab..4bf9ba847b44 100644
> --- a/include/linux/build_bug.h
> +++ b/include/linux/build_bug.h
> @@ -58,4 +58,23 @@
> #define BUILD_BUG() BUILD_BUG_ON_MSG(1, "BUILD_BUG failed")
> + * static_assert - check integer constant expression at build time
> + *
> + * static_assert() is a wrapper for the C11 _Static_assert, with a
> + * little macro magic to make the message optional (defaulting to the
> + * stringification of the tested expression).
> + *
> + * Contrary to BUILD_BUG_ON(), static_assert() can be used at global
> + * scope, but requires the expression to be an integer constant
> + * expression (i.e., it is not enough that __builtin_constant_p() is
> + * true for expr).
> + *
> + * Also note that BUILD_BUG_ON() fails the build if the condition is
> + * true, while static_assert() fails the build if the expression is
> + * false.
> + */
> +#define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
> +#define __static_assert(expr, msg, ...) _Static_assert(expr, "" msg "")
What is the "" "" for?
There might be room for argument about
where this macro should go.
Another possible place is <linux/compiler.h>
where compiletime_assert() is defined.