On Mon, Feb 4, 2019 at 4:24 AM Rasmus Villemoes <li...@rasmusvillemoes.dk> wrote: > > 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 <li...@rasmusvillemoes.dk> > --- > 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? Bikeshed: There might be room for argument about where this macro should go. Another possible place is <linux/compiler.h> where compiletime_assert() is defined. Thanks. -- Best Regards Masahiro Yamada
