On Tue, Feb 5, 2019 at 6:39 PM Rasmus Villemoes <li...@rasmusvillemoes.dk> wrote: > > On 05/02/2019 09.05, Masahiro Yamada wrote: > > 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? > > Good point. It's a leftover from when I had a fallback-implementation of > _Static_assert for gcc < 4.6, where I wanted to ensure that the second > argument was a string literal, even if my fallback implementation > ignored that argument. Now it's actually a little harmful, because > > foobar.c:5:34: error: expected string literal before ‘expected’ > static_assert(sizeof(long) == 8, expected 64 bit machine); > > is better than > > foobar.c:4:34: error: expected ‘)’ before ‘expected’ > static_assert(sizeof(long) == 8, expected 64 bit machine); > > > 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. > > I'd rather move compiletime_assert to build_bug.h, and rename it so that > it becomes an implementation detail of BUILD_BUG. There are not that > many direct users of compiletime_assert(), and I think we should > standardize on fewer ways of achieving the same thing. static_assert() > for checking ICEs, usable at any scope, and BUILD_BUG_* for checking > that the optimizer is sufficiently smart. > > This would also be a step towards another cleanup I'd like to do: make > build_bug.h not depend on compiler.h,
Probably, you cannot do this. compiletime_assert relies on __attribute__((__error__(...))) that is only supported by GCC. > because we already have a > dependency in the other direction (ARRAY_SIZE using BUILD_BUG_ON_ZERO). > > Rasmus -- Best Regards Masahiro Yamada
