linux-kernel - Re: [PATCH] build_bug.h: add wrapper for _Static

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Message-ID: <CAKwvOdn0ssTmUue3YVU_2VJKR=56X4j2JJxNAoZ6x3-inCwwhQ@mail.gmail.com>
Date:   Wed, 6 Feb 2019 16:14:55 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] build_bug.h: add wrapper for _Static_assert

On Tue, Feb 5, 2019 at 1:38 AM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> On 05/02/2019 09.05, Masahiro Yamada wrote:
> > On Mon, Feb 4, 2019 at 4:24 AM Rasmus Villemoes
> > <linux@...musvillemoes.dk> wrote:
> >> +#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

I had assumed it was for the "optional" part of the error message,
since whether you pass a string error message or not, the C
preprocessor should join them all together?

>
> foobar.c:5:34: error: expected string literal before ‘expected’
>  static_assert(sizeof(long) == 8, expected 64 bit machine);

Hopefully you'd put `expected` in double quotes?

Note: I'm _very_ happy to see this being added.  Once it's landed, I
too can think of some places that it would work better than
BUILD_BUG_ON().
-- 
Thanks,
~Nick Desaulniers