freebsd stable - Aug 2017 - svn commit: r322875 - head/sys/dev/nvme

> Author: imp
> Date: Fri Aug 25 04:33:06 2017
> New Revision: 322875
> URL: 
> https://svnweb.freebsd.org/changeset/base/322875
> 
> 
> Log:
>   Use _Static_assert
>   
>   These files are compiled in userland too, so we can't use sys/systm.h
>   and rely on CTASSERT. Switch to using _Static_assert instead.
>   
>   MFC After: 3 days
>   Sponsored by: Netflix
> 
> Modified:
>   head/sys/dev/nvme/nvme.h
>   head/sys/dev/nvme/nvme_util.c
As I remember _Static_assert is from C11, not
the older C99.

As I understand head/sys/dev/nvme/nvme.h use by
C++ code could now reject attempts to use
_Static_assert .

There have been at least one old bugzilla report
for such. An example is 205453 (back around
2015-Dec).

From back then:
> # more main.cc
> #include "/usr/include/sys/cdefs.h"
> _Static_assert(1,"Test");
> int main(void)
> {
>     return 0;
> }
> 
> For example:
> 
> # g++49 main.cc
> main.cc:2:15: error: expected constructor, destructor, or type conversion
before '(' token
>  _Static_assert(1,"Test");
> . . .
> g++49, g++5, and powerpc64-portbld-freebsd11.0-g++ all reject the above
source the same way that libcxxrt/guard.cc compiles are rejected during
powerpc64-portbld-freebsd11.0-g++ based buildworld lib32 -m32 compiles.
> 
> gcc49, gcc5, and powerpc64-portbld-freebsd11.0-gcc all accept the above
instead (when in main.c instead of main.cc so it is handle as C code), with or
without the include. _Static_assert is specific to C11 and is not part of C++.
It takes explicit definitions to make the syntax acceptable as C++.
> 
> Note: clang++ (3.7) accepts the use of the C11 _Static_assert, with or
without the include, going well outside the C++ language definition.
> 
> . . .
> 
> Fixed in r297299 .
(The context was a C++ file head/contrib/libcxxrt/guard.cc so C++'s
static_assert was used instead and -std=c++11 was added for the
library in question [libcxxrt].)

Unless head/sys/dev/nvme/nvme.h is not to be used from
C++ code: use of _Static_assert in the header would appear
to be a problem.

==Mark Millard
markmi at dsl-only.net

2017-08-25 8:32 GMT+02:00 Mark Millard <markmi at
dsl-only.net>:
>> # g++49 main.cc
>> main.cc:2:15: error: expected constructor, destructor, or type
conversion before '(' token
>>  _Static_assert(1,"Test");
Yeah, that's because GCC is such a pain in the neck compiler that it
doesn't want to expose these C11 keywords when building C++, even
though they are in the reserved namespace (_[A-Z]). GCC would be
permitted to expose these and still comply to standards. Doing so
would make things so much easier for operating system implementors,
like us. Clang does get it right, in my opinion.

We should just extend <sys/cdefs.h> to define _Static_assert() when
using GCC in C++ mode (if we're not doing so already).

-- 
Ed Schouten <ed at nuxi.nl>
Nuxi, 's-Hertogenbosch, the Netherlands
KvK-nr.: 62051717

On 25 Aug 2017, at 07:32, Mark Millard <markmi at dsl-only.net>
wrote:
> 
> As I remember _Static_assert is from C11, not
> the older C99.
In pre-C11 dialects of C, _Static_assert is an identifier reserved for the
implementation.  sys/cdefs.h defines it to generate a zero-length array if the
condition is true or a negative-length array if it is false, emulating the
behaviour (though giving less helpful error messages)
> 
> As I understand head/sys/dev/nvme/nvme.h use by
> C++ code could now reject attempts to use
> _Static_assert .
In C++, _Static_assert is an identifier reserved for the implementation, but in
C++11 or newer static_assert is a keyword.  sys/cdefs.h defines _Static_assert
to static_assert for newer versions of C++ and defines it to the
C-before-11-compatible version for C++-before-11.

TL;DR: We have gone to a lot of effort to ensure that these keywords work in all
C/C++ dialects, please use them, please report bugs if you find a case where
they don?t work.

David