svn commit: r317755 - head/sbin/ifconfig

[Alan Somers]

On Wed, May 3, 2017 at 2:51 PM, Bruce Evans <brde at optusnet.com.au> wrote:
> On Wed, 3 May 2017, Eric van Gyzen wrote:
>
>> On 05/03/2017 14:38, Alan Somers wrote:
>>>
>>> On Wed, May 3, 2017 at 1:34 PM, Warner Losh <imp at bsdimp.com> wrote:
>>>>
>>>> On Wed, May 3, 2017 at 1:32 PM, Alan Somers <asomers at freebsd.org> wrote:
>>>>>
>>>>> On Wed, May 3, 2017 at 12:16 PM, Ngie Cooper <yaneurabeya at gmail.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>>> On May 3, 2017, at 10:21, Alan Somers <asomers at FreeBSD.org> wrote:
>>>>>>>
>>>>>>> Author: asomers
>>>>>>> Date: Wed May  3 17:21:01 2017
>>>>>>> New Revision: 317755
>>>>>>> URL: https://svnweb.freebsd.org/changeset/base/317755
>>>>>>>
>>>>>>> Log:
>>>>>>>  Various Coverity fixes in ifconfig(8)
>>>>>>
>>>>>>
>>>>>> ...
>>>>>>
>>>>>>>  * Mark usage() as _Noreturn (1305806, 1305750)
>>>>>>
>>>>>>
>>>>>> ...
>>>>>>
>>>>>>> -static    void usage(void);
>>>>>>> +static    void usage(void) _Noreturn;
>>>>>>
>>>>>>
>>>>>> Hi Alan,
>>>>>>     Please use __dead2 instead to be consistent with legacy use of
>>>>>> similar gcc attributes.
>>>>>> Thanks,
>>>>>> -Ngie
>>>>>
>>>>>
>>>>> Why not use _Noreturn?  It's standardized by C11, so tools understand
>>>>> it better than __dead2.
>>>>
>>>>
>>>> Tools that can't understand #define __dead2 _Noreturn aren't worth
>>>> supporting.
>>>
>>> Some tools don't expand preprocessor macros.  Like my editor, for
>>> example, which highlights _Noreturn as a keyword but not __dead2.
>>
>>
>> Please use _Noreturn, because it's standard.  sys/cdefs.h already
>> defines it appropriately for C < C11.
>
>
> _Noreturn is far too hard to use.  The above use of it is a syntax error:
>
>     pts/12:bde at freefall:~/u3> cat z.c
>     void foo(void) _Noreturn;
>     _Noreturn void foo(void);
>     pts/12:bde at freefall:~/u3> cc -std=c11 z.c
>     z.c:1:16: error: '_Noreturn' keyword must precede function declarator
>     void foo(void) _Noreturn;
>                    ^~~~~~~~~
>     _Noreturn
>     1 error generated.
>
> sys/cdefs.h defines might define it appropropriately for C < C11, but
> it defines it as __dead2 for all C, so prevents the C11 _Noreturn
> keyword being used.  This normally breaks detection of the syntax error.
> Normally <sys/cdefs.h> is included first, so you __dead2 obfuscated by
> spelling it _Noreturn instead of C11 _Noreturn.
>
> Defining _Noreturn as __dead2 is wrong because it gives the opposite
> syntax error.  __dead2 can now be placed anywhere, but everything in
> sys/cdefs.h is supposed to be portable back to gcc-1.  __dead2 must
> be placed after the function for gcc-2.0, since __attribute__(()) had
> more restrictions then.  So if you write:
>
>    #include <sys/cdefs.h>
>    _Noreturn void foo(void);
>
> to satisfy the C11 syntax, then you get a syntax error for old gcc (> 1).
>
> This is just the start of the complications for soft-coded C11'isms.
> C11 also has noreturn.  You have to include <stdnoreturn.h> to get that.
> But you actiually get the _Noreturn macro which expands to __dead2.
>
> There are further complications for C++11.  sys/cdefs.h does have a
> correct-looking ifdef for C+11.  This gives the [[noreturn]] keyward
> instead of __dead2.   C11 doesn't have <stdnoreturn.h>.  I think its
> keyword must be spelled [[noreturn]].  This spelling is completely
> incompatibly with C.
>
> Bruce

Why do you say that cdefs.h should be compatible with gcc-1?  gcc-2
was released more than 25 years ago.  gcc-1 isn't the default compiler
for any architecture and isn't available in ports.  If anybody can
find a copy of gcc-1, I doubt that much of our codebase would compile.
It sounds to me that the best practice would be to place both __dead2
and _Noreturn before the function name.

-Alan