svn commit: r317755 - head/sbin/ifconfig

[Bruce Evans]

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