Re: [[GCC13][Patch][V3] 1/2] Add a new option -fstrict-flex-array[=n] and new attribute strict_flex_array

[Qing Zhao <qing.zhao@oracle.com>]

> On Aug 31, 2022, at 2:55 PM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
> 
> 
>> On Aug 31, 2022, at 1:21 PM, Joseph Myers <joseph@codesourcery.com> wrote:
>> 
>> On Wed, 31 Aug 2022, Qing Zhao via Gcc-patches wrote:
>> 
>>>> "a GNU extension" suggests a particular language feature, but I think 
>>>> you're actually referring here to a whole language version rather than an 
>>>> individual feature.
>>> 
>>> Is “not supported by GNU extension GNU89” better?
>> 
>> There are no existing diagnostics referring to GNU89 at all.  I don't 
>> think "GNU extension" needs to be mentioned in that diagnostic, but I also 
>> think that having that diagnostic at all is ill-conceived.
>> 
>>>> In any case, -std=gnu89 supports flexible array members.
>>> 
>>> Yes, but only [0],[1] are supported as flexible array members.  The C99 
>>> flexible array member [] is not supported by GNU89, right?
>> 
>> C99 flexible array members are fully supported in GNU89 mode.  In general, 
>> any feature from a new language version that doesn't affect code that was 
>> valid in previous versions is likely to be accepted as an extension with 
>> options for older language versions.
> 
> 
> We have a previous discussion on this: (https://gcc.gnu.org/pipermail/gcc-patches/2022-July/599067.html)
> 
> And looks like that the previous conclusion was wrong… please see the following:
> 
> ======
> 
>> How is level 3 (thus -fstrict-flex-array) interpreted when you specify 
>> -std=c89?  How for -std=gnu89?
> 
> 1. what’s the major difference between -std=c89 and -std=gnu89 on flexible array? (Checked online, cannot find a concrete answer on this).
> 	** my understanding is:   -std=c89 will not support any flexible array (neither [], [0], [1]), but -std=gnu89 will support [0] and [1], but not [].
>        Is this correct?
> 
> If my answer to the first question is correct, then:
> 
> 2. When -fstrict-flex-array=n and -std=c89 present at the same time, which one has the higher priority? 
>    	** I think that -std=c89 should be honored over -fstrict-flex-array, therefore we should disable -fstrict-flex-array=n when n > 0  and issue warnings to the user.
> 
> 
> 3. how about -fstrict-flex-array=n and -std=gnu89 present at the same time? 
>    	** When -std=gnu89 present, [] is not supported. So, we need to issue an warning to disable -fstrict-flex-array=3; but level 1 and level 2 is Okay.
> 
> We also need to document the above.
> ====
> 
> So, from my understanding from what you said so far, 
> 
> -std=c89 will not support any flexible array (neither [], [0], [1]),  but -std=gnu89 will support ALL flexible array including [0], [1], and [].
> 
> Is this understanding correct?

And also for C++:

-std=c++98 will not support any flexible array, but -std=gnu++98 will support ALL flexible array ([0],[1].[])?

Qing
> 
> thanks.
> 
> Qing
> 
> 
>> 
>> -- 
>> Joseph S. Myers
>> joseph@codesourcery.com