Well, yes and no. The code I linked to in the pastebin is what demonstrates the issue. The
code in question is Apple’s libdispatch which I package separately as well as part of
Swift. In that situation they’re using a C++ file that uses the underlying primitives in
stdatomic in macros for their own higher-level functions, thus why stdatomic is ultimately
being invoked.
On 1 Feb 2022, at 3:26, Jonathan Wakely wrote:
> On Tue, 1 Feb 2022 at 09:15, Jonathan Wakely <jwakely(a)redhat.com> wrote:
>>
>> On Mon, 31 Jan 2022 at 22:45, Ron Olson <tachoknight(a)gmail.com> wrote:
>>>
>>> Hey all,
>>>
>>> I’m troubleshooting an issue and came up with this sample program:
https://pastebin.com/g9S8Z64q to demonstrate the problem. Basically, clang 13, on Rawhide,
won’t compile that program, while on Fedora 35 it does.
>>>
>>> The reason why is that on Rawhide, stdatomic.h exists under
/usr/include/c++/12 while it does not exist on 35, so clang uses its built-in stdatomic
per
https://clang.llvm.org/docs/LanguageExtensions.html#c11-atomic-operations.
>>
>> But that's about C, not C++.
>>
>>> If it uses its internal version, the sample program compiles fine.
>>>
>>> Looking at stdatomic.h on Rawhide, I see it’s gated by “#if __cplusplus >
202002L”, so that means C++2b or later, not C++20. This seems to create a problem for
clang which, since the file is present, wants to use it, but since it’s effectively empty
due to the #ifdef, compiling of the sample program fails.
>>>
>>> Is it possible to disable clang’s use of the header file as a flag? I’ve been
unable to find anything like that, and obviously renaming the header is out of the
question. Also, is it correct to the setting stdatomic.h to only be used by c++2b?
>>
>> Yes, that's 100% correct.
>>
>> Clang is at fault here. <stdatomic.h> does not exist in C++ up to and
>> including the current C++20 standard, so Clang should not assume it's
>> present or usable.
>>
>> In C++23 there is now a <stdatomic.h> header in the C++ library for
>> compatibility. That's what I added to GCC, and that's what Clang is
>> now picking up.
>>
>> Why is C++ code in Clang using a C header that isn't part of C++?
>
> I think I misread, this isn't code in Clang, this is your code and
> you're just using Clang, right?
>
> So then you can fix your code fairly easily.
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