[cfe-dev] [RFC] Make <atomic> and <stdatomic.h> explicitly incompatible

Wed Mar 7 13:38:54 PST 2018

It is occasionally necessary to write a C header that uses atomic types
then include it in a C++ file. For example, implementing an abi defined in
C using C++.

This didn't quite work out of the box - I think I needed to #include
<atomic> inside extern "C" or some similar absurdity - but I did get valid
asm out of the result without modifying clang. Sadly I can't currently
access the source to check the details.

If this change goes ahead, would I no longer be able to use atomic types
from a C header in C++?

Cheers!

Jon

On 7 Mar 2018 16:15, "via cfe-dev" <cfe-dev at lists.llvm.org> wrote:

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Today's Topics:

   1. [RFC] Make <atomic> and <stdatomic.h> explicitly  incompatible
      (Volodymyr Sapsai via cfe-dev)
   2. Re: [RFC] Make <atomic> and <stdatomic.h> explicitly
      incompatible (Shoaib Meenai via cfe-dev)

----------------------------------------------------------------------

Message: 1
Date: Wed, 07 Mar 2018 12:32:05 -0800
From: Volodymyr Sapsai via cfe-dev <cfe-dev at lists.llvm.org>
To: Clang Dev <cfe-dev at lists.llvm.org>
Subject: [cfe-dev] [RFC] Make <atomic> and <stdatomic.h> explicitly
        incompatible
Message-ID: <C1343C23-7BE7-43A2-95BC-239F410913D8 at apple.com>
Content-Type: text/plain; charset="utf-8"

Hello all,

Currently atomics are available in C through <stdatomic.h> and in C++
through <atomic>. These headers have some common names, so using both of
them in the same translation unit usually doesn’t work. The downside is
that in some cases diagnostic can be overly verbose and overwhelming. For
example, compiling just 2 lines

#include <stdatomic.h>
#include <atomic>

results in 68 errors.

I am proposing to make these headers explicitly incompatible, so we have
just a single error telling so instead of multiple errors. What do you
think?

Alternative solution is to make these headers compatible. But the consensus
seems to be that it’s not the right solution. See https://reviews.llvm.org/
D26376 <https://reviews.llvm.org/D26376> for more details. There is also a
proposal P0943R0: Support C atomics in C++ <http://open-std.org/jtc1/
sc22/wg21/docs/papers/2018/p0943r0.html>.  As far as I can tell, my
proposal doesn’t interfere with P0943R0 but will be obsolete if that
proposal is accepted. Regardless of the future development I think it is
still beneficial to have better diagnostics in the short term.

I am not providing concrete implementation yet, at first I want to know the
opinion of the community on the matter.

Thanks,
Volodymyr
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Message: 2
Date: Wed, 7 Mar 2018 21:15:02 +0000
From: Shoaib Meenai via cfe-dev <cfe-dev at lists.llvm.org>
To: Volodymyr Sapsai <vsapsai at apple.com>, Clang Dev
        <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [RFC] Make <atomic> and <stdatomic.h>
        explicitly incompatible
Message-ID: <43BFBEC0-0C03-4B7A-874F-62F562C9FDB5 at fb.com>
Content-Type: text/plain; charset="utf-8"

I'd be in favor of making them explicitly incompatible. I ran into this
recently, found the incompatibility quite surprising, and had to do a bunch
of research (looking at the relevant standards, doing experiments across a
bunch of compilers and standard libraries, etc.) to convince myself that it
was expected behavior. An explicit error would be super nice.

From: cfe-dev <cfe-dev-bounces at lists.llvm.org> on behalf of cfe-dev <
cfe-dev at lists.llvm.org>
Reply-To: Volodymyr Sapsai <vsapsai at apple.com>
Date: Wednesday, March 7, 2018 at 12:32 PM
To: cfe-dev <cfe-dev at lists.llvm.org>
Subject: [cfe-dev] [RFC] Make <atomic> and <stdatomic.h> explicitly
incompatible

Hello all,

Currently atomics are available in C through <stdatomic.h> and in C++
through <atomic>. These headers have some common names, so using both of
them in the same translation unit usually doesn’t work. The downside is
that in some cases diagnostic can be overly verbose and overwhelming. For
example, compiling just 2 lines

#include <stdatomic.h>
#include <atomic>

results in 68 errors.

I am proposing to make these headers explicitly incompatible, so we have
just a single error telling so instead of multiple errors. What do you
think?

Alternative solution is to make these headers compatible. But the consensus
seems to be that it’s not the right solution. See https://reviews.llvm.org/
D26376<https://urldefense.proofpoint.com/v2/url?u=https-
3A__reviews.llvm.org_D26376&d=DwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=
o3kDXzdBUE3ljQXKeTWOMw&m=f2XAChgmKGyURiNTGeNTF1t1Fpo5BL
SqNdUKyYAZYeM&s=O1AIlpTPrgXe3-fXW3-VqVGTAUhg2qKBozM1iOtQhbE&e=> for more
details. There is also a proposal P0943R0: Support C atomics in C++<
https://urldefense.proofpoint.com/v2/url?u=http-
3A__open-2Dstd.org_jtc1_sc22_wg21_docs_papers_2018_p0943r0.html&d=DwMFaQ&c=
5VD0RTtNlTh3ycd41b3MUw&r=o3kDXzdBUE3ljQXKeTWOMw&m=
f2XAChgmKGyURiNTGeNTF1t1Fpo5BLSqNdUKyYAZYeM&s=iHKcnTreK84QBWD-j_3_n7ww5ON1-
EzKzni3NgSgiaY&e=>.  As far as I can tell, my proposal doesn’t interfere
with P0943R0 but will be obsolete if that proposal is accepted. Regardless
of the future development I think it is still beneficial to have better
diagnostics in the short term.

I am not providing concrete implementation yet, at first I want to know the
opinion of the community on the matter.

Thanks,
Volodymyr
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