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Re: [wg14/wg21 liaison] On _Thread_local for better C++ interoperability with C (P2478)

From: Jens Maurer <Jens.Maurer_at_[hidden]>
Date: Fri, 11 Feb 2022 22:33:25 +0100
On 11/02/2022 22.20, Corentin via Liaison wrote:

> I think the most intuitive behavior is if
> extern "C" thread_local S foo;
> behaves just like _Thread_local would do in C and a C++ type
> that requires non-trivial initialization would simply not
> be allowed, i.e. it behaves like _Thread_local in clang
> in c++ mode.

Sounds good to me.

 - Survives WG14 making thread_local a real keyword.

 - Requires no collaboration from WG14.

 - Your header shared between C++ and C already ought to use 'extern "C"',
so this reduces the footgun surface.

 - thread_local in C++ with dynamic libraries is already a nightmare
(dynamic initialization order, squared), and this nicely sidesteps the

 - If C++ wants to do something on the C++ side (e.g. constdestroy or so),
it can do so at its own pace.

> I agree. Why would you be trying to use a type with non-trivial init in common code defined in a header, but so that it does different things in C and C++? If you need non-trivial init, define the code in a separate C++ transition unit, not in a header.
> But by that logic, do we want to change the grammar of C++ for this narrow scenario?

We're not changing the grammar. We're just adding a paragraph of restrictions
for thread_local. That seems palatable, given that 'extern "C"' already
causes restrictions for other areas of the C++ syntax.


Received on 2022-02-11 21:33:28