On Fri, Feb 11, 2022 at 4:44 PM Jens Maurer via Liaison <liaison@lists.isocpp.org> wrote:

On 11/02/2022 22.39, Corentin wrote:
>
>
> On Fri, Feb 11, 2022 at 10:33 PM Jens Maurer <Jens.Maurer@gmx.net <mailto:Jens.Maurer@gmx.net>> wrote:
>
> 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
> problem.
>
>   - 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.
>
>
> Don't we need to allow extern "C" static? Afaict this would be novel

extern "C" {
static int x = 1;
}

works with gcc today.

Jens

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