On Fri, 3 Mar 2023 at 08:20, Alexandre Oliva via Libstdc++ <
libstd...@gcc.gnu.org> wrote:
> On Feb 17, 2023, Jonathan Wakely <jwakely....@gmail.com> wrote:
>
> > The proper fix is to ensure the program has a non-weak reference to
> > pthread_join without extra help (or use a recent glibc where it always
> > works).
>
> Indeed! How about this? Regstrapped on x86_64-linux-gnu, also tested
> on arm-vx7r2 (gcc-12); verified that strong references are present in
> 95989.o, but not in libstdc++.a or .so. Ok to install?
>
> (nit: the static local in a ctor of a template class may make for
> multiple copies. Maybe a non-template always-inline function called by
> all instantiations would be better.)
>
Yeah, that does seem less than ideal.
Jakub previously suggested doing this for PR 61841, which was a similar
problem with pthread_create:
__asm ("" : : "r" (&pthread_create)); would not be optimized away.
That would avoid the multiple copies.
Alternatively we could get really creative and cast the addresses of both
&pthread_create and &pthread_join to uintptr_t and XOR them, and pass that
as the __depend argument (which is never actually dereferenced, it's only
there to create a link-time dependency).
>
>
> link pthread_join from std::thread ctor
>
> Like pthread_create, pthread_join may fail to be statically linked in
> absent strong uses, so add to user code strong references to both when
> std::thread objects are created.
>
>
> for libstdc++-v3/ChangeLog
>
> * include/bits/std_thread.h (std::thread ctor): Add strong
> reference to pthread_join.
>
> ---
> libstdc++-v3/include/bits/std_thread.h | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/libstdc++-v3/include/bits/std_thread.h
> b/libstdc++-v3/include/bits/std_thread.h
> index adbd3928ff783..4bda350fa2c7b 100644
> --- a/libstdc++-v3/include/bits/std_thread.h
> +++ b/libstdc++-v3/include/bits/std_thread.h
> @@ -145,6 +145,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> #ifdef GTHR_ACTIVE_PROXY
> // Create a reference to pthread_create, not just the gthr weak
> symbol.
> auto __depend = reinterpret_cast<void(*)()>(&pthread_create);
> + static auto __attribute__((__used__)) __depend_join =
> &pthread_join;
> #else
> auto __depend = nullptr;
> #endif
>
>
> --
> Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/
> Free Software Activist GNU Toolchain Engineer
> Disinformation flourishes because many people care deeply about injustice
> but very few check the facts. Ask me about <https://stallmansupport.org>
>
>
