[PATCHv2] libstdc++: Make syncbuf _S_get_mutex definition extern

[Tomasz Kaminski]

On Fri, Aug 29, 2025 at 4:53 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> On Fri, 29 Aug 2025 at 14:31, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
> >
> >
> >
> > On Fri, Aug 29, 2025 at 2:06 AM Nathan Myers <ncm@cantrip.org> wrote:
> >>
> >> Changes in v2:
> >>  * Make a global function __syncbuf_get_mutex, not an extern template
> >>  instantiation.
> >>
> >> This patch creates a global function __syncbuf_get_mutex, gated by
> >> _GLIBCXX_HAS_GTHREADS, replacing a static instantiated member
> >> _S_get_mutex used in syncbuf<> construction, and makes the global
> >> symbol visible. A static local table of 16 mutexes is shared among
> >> all specializations of syncbuf<>, chosen on construction by a hash
> >> of the wrapped streambuf's address.
> >>
> >> It detaches the implementation of _S_get_mutex from the C++20 ABI.
> >
> > Wouldn't it be more extensible, if we always would define  _S_get_mutex
>
> Do you mean when ! HAS_GTHREADS? Is that useful?
>
Oh, I didn't mean to send this comment, after reading more context. Please
ignore it.

>
> >>
> >>
> >> libstdc++-v3/ChangeLog:
> >>         * include/std/syncstream: (syncbuf<>::__mutex) Remove
> _S_get_mutex,
> >>         use extern function instead.
> >>         * src/c++20/syncbuf.cc: Define global __syncbuf_get_mutex.
> >>         * src/c++20/Makefile.am: Mention syncbuf.cc.
> >>         * src/c++20/Makefile.in: Regenerate.
> >>         * config/abi/pre/gnu.ver: Mention mangled __syncbuf_get_mutex.
> >> ---
> >>  libstdc++-v3/config/abi/pre/gnu.ver |  3 ++
> >>  libstdc++-v3/include/std/syncstream | 19 ++++--------
> >>  libstdc++-v3/src/c++20/Makefile.am  |  2 +-
> >>  libstdc++-v3/src/c++20/Makefile.in  |  2 +-
> >>  libstdc++-v3/src/c++20/syncbuf.cc   | 45 +++++++++++++++++++++++++++++
> >>  5 files changed, 56 insertions(+), 15 deletions(-)
> >>  create mode 100644 libstdc++-v3/src/c++20/syncbuf.cc
> >>
> >> diff --git a/libstdc++-v3/config/abi/pre/gnu.ver
> b/libstdc++-v3/config/abi/pre/gnu.ver
> >> index e1601dc39d2..2e48241d51f 100644
> >> --- a/libstdc++-v3/config/abi/pre/gnu.ver
> >> +++ b/libstdc++-v3/config/abi/pre/gnu.ver
> >> @@ -2559,6 +2559,9 @@ GLIBCXX_3.4.35 {
> >>      _ZNSt6chrono9gps_clock3nowEv;
> >>      _ZNSt6chrono9tai_clock3nowEv;
> >>
> >> +    # mutex& __syncbuf_get_mutex(void*)
> >> +    _ZSt19__syncbuf_get_mutexPv;
> >> +
> >>      # __gnu_debug::_Safe_iterator_base and _Safe_sequence_base const
> >>
> _ZN11__gnu_debug19_Safe_iterator_base9_M_attachEPKNS_19_Safe_sequence_baseEb;
> >>
> _ZN11__gnu_debug19_Safe_iterator_base16_M_attach_singleEPKNS_19_Safe_sequence_baseEb;
> >> diff --git a/libstdc++-v3/include/std/syncstream
> b/libstdc++-v3/include/std/syncstream
> >> index e2b5a199ec9..649756b2581 100644
> >> --- a/libstdc++-v3/include/std/syncstream
> >> +++ b/libstdc++-v3/include/std/syncstream
> >> @@ -46,13 +46,17 @@
> >>  #include <bits/alloc_traits.h>
> >>  #include <bits/allocator.h>
> >>  #include <bits/functexcept.h>
> >> -#include <bits/functional_hash.h>
> >>  #include <bits/std_mutex.h>
> >>
> >>  namespace std _GLIBCXX_VISIBILITY(default)
> >>  {
> >>  _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>
> >> +#if _GLIBCXX_HAS_GTHREADS
> >> +  extern mutex&
> >> +  __syncbuf_get_mutex(void*);  // in src/c++20/syncbuf.cc
> >> +#endif
> >> +
> >>    template<typename _CharT, typename _Traits, typename _Alloc>
> >>      class basic_syncbuf : public __syncbuf_base<_CharT, _Traits>
> >>      {
> >> @@ -202,7 +206,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>         mutex* _M_mtx;
> >>
> >>         __mutex(void* __t)
> >> -         : _M_mtx(__t ? &_S_get_mutex(__t) : nullptr)
> >> +         : _M_mtx(__t ? &__syncbuf_get_mutex(__t) : nullptr)
> >>         { }
> >
> > You could put the declaration of the __syncbuf_get_mutex function inside
> the constructor,
> > to avoid having a separate _GLIBCXX_HAS_GTHREADS block.
> >          __mutex(void* __t)
> >           : _M_mtx(nullptr)
> >         {
> >            extern mutex&  __syncbuf_get_mutex(void*);  // in
> src/c++20/syncbuf.cc
> >            if (__t)
> >              _M_mtx = &_S_get_mutex(__t);
> >         }
> > But let's see what other think.
>
> Yes, we do that in a few other places, so that there's no
> user-accessible declaration of the function.
>
>
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