https://gcc.gnu.org/g:4eb12ddd32d53ef55c2bd9bb03db057f096fff8f
commit r16-3668-g4eb12ddd32d53ef55c2bd9bb03db057f096fff8f
Author: Nathan Myers <n...@cantrip.org>
Date: Thu Aug 28 13:11:57 2025 -0400
libstdc++: Make syncbuf _S_get_mutex definition extern.
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.
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.
Diff:
---
libstdc++-v3/config/abi/pre/gnu.ver | 3 +++
libstdc++-v3/include/std/syncstream | 22 +++++-------------
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(+), 18 deletions(-)
diff --git a/libstdc++-v3/config/abi/pre/gnu.ver
b/libstdc++-v3/config/abi/pre/gnu.ver
index e1601dc39d25..2e48241d51f9 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 e2b5a199ec93..92fbbdc760e7 100644
--- a/libstdc++-v3/include/std/syncstream
+++ b/libstdc++-v3/include/std/syncstream
@@ -46,7 +46,6 @@
#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)
@@ -199,11 +198,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
struct __mutex
{
#if _GLIBCXX_HAS_GTHREADS
- mutex* _M_mtx;
+ mutex* _M_mtx = nullptr;
- __mutex(void* __t)
- : _M_mtx(__t ? &_S_get_mutex(__t) : nullptr)
- { }
+ __mutex(void* __t) // __t is the underlying sbuf, as hash seed.
+ {
+ extern mutex& __syncbuf_get_mutex(void*); // in src/c++20/syncbuf.cc
+ if (__t) _M_mtx = &__syncbuf_get_mutex(__t);
+ }
void
swap(__mutex& __other) noexcept
@@ -220,17 +221,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
_M_mtx->unlock();
}
-
- // FIXME: This should be put in the .so
- static mutex&
- _S_get_mutex(void* __t)
- {
- const unsigned char __mask = 0xf;
- static mutex __m[__mask + 1];
-
- auto __key = _Hash_impl::hash(__t) & __mask;
- return __m[__key];
- }
#else
__mutex(void*) { }
void swap(__mutex&&) noexcept { }
diff --git a/libstdc++-v3/src/c++20/Makefile.am
b/libstdc++-v3/src/c++20/Makefile.am
index 736558ff24a7..384990aed776 100644
--- a/libstdc++-v3/src/c++20/Makefile.am
+++ b/libstdc++-v3/src/c++20/Makefile.am
@@ -36,7 +36,7 @@ else
inst_sources =
endif
-sources = tzdb.cc format.cc atomic.cc clock.cc
+sources = tzdb.cc format.cc atomic.cc clock.cc syncbuf.cc
vpath % $(top_srcdir)/src/c++20
diff --git a/libstdc++-v3/src/c++20/Makefile.in
b/libstdc++-v3/src/c++20/Makefile.in
index 3cb6d6fea5f3..1e005ae3c14e 100644
--- a/libstdc++-v3/src/c++20/Makefile.in
+++ b/libstdc++-v3/src/c++20/Makefile.in
@@ -432,7 +432,7 @@ headers =
@ENABLE_EXTERN_TEMPLATE_TRUE@inst_sources = \
@ENABLE_EXTERN_TEMPLATE_TRUE@ sstream-inst.cc
-sources = tzdb.cc format.cc atomic.cc clock.cc
+sources = tzdb.cc format.cc atomic.cc clock.cc syncbuf.cc
@GLIBCXX_HOSTED_FALSE@libc__20convenience_la_SOURCES =
@GLIBCXX_HOSTED_TRUE@libc__20convenience_la_SOURCES = $(sources)
$(inst_sources)
diff --git a/libstdc++-v3/src/c++20/syncbuf.cc
b/libstdc++-v3/src/c++20/syncbuf.cc
new file mode 100644
index 000000000000..150ba5e3ce4d
--- /dev/null
+++ b/libstdc++-v3/src/c++20/syncbuf.cc
@@ -0,0 +1,45 @@
+// Explicit instantiation file.
+
+// Copyright (C) 2020-2025 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library. This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
+// <http://www.gnu.org/licenses/>.
+
+#include <syncstream>
+#include <bits/functional_hash.h>
+
+#if _GLIBCXX_HAS_GTHREADS
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+mutex&
+__syncbuf_get_mutex(void* __t)
+ {
+ const unsigned char __mask = 0xf;
+ static mutex __m[__mask + 1];
+
+ auto __key = _Hash_impl::hash(__t) & __mask;
+ return __m[__key];
+ }
+
+_GLIBCXX_END_NAMESPACE_VERSION
+}
+#endif // GTHREADS
