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* [gcc r13-4857] libstdc++: Avoid recursion in __nothrow_wait_cv::wait [PR105730]
@ 2022-12-22 23:35 Jonathan Wakely
  0 siblings, 0 replies; only message in thread
From: Jonathan Wakely @ 2022-12-22 23:35 UTC (permalink / raw)
  To: gcc-cvs, libstdc++-cvs

commit r13-4857-gee4af2ed0b7322884ec4ff537564683c3749b813
Author: Jonathan Wakely <>
Date:   Thu Dec 22 09:56:47 2022 +0000

    libstdc++: Avoid recursion in __nothrow_wait_cv::wait [PR105730]
    The commit r12-5877-g9e18a25331fa25 removed the incorrect
    noexcept-specifier from std::condition_variable::wait and gave the new
    symbol version @@GLIBCXX_3.4.30. It also redefined the original symbol
    std::condition_variable::wait(unique_lock<mutex>&)@GLIBCXX_3.4.11 as an
    alias for a new symbol, __gnu_cxx::__nothrow_wait_cv::wait, which still
    has the incorrect noexcept guarantee. That __nothrow_wait_cv::wait is
    just a wrapper around the real condition_variable::wait which adds
    noexcept and so terminates on a __forced_unwind exception.
    This doesn't work on uclibc, possibly due to a dynamic linker bug. When
    __nothrow_wait_cv::wait calls the condition_variable::wait function it
    binds to the alias symbol, which means it just calls itself recursively
    until the stack overflows.
    This change avoids the possibility of a recursive call by changing the
    __nothrow_wait_cv::wait function so that instead of calling
    condition_variable::wait it re-implements it. This requires accessing
    the private _M_cond member of condition_variable, so we need to use the
    trick of instantiating a template with the member-pointer of the private
            PR libstdc++/105730
            * src/c++11/ (__nothrow_wait_cv::wait):
            Access private data member of base class and call its wait

 libstdc++-v3/src/c++11/ | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/c++11/ b/libstdc++-v3/src/c++11/
index e3a8b8403ca..3cef3bc0714 100644
--- a/libstdc++-v3/src/c++11/
+++ b/libstdc++-v3/src/c++11/
 namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
+  // Pointer-to-member for private std::condition_variable::_M_cond member.
+  std::__condvar std::condition_variable::* __base_member;
+  template<std::__condvar std::condition_variable::*X>
+    struct cracker
+    { static std::__condvar std::condition_variable::* value; };
+  // Initializer for this static member also initializes __base_member.
+  template<std::__condvar std::condition_variable::*X>
+    std::__condvar std::condition_variable::*
+      cracker<X>::value = __base_member = X;
+  // Explicit instantiation is allowed to access the private member.
+  template class cracker<&std::condition_variable::_M_cond>;
 struct __nothrow_wait_cv : std::condition_variable
   void wait(std::unique_lock<std::mutex>&) noexcept;
@@ -76,7 +94,9 @@ __attribute__((used))
 __nothrow_wait_cv::wait(std::unique_lock<std::mutex>& lock) noexcept
-  this->condition_variable::wait(lock);
+  // In theory this could be simply this->std::condition_variable::wait(lock)
+  // but with uclibc that binds to the @GLIBCXX_3.4.11 symbol, see PR 105730.
+  (this->*__base_member).wait(*lock.mutex());
 } // namespace __gnu_cxx

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