libstdc++-v3/src/c++11/mutex.cc

// mutex -*- C++ -*-

// Copyright (C) 2008-2026 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 <mutex>

#ifdef _GLIBCXX_HAS_GTHREADS

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

#ifdef _GLIBCXX_HAVE_TLS
  __thread void* __once_callable;
  __thread void (*__once_call)();

# ifdef _GLIBCXX_NO_EXTERN_THREAD_LOCAL

  // When thread-local variables can't be exported, these functions are called
  // to retrieve these variables.
  void*&
  __get_once_callable() noexcept
  { return __once_callable; }

  std::add_lvalue_reference<void (*)()>::type
  __get_once_call() noexcept
  { return __once_call; }

# endif // _GLIBCXX_NO_EXTERN_THREAD_LOCAL

  extern "C" void __once_proxy()
  {
    // The caller stored a function pointer in __once_call. If it requires
    // any state, it gets it from __once_callable.
    __once_call();
  }

#else // ! TLS

  // Explicit instantiation due to -fno-implicit-instantiation.
  template class function<void()>;

  function<void()> __once_functor;

  mutex&
  __get_once_mutex()
  {
    static mutex once_mutex;
    return once_mutex;
  }

namespace
{
  // Store ptr in a global variable and return the previous value.
  inline unique_lock<mutex>*
  set_lock_ptr(unique_lock<mutex>* ptr)
  {
    static unique_lock<mutex>* __once_functor_lock_ptr = nullptr;
    return std::__exchange(__once_functor_lock_ptr, ptr);
  }
}

  // code linked against ABI 3.4.12 and later uses this
  void
  __set_once_functor_lock_ptr(unique_lock<mutex>* __ptr)
  {
    (void) set_lock_ptr(__ptr);
  }

  // unsafe - retained for compatibility with ABI 3.4.11
  unique_lock<mutex>&
  __get_once_functor_lock()
  {
    static unique_lock<mutex> once_functor_lock(__get_once_mutex(), defer_lock);
    return once_functor_lock;
  }

  // This is called via pthread_once while __get_once_mutex() is locked.
  extern "C" void
  __once_proxy()
  {
    // Get the callable out of the global functor.
    function<void()> callable = std::move(__once_functor);

    // Then unlock the global mutex
    if (unique_lock<mutex>* lock = set_lock_ptr(nullptr))
    {
      // Caller is using the new ABI and provided a pointer to its lock.
      lock->unlock();
    }
    else
      __get_once_functor_lock().unlock();  // global lock

    // Finally, invoke the callable.
    callable();
  }
#endif // ! TLS

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // _GLIBCXX_HAS_GTHREADS