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Edit File : shared_mutex \| Size : 18.95 KB
// <shared_mutex> -- C++ -- // Copyright (C) 2013-2018 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/>. /** @file include/shared_mutex * This is a Standard C++ Library header. / #ifndef _GLIBCXX_SHARED_MUTEX #define _GLIBCXX_SHARED_MUTEX 1 #pragma GCC system_header #if __cplusplus >= 201402L #include <bits/c++config.h> #include <condition_variable> #include <bits/functexcept.h> namespace std _GLIBCXX_VISIBILITY(default) { _GLIBCXX_BEGIN_NAMESPACE_VERSION /* * @ingroup mutexes * @{ / #ifdef _GLIBCXX_USE_C99_STDINT_TR1 #ifdef _GLIBCXX_HAS_GTHREADS #if __cplusplus >= 201703L #define __cpp_lib_shared_mutex 201505 class shared_mutex; #endif #define __cpp_lib_shared_timed_mutex 201402 class shared_timed_mutex; #if _GLIBCXX_USE_PTHREAD_RWLOCK_T /// A shared mutex type implemented using pthread_rwlock_t. class __shared_mutex_pthread { friend class shared_timed_mutex; #ifdef PTHREAD_RWLOCK_INITIALIZER pthread_rwlock_t _M_rwlock = PTHREAD_RWLOCK_INITIALIZER; public: __shared_mutex_pthread() = default; ~__shared_mutex_pthread() = default; #else pthread_rwlock_t _M_rwlock; public: __shared_mutex_pthread() { int __ret = pthread_rwlock_init(&_M_rwlock, NULL); if (__ret == ENOMEM) __throw_bad_alloc(); else if (__ret == EAGAIN) __throw_system_error(int(errc::resource_unavailable_try_again)); else if (__ret == EPERM) __throw_system_error(int(errc::operation_not_permitted)); // Errors not handled: EBUSY, EINVAL __glibcxx_assert(__ret == 0); } ~__shared_mutex_pthread() { int __ret __attribute((__unused__)) = pthread_rwlock_destroy(&_M_rwlock); // Errors not handled: EBUSY, EINVAL __glibcxx_assert(__ret == 0); } #endif __shared_mutex_pthread(const __shared_mutex_pthread&) = delete; __shared_mutex_pthread& operator=(const __shared_mutex_pthread&) = delete; void lock() { int __ret = pthread_rwlock_wrlock(&_M_rwlock); if (__ret == EDEADLK) __throw_system_error(int(errc::resource_deadlock_would_occur)); // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); } bool try_lock() { int __ret = pthread_rwlock_trywrlock(&_M_rwlock); if (__ret == EBUSY) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } void unlock() { int __ret __attribute((__unused__)) = pthread_rwlock_unlock(&_M_rwlock); // Errors not handled: EPERM, EBUSY, EINVAL __glibcxx_assert(__ret == 0); } // Shared ownership void lock_shared() { int __ret; // We retry if we exceeded the maximum number of read locks supported by // the POSIX implementation; this can result in busy-waiting, but this // is okay based on the current specification of forward progress // guarantees by the standard. do __ret = pthread_rwlock_rdlock(&_M_rwlock); while (__ret == EAGAIN); if (__ret == EDEADLK) __throw_system_error(int(errc::resource_deadlock_would_occur)); // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); } bool try_lock_shared() { int __ret = pthread_rwlock_tryrdlock(&_M_rwlock); // If the maximum number of read locks has been exceeded, we just fail // to acquire the lock. Unlike for lock(), we are not allowed to throw // an exception. if (__ret == EBUSY \|\| __ret == EAGAIN) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } void unlock_shared() { unlock(); } void native_handle() { return &_M_rwlock; } }; #endif #if ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK) /// A shared mutex type implemented using std::condition_variable. class __shared_mutex_cv { friend class shared_timed_mutex; // Based on Howard Hinnant's reference implementation from N2406. // The high bit of _M_state is the write-entered flag which is set to // indicate a writer has taken the lock or is queuing to take the lock. // The remaining bits are the count of reader locks. // // To take a reader lock, block on gate1 while the write-entered flag is // set or the maximum number of reader locks is held, then increment the // reader lock count. // To release, decrement the count, then if the write-entered flag is set // and the count is zero then signal gate2 to wake a queued writer, // otherwise if the maximum number of reader locks was held signal gate1 // to wake a reader. // // To take a writer lock, block on gate1 while the write-entered flag is // set, then set the write-entered flag to start queueing, then block on // gate2 while the number of reader locks is non-zero. // To release, unset the write-entered flag and signal gate1 to wake all // blocked readers and writers. // // This means that when no reader locks are held readers and writers get // equal priority. When one or more reader locks is held a writer gets // priority and no more reader locks can be taken while the writer is // queued. // Only locked when accessing _M_state or waiting on condition variables. mutex _M_mut; // Used to block while write-entered is set or reader count at maximum. condition_variable _M_gate1; // Used to block queued writers while reader count is non-zero. condition_variable _M_gate2; // The write-entered flag and reader count. unsigned _M_state; static constexpr unsigned _S_write_entered = 1U << (sizeof(unsigned)__CHAR_BIT__ - 1); static constexpr unsigned _S_max_readers = ~_S_write_entered; // Test whether the write-entered flag is set. _M_mut must be locked. bool _M_write_entered() const { return _M_state & _S_write_entered; } // The number of reader locks currently held. _M_mut must be locked. unsigned _M_readers() const { return _M_state & _S_max_readers; } public: __shared_mutex_cv() : _M_state(0) {} ~__shared_mutex_cv() { __glibcxx_assert( _M_state == 0 ); } __shared_mutex_cv(const __shared_mutex_cv&) = delete; __shared_mutex_cv& operator=(const __shared_mutex_cv&) = delete; // Exclusive ownership void lock() { unique_lock<mutex> __lk(_M_mut); // Wait until we can set the write-entered flag. _M_gate1.wait(__lk, [=]{ return !_M_write_entered(); }); _M_state \|= _S_write_entered; // Then wait until there are no more readers. _M_gate2.wait(__lk, [=]{ return _M_readers() == 0; }); } bool try_lock() { unique_lock<mutex> __lk(_M_mut, try_to_lock); if (__lk.owns_lock() && _M_state == 0) { _M_state = _S_write_entered; return true; } return false; } void unlock() { lock_guard<mutex> __lk(_M_mut); __glibcxx_assert( _M_write_entered() ); _M_state = 0; // call notify_all() while mutex is held so that another thread can't // lock and unlock the mutex then destroy this before we make the call. _M_gate1.notify_all(); } // Shared ownership void lock_shared() { unique_lock<mutex> __lk(_M_mut); _M_gate1.wait(__lk, [=]{ return _M_state < _S_max_readers; }); ++_M_state; } bool try_lock_shared() { unique_lock<mutex> __lk(_M_mut, try_to_lock); if (!__lk.owns_lock()) return false; if (_M_state < _S_max_readers) { ++_M_state; return true; } return false; } void unlock_shared() { lock_guard<mutex> __lk(_M_mut); __glibcxx_assert( _M_readers() > 0 ); auto __prev = _M_state--; if (_M_write_entered()) { // Wake the queued writer if there are no more readers. if (_M_readers() == 0) _M_gate2.notify_one(); // No need to notify gate1 because we give priority to the queued // writer, and that writer will eventually notify gate1 after it // clears the write-entered flag. } else { // Wake any thread that was blocked on reader overflow. if (__prev == _S_max_readers) _M_gate1.notify_one(); } } }; #endif #if __cplusplus > 201402L /// The standard shared mutex type. class shared_mutex { public: shared_mutex() = default; ~shared_mutex() = default; shared_mutex(const shared_mutex&) = delete; shared_mutex& operator=(const shared_mutex&) = delete; // Exclusive ownership void lock() { _M_impl.lock(); } bool try_lock() { return _M_impl.try_lock(); } void unlock() { _M_impl.unlock(); } // Shared ownership void lock_shared() { _M_impl.lock_shared(); } bool try_lock_shared() { return _M_impl.try_lock_shared(); } void unlock_shared() { _M_impl.unlock_shared(); } #if _GLIBCXX_USE_PTHREAD_RWLOCK_T typedef void* native_handle_type; native_handle_type native_handle() { return _M_impl.native_handle(); } private: __shared_mutex_pthread _M_impl; #else private: __shared_mutex_cv _M_impl; #endif }; #endif // C++17 #if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK using __shared_timed_mutex_base = __shared_mutex_pthread; #else using __shared_timed_mutex_base = __shared_mutex_cv; #endif /// The standard shared timed mutex type. class shared_timed_mutex : private __shared_timed_mutex_base { using _Base = __shared_timed_mutex_base; // Must use the same clock as condition_variable for __shared_mutex_cv. typedef chrono::system_clock __clock_t; public: shared_timed_mutex() = default; ~shared_timed_mutex() = default; shared_timed_mutex(const shared_timed_mutex&) = delete; shared_timed_mutex& operator=(const shared_timed_mutex&) = delete; // Exclusive ownership void lock() { _Base::lock(); } bool try_lock() { return _Base::try_lock(); } void unlock() { _Base::unlock(); } template<typename _Rep, typename _Period> bool try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time) { return try_lock_until(__clock_t::now() + __rel_time); } // Shared ownership void lock_shared() { _Base::lock_shared(); } bool try_lock_shared() { return _Base::try_lock_shared(); } void unlock_shared() { _Base::unlock_shared(); } template<typename _Rep, typename _Period> bool try_lock_shared_for(const chrono::duration<_Rep, _Period>& __rel_time) { return try_lock_shared_until(__clock_t::now() + __rel_time); } #if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK // Exclusive ownership template<typename _Duration> bool try_lock_until(const chrono::time_point<__clock_t, _Duration>& __atime) { auto __s = chrono::time_point_cast<chrono::seconds>(__atime); auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s); __gthread_time_t __ts = { static_cast<std::time_t>(__s.time_since_epoch().count()), static_cast<long>(__ns.count()) }; int __ret = pthread_rwlock_timedwrlock(&_M_rwlock, &__ts); // On self-deadlock, we just fail to acquire the lock. Technically, // the program violated the precondition. if (__ret == ETIMEDOUT \|\| __ret == EDEADLK) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } template<typename _Clock, typename _Duration> bool try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { // DR 887 - Sync unknown clock to known clock. const typename _Clock::time_point __c_entry = _Clock::now(); const __clock_t::time_point __s_entry = __clock_t::now(); const auto __delta = __abs_time - __c_entry; const auto __s_atime = __s_entry + __delta; return try_lock_until(__s_atime); } // Shared ownership template<typename _Duration> bool try_lock_shared_until(const chrono::time_point<__clock_t, _Duration>& __atime) { auto __s = chrono::time_point_cast<chrono::seconds>(__atime); auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s); __gthread_time_t __ts = { static_cast<std::time_t>(__s.time_since_epoch().count()), static_cast<long>(__ns.count()) }; int __ret; // Unlike for lock(), we are not allowed to throw an exception so if // the maximum number of read locks has been exceeded, or we would // deadlock, we just try to acquire the lock again (and will time out // eventually). // In cases where we would exceed the maximum number of read locks // throughout the whole time until the timeout, we will fail to // acquire the lock even if it would be logically free; however, this // is allowed by the standard, and we made a "strong effort" // (see C++14 30.4.1.4p26). // For cases where the implementation detects a deadlock we // intentionally block and timeout so that an early return isn't // mistaken for a spurious failure, which might help users realise // there is a deadlock. do __ret = pthread_rwlock_timedrdlock(&_M_rwlock, &__ts); while (__ret == EAGAIN \|\| __ret == EDEADLK); if (__ret == ETIMEDOUT) return false; // Errors not handled: EINVAL __glibcxx_assert(__ret == 0); return true; } template<typename _Clock, typename _Duration> bool try_lock_shared_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { // DR 887 - Sync unknown clock to known clock. const typename _Clock::time_point __c_entry = _Clock::now(); const __clock_t::time_point __s_entry = __clock_t::now(); const auto __delta = __abs_time - __c_entry; const auto __s_atime = __s_entry + __delta; return try_lock_shared_until(__s_atime); } #else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK) // Exclusive ownership template<typename _Clock, typename _Duration> bool try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { unique_lock<mutex> __lk(_M_mut); if (!_M_gate1.wait_until(__lk, __abs_time, [=]{ return !_M_write_entered(); })) { return false; } _M_state \|= _S_write_entered; if (!_M_gate2.wait_until(__lk, __abs_time, [=]{ return _M_readers() == 0; })) { _M_state ^= _S_write_entered; // Wake all threads blocked while the write-entered flag was set. _M_gate1.notify_all(); return false; } return true; } // Shared ownership template <typename _Clock, typename _Duration> bool try_lock_shared_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { unique_lock<mutex> __lk(_M_mut); if (!_M_gate1.wait_until(__lk, __abs_time, [=]{ return _M_state < _S_max_readers; })) { return false; } ++_M_state; return true; } #endif // _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK }; #endif // _GLIBCXX_HAS_GTHREADS /// shared_lock template<typename _Mutex> class shared_lock { public: typedef _Mutex mutex_type; // Shared locking shared_lock() noexcept : _M_pm(nullptr), _M_owns(false) { } explicit shared_lock(mutex_type& __m) : _M_pm(std::__addressof(__m)), _M_owns(true) { __m.lock_shared(); } shared_lock(mutex_type& __m, defer_lock_t) noexcept : _M_pm(std::__addressof(__m)), _M_owns(false) { } shared_lock(mutex_type& __m, try_to_lock_t) : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared()) { } shared_lock(mutex_type& __m, adopt_lock_t) : _M_pm(std::__addressof(__m)), _M_owns(true) { } template<typename _Clock, typename _Duration> shared_lock(mutex_type& __m, const chrono::time_point<_Clock, _Duration>& __abs_time) : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared_until(__abs_time)) { } template<typename _Rep, typename _Period> shared_lock(mutex_type& __m, const chrono::duration<_Rep, _Period>& __rel_time) : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared_for(__rel_time)) { } ~shared_lock() { if (_M_owns) _M_pm->unlock_shared(); } shared_lock(shared_lock const&) = delete; shared_lock& operator=(shared_lock const&) = delete; shared_lock(shared_lock&& __sl) noexcept : shared_lock() { swap(__sl); } shared_lock& operator=(shared_lock&& __sl) noexcept { shared_lock(std::move(__sl)).swap(this); return this; } void lock() { _M_lockable(); _M_pm->lock_shared(); _M_owns = true; } bool try_lock() { _M_lockable(); return _M_owns = _M_pm->try_lock_shared(); } template<typename _Rep, typename _Period> bool try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time) { _M_lockable(); return _M_owns = _M_pm->try_lock_shared_for(__rel_time); } template<typename _Clock, typename _Duration> bool try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time) { _M_lockable(); return _M_owns = _M_pm->try_lock_shared_until(__abs_time); } void unlock() { if (!_M_owns) __throw_system_error(int(errc::resource_deadlock_would_occur)); _M_pm->unlock_shared(); _M_owns = false; } // Setters void swap(shared_lock& __u) noexcept { std::swap(_M_pm, __u._M_pm); std::swap(_M_owns, __u._M_owns); } mutex_type* release() noexcept { _M_owns = false; return std::exchange(_M_pm, nullptr); } // Getters bool owns_lock() const noexcept { return _M_owns; } explicit operator bool() const noexcept { return _M_owns; } mutex_type* mutex() const noexcept { return _M_pm; } private: void _M_lockable() const { if (_M_pm == nullptr) __throw_system_error(int(errc::operation_not_permitted)); if (_M_owns) __throw_system_error(int(errc::resource_deadlock_would_occur)); } mutex_type* _M_pm; bool _M_owns; }; /// Swap specialization for shared_lock template<typename _Mutex> void swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) noexcept { __x.swap(__y); } #endif // _GLIBCXX_USE_C99_STDINT_TR1 // @} group mutexes _GLIBCXX_END_NAMESPACE_VERSION } // namespace #endif // C++14 #endif // _GLIBCXX_SHARED_MUTEX Back

Edit File : shared_mutex | Size : 18.95 KB

// <shared_mutex> -*- C++ -*-

// Copyright (C) 2013-2018 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/>.

/** @file include/shared_mutex
 *  This is a Standard C++ Library header.
 */

#ifndef _GLIBCXX_SHARED_MUTEX
#define _GLIBCXX_SHARED_MUTEX 1

#pragma GCC system_header

#if __cplusplus >= 201402L

#include <bits/c++config.h>
#include <condition_variable>
#include <bits/functexcept.h>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

/**
   * @ingroup mutexes
   * @{
   */

#ifdef _GLIBCXX_USE_C99_STDINT_TR1
#ifdef _GLIBCXX_HAS_GTHREADS

#if __cplusplus >= 201703L
#define __cpp_lib_shared_mutex 201505
  class shared_mutex;
#endif

#define __cpp_lib_shared_timed_mutex 201402
  class shared_timed_mutex;

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
  /// A shared mutex type implemented using pthread_rwlock_t.
  class __shared_mutex_pthread
  {
    friend class shared_timed_mutex;

#ifdef PTHREAD_RWLOCK_INITIALIZER
    pthread_rwlock_t	_M_rwlock = PTHREAD_RWLOCK_INITIALIZER;

public:
    __shared_mutex_pthread() = default;
    ~__shared_mutex_pthread() = default;
#else
    pthread_rwlock_t	_M_rwlock;

public:
    __shared_mutex_pthread()
    {
      int __ret = pthread_rwlock_init(&_M_rwlock, NULL);
      if (__ret == ENOMEM)
	__throw_bad_alloc();
      else if (__ret == EAGAIN)
	__throw_system_error(int(errc::resource_unavailable_try_again));
      else if (__ret == EPERM)
	__throw_system_error(int(errc::operation_not_permitted));
      // Errors not handled: EBUSY, EINVAL
      __glibcxx_assert(__ret == 0);
    }

~__shared_mutex_pthread()
    {
      int __ret __attribute((__unused__)) = pthread_rwlock_destroy(&_M_rwlock);
      // Errors not handled: EBUSY, EINVAL
      __glibcxx_assert(__ret == 0);
    }
#endif

__shared_mutex_pthread(const __shared_mutex_pthread&) = delete;
    __shared_mutex_pthread& operator=(const __shared_mutex_pthread&) = delete;

void
    lock()
    {
      int __ret = pthread_rwlock_wrlock(&_M_rwlock);
      if (__ret == EDEADLK)
	__throw_system_error(int(errc::resource_deadlock_would_occur));
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
    }

bool
    try_lock()
    {
      int __ret = pthread_rwlock_trywrlock(&_M_rwlock);
      if (__ret == EBUSY) return false;
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
      return true;
    }

void
    unlock()
    {
      int __ret __attribute((__unused__)) = pthread_rwlock_unlock(&_M_rwlock);
      // Errors not handled: EPERM, EBUSY, EINVAL
      __glibcxx_assert(__ret == 0);
    }

// Shared ownership

void
    lock_shared()
    {
      int __ret;
      // We retry if we exceeded the maximum number of read locks supported by
      // the POSIX implementation; this can result in busy-waiting, but this
      // is okay based on the current specification of forward progress
      // guarantees by the standard.
      do
	__ret = pthread_rwlock_rdlock(&_M_rwlock);
      while (__ret == EAGAIN);
      if (__ret == EDEADLK)
	__throw_system_error(int(errc::resource_deadlock_would_occur));
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
    }

bool
    try_lock_shared()
    {
      int __ret = pthread_rwlock_tryrdlock(&_M_rwlock);
      // If the maximum number of read locks has been exceeded, we just fail
      // to acquire the lock.  Unlike for lock(), we are not allowed to throw
      // an exception.
      if (__ret == EBUSY || __ret == EAGAIN) return false;
      // Errors not handled: EINVAL
      __glibcxx_assert(__ret == 0);
      return true;
    }

void
    unlock_shared()
    {
      unlock();
    }

void* native_handle() { return &_M_rwlock; }
  };
#endif

#if ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)
  /// A shared mutex type implemented using std::condition_variable.
  class __shared_mutex_cv
  {
    friend class shared_timed_mutex;

// Based on Howard Hinnant's reference implementation from N2406.

// The high bit of _M_state is the write-entered flag which is set to
    // indicate a writer has taken the lock or is queuing to take the lock.
    // The remaining bits are the count of reader locks.
    //
    // To take a reader lock, block on gate1 while the write-entered flag is
    // set or the maximum number of reader locks is held, then increment the
    // reader lock count.
    // To release, decrement the count, then if the write-entered flag is set
    // and the count is zero then signal gate2 to wake a queued writer,
    // otherwise if the maximum number of reader locks was held signal gate1
    // to wake a reader.
    //
    // To take a writer lock, block on gate1 while the write-entered flag is
    // set, then set the write-entered flag to start queueing, then block on
    // gate2 while the number of reader locks is non-zero.
    // To release, unset the write-entered flag and signal gate1 to wake all
    // blocked readers and writers.
    //
    // This means that when no reader locks are held readers and writers get
    // equal priority. When one or more reader locks is held a writer gets
    // priority and no more reader locks can be taken while the writer is
    // queued.

// Only locked when accessing _M_state or waiting on condition variables.
    mutex		_M_mut;
    // Used to block while write-entered is set or reader count at maximum.
    condition_variable	_M_gate1;
    // Used to block queued writers while reader count is non-zero.
    condition_variable	_M_gate2;
    // The write-entered flag and reader count.
    unsigned		_M_state;

static constexpr unsigned _S_write_entered
      = 1U << (sizeof(unsigned)*__CHAR_BIT__ - 1);
    static constexpr unsigned _S_max_readers = ~_S_write_entered;

// Test whether the write-entered flag is set. _M_mut must be locked.
    bool _M_write_entered() const { return _M_state & _S_write_entered; }

// The number of reader locks currently held. _M_mut must be locked.
    unsigned _M_readers() const { return _M_state & _S_max_readers; }

public:
    __shared_mutex_cv() : _M_state(0) {}

~__shared_mutex_cv()
    {
      __glibcxx_assert( _M_state == 0 );
    }

__shared_mutex_cv(const __shared_mutex_cv&) = delete;
    __shared_mutex_cv& operator=(const __shared_mutex_cv&) = delete;

// Exclusive ownership

void
    lock()
    {
      unique_lock<mutex> __lk(_M_mut);
      // Wait until we can set the write-entered flag.
      _M_gate1.wait(__lk, [=]{ return !_M_write_entered(); });
      _M_state |= _S_write_entered;
      // Then wait until there are no more readers.
      _M_gate2.wait(__lk, [=]{ return _M_readers() == 0; });
    }

bool
    try_lock()
    {
      unique_lock<mutex> __lk(_M_mut, try_to_lock);
      if (__lk.owns_lock() && _M_state == 0)
	{
	  _M_state = _S_write_entered;
	  return true;
	}
      return false;
    }

void
    unlock()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_write_entered() );
      _M_state = 0;
      // call notify_all() while mutex is held so that another thread can't
      // lock and unlock the mutex then destroy *this before we make the call.
      _M_gate1.notify_all();
    }

// Shared ownership

void
    lock_shared()
    {
      unique_lock<mutex> __lk(_M_mut);
      _M_gate1.wait(__lk, [=]{ return _M_state < _S_max_readers; });
      ++_M_state;
    }

bool
    try_lock_shared()
    {
      unique_lock<mutex> __lk(_M_mut, try_to_lock);
      if (!__lk.owns_lock())
	return false;
      if (_M_state < _S_max_readers)
	{
	  ++_M_state;
	  return true;
	}
      return false;
    }

void
    unlock_shared()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_readers() > 0 );
      auto __prev = _M_state--;
      if (_M_write_entered())
	{
	  // Wake the queued writer if there are no more readers.
	  if (_M_readers() == 0)
	    _M_gate2.notify_one();
	  // No need to notify gate1 because we give priority to the queued
	  // writer, and that writer will eventually notify gate1 after it
	  // clears the write-entered flag.
	}
      else
	{
	  // Wake any thread that was blocked on reader overflow.
	  if (__prev == _S_max_readers)
	    _M_gate1.notify_one();
	}
    }
  };
#endif

#if __cplusplus > 201402L
  /// The standard shared mutex type.
  class shared_mutex
  {
  public:
    shared_mutex() = default;
    ~shared_mutex() = default;

shared_mutex(const shared_mutex&) = delete;
    shared_mutex& operator=(const shared_mutex&) = delete;

// Exclusive ownership

void lock() { _M_impl.lock(); }
    bool try_lock() { return _M_impl.try_lock(); }
    void unlock() { _M_impl.unlock(); }

// Shared ownership

void lock_shared() { _M_impl.lock_shared(); }
    bool try_lock_shared() { return _M_impl.try_lock_shared(); }
    void unlock_shared() { _M_impl.unlock_shared(); }

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
    typedef void* native_handle_type;
    native_handle_type native_handle() { return _M_impl.native_handle(); }

private:
    __shared_mutex_pthread _M_impl;
#else
  private:
    __shared_mutex_cv _M_impl;
#endif
  };
#endif // C++17

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
  using __shared_timed_mutex_base = __shared_mutex_pthread;
#else
  using __shared_timed_mutex_base = __shared_mutex_cv;
#endif

/// The standard shared timed mutex type.
  class shared_timed_mutex
  : private __shared_timed_mutex_base
  {
    using _Base = __shared_timed_mutex_base;

// Must use the same clock as condition_variable for __shared_mutex_cv.
    typedef chrono::system_clock	__clock_t;

public:
    shared_timed_mutex() = default;
    ~shared_timed_mutex() = default;

shared_timed_mutex(const shared_timed_mutex&) = delete;
    shared_timed_mutex& operator=(const shared_timed_mutex&) = delete;

// Exclusive ownership

void lock() { _Base::lock(); }
    bool try_lock() { return _Base::try_lock(); }
    void unlock() { _Base::unlock(); }

template<typename _Rep, typename _Period>
      bool
      try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time)
      {
	return try_lock_until(__clock_t::now() + __rel_time);
      }

// Shared ownership

void lock_shared() { _Base::lock_shared(); }
    bool try_lock_shared() { return _Base::try_lock_shared(); }
    void unlock_shared() { _Base::unlock_shared(); }

template<typename _Rep, typename _Period>
      bool
      try_lock_shared_for(const chrono::duration<_Rep, _Period>& __rel_time)
      {
	return try_lock_shared_until(__clock_t::now() + __rel_time);
      }

#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK

// Exclusive ownership

template<typename _Duration>
      bool
      try_lock_until(const chrono::time_point<__clock_t, _Duration>& __atime)
      {
	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

__gthread_time_t __ts =
	  {
	    static_cast<std::time_t>(__s.time_since_epoch().count()),
	    static_cast<long>(__ns.count())
	  };

int __ret = pthread_rwlock_timedwrlock(&_M_rwlock, &__ts);
	// On self-deadlock, we just fail to acquire the lock.  Technically,
	// the program violated the precondition.
	if (__ret == ETIMEDOUT || __ret == EDEADLK)
	  return false;
	// Errors not handled: EINVAL
	__glibcxx_assert(__ret == 0);
	return true;
      }

template<typename _Clock, typename _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
      {
	// DR 887 - Sync unknown clock to known clock.
	const typename _Clock::time_point __c_entry = _Clock::now();
	const __clock_t::time_point __s_entry = __clock_t::now();
	const auto __delta = __abs_time - __c_entry;
	const auto __s_atime = __s_entry + __delta;
	return try_lock_until(__s_atime);
      }

// Shared ownership

template<typename _Duration>
      bool
      try_lock_shared_until(const chrono::time_point<__clock_t,
			    _Duration>& __atime)
      {
	auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
	auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);

__gthread_time_t __ts =
	  {
	    static_cast<std::time_t>(__s.time_since_epoch().count()),
	    static_cast<long>(__ns.count())
	  };

int __ret;
	// Unlike for lock(), we are not allowed to throw an exception so if
	// the maximum number of read locks has been exceeded, or we would
	// deadlock, we just try to acquire the lock again (and will time out
	// eventually).
	// In cases where we would exceed the maximum number of read locks
	// throughout the whole time until the timeout, we will fail to
	// acquire the lock even if it would be logically free; however, this
	// is allowed by the standard, and we made a "strong effort"
	// (see C++14 30.4.1.4p26).
	// For cases where the implementation detects a deadlock we
	// intentionally block and timeout so that an early return isn't
	// mistaken for a spurious failure, which might help users realise
	// there is a deadlock.
	do
	  __ret = pthread_rwlock_timedrdlock(&_M_rwlock, &__ts);
	while (__ret == EAGAIN || __ret == EDEADLK);
	if (__ret == ETIMEDOUT)
	  return false;
	// Errors not handled: EINVAL
	__glibcxx_assert(__ret == 0);
	return true;
      }

template<typename _Clock, typename _Duration>
      bool
      try_lock_shared_until(const chrono::time_point<_Clock,
						     _Duration>& __abs_time)
      {
	// DR 887 - Sync unknown clock to known clock.
	const typename _Clock::time_point __c_entry = _Clock::now();
	const __clock_t::time_point __s_entry = __clock_t::now();
	const auto __delta = __abs_time - __c_entry;
	const auto __s_atime = __s_entry + __delta;
	return try_lock_shared_until(__s_atime);
      }

#else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)

// Exclusive ownership

template<typename _Clock, typename _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
      {
	unique_lock<mutex> __lk(_M_mut);
	if (!_M_gate1.wait_until(__lk, __abs_time,
				 [=]{ return !_M_write_entered(); }))
	  {
	    return false;
	  }
	_M_state |= _S_write_entered;
	if (!_M_gate2.wait_until(__lk, __abs_time,
				 [=]{ return _M_readers() == 0; }))
	  {
	    _M_state ^= _S_write_entered;
	    // Wake all threads blocked while the write-entered flag was set.
	    _M_gate1.notify_all();
	    return false;
	  }
	return true;
      }

// Shared ownership

template <typename _Clock, typename _Duration>
      bool
      try_lock_shared_until(const chrono::time_point<_Clock,
						     _Duration>& __abs_time)
      {
	unique_lock<mutex> __lk(_M_mut);
	if (!_M_gate1.wait_until(__lk, __abs_time,
				 [=]{ return _M_state < _S_max_readers; }))
	  {
	    return false;
	  }
	++_M_state;
	return true;
      }

#endif // _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
  };
#endif // _GLIBCXX_HAS_GTHREADS

/// shared_lock
  template<typename _Mutex>
    class shared_lock
    {
    public:
      typedef _Mutex mutex_type;

// Shared locking

shared_lock() noexcept : _M_pm(nullptr), _M_owns(false) { }

explicit
      shared_lock(mutex_type& __m)
      : _M_pm(std::__addressof(__m)), _M_owns(true)
      { __m.lock_shared(); }

shared_lock(mutex_type& __m, defer_lock_t) noexcept
      : _M_pm(std::__addressof(__m)), _M_owns(false) { }

shared_lock(mutex_type& __m, try_to_lock_t)
      : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared()) { }

shared_lock(mutex_type& __m, adopt_lock_t)
      : _M_pm(std::__addressof(__m)), _M_owns(true) { }

template<typename _Clock, typename _Duration>
	shared_lock(mutex_type& __m,
		    const chrono::time_point<_Clock, _Duration>& __abs_time)
      : _M_pm(std::__addressof(__m)),
	_M_owns(__m.try_lock_shared_until(__abs_time)) { }

template<typename _Rep, typename _Period>
	shared_lock(mutex_type& __m,
		    const chrono::duration<_Rep, _Period>& __rel_time)
      : _M_pm(std::__addressof(__m)),
	_M_owns(__m.try_lock_shared_for(__rel_time)) { }

~shared_lock()
      {
	if (_M_owns)
	  _M_pm->unlock_shared();
      }

shared_lock(shared_lock const&) = delete;
      shared_lock& operator=(shared_lock const&) = delete;

shared_lock(shared_lock&& __sl) noexcept : shared_lock()
      { swap(__sl); }

shared_lock&
      operator=(shared_lock&& __sl) noexcept
      {
	shared_lock(std::move(__sl)).swap(*this);
	return *this;
      }

void
      lock()
      {
	_M_lockable();
	_M_pm->lock_shared();
	_M_owns = true;
      }

bool
      try_lock()
      {
	_M_lockable();
	return _M_owns = _M_pm->try_lock_shared();
      }

template<typename _Rep, typename _Period>
	bool
	try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time)
	{
	  _M_lockable();
	  return _M_owns = _M_pm->try_lock_shared_for(__rel_time);
	}

template<typename _Clock, typename _Duration>
	bool
	try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
	{
	  _M_lockable();
	  return _M_owns = _M_pm->try_lock_shared_until(__abs_time);
	}

void
      unlock()
      {
	if (!_M_owns)
	  __throw_system_error(int(errc::resource_deadlock_would_occur));
	_M_pm->unlock_shared();
	_M_owns = false;
      }

// Setters

void
      swap(shared_lock& __u) noexcept
      {
	std::swap(_M_pm, __u._M_pm);
	std::swap(_M_owns, __u._M_owns);
      }

mutex_type*
      release() noexcept
      {
	_M_owns = false;
	return std::exchange(_M_pm, nullptr);
      }

// Getters

bool owns_lock() const noexcept { return _M_owns; }

explicit operator bool() const noexcept { return _M_owns; }

mutex_type* mutex() const noexcept { return _M_pm; }

private:
      void
      _M_lockable() const
      {
	if (_M_pm == nullptr)
	  __throw_system_error(int(errc::operation_not_permitted));
	if (_M_owns)
	  __throw_system_error(int(errc::resource_deadlock_would_occur));
      }

mutex_type*	_M_pm;
      bool		_M_owns;
    };

/// Swap specialization for shared_lock
  template<typename _Mutex>
    void
    swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) noexcept
    { __x.swap(__y); }

#endif // _GLIBCXX_USE_C99_STDINT_TR1

// @} group mutexes
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace

#endif // C++14

#endif // _GLIBCXX_SHARED_MUTEX

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