1 // <stop_token> -*- C++ -*-
3 // Copyright (C) 2019-2023 Free Software Foundation, Inc.
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
25 /** @file include/stop_token
26 * This is a Standard C++ Library header.
29 #ifndef _GLIBCXX_STOP_TOKEN
30 #define _GLIBCXX_STOP_TOKEN
32 #include <bits/requires_hosted.h> // concurrency
34 #if __cplusplus > 201703L
37 #include <bits/std_thread.h>
41 #define __cpp_lib_jthread 201911L
43 namespace std _GLIBCXX_VISIBILITY(default)
45 _GLIBCXX_BEGIN_NAMESPACE_VERSION
47 /// Tag type indicating a stop_source should have no shared-stop-state.
48 struct nostopstate_t { explicit nostopstate_t() = default; };
49 inline constexpr nostopstate_t nostopstate{};
53 /// Allow testing whether a stop request has been made on a `stop_source`.
57 stop_token() noexcept = default;
59 stop_token(const stop_token&) noexcept = default;
60 stop_token(stop_token&&) noexcept = default;
62 ~stop_token() = default;
65 operator=(const stop_token&) noexcept = default;
68 operator=(stop_token&&) noexcept = default;
72 stop_possible() const noexcept
74 return static_cast<bool>(_M_state) && _M_state->_M_stop_possible();
79 stop_requested() const noexcept
81 return static_cast<bool>(_M_state) && _M_state->_M_stop_requested();
85 swap(stop_token& __rhs) noexcept
86 { _M_state.swap(__rhs._M_state); }
90 operator==(const stop_token& __a, const stop_token& __b)
91 { return __a._M_state == __b._M_state; }
94 swap(stop_token& __lhs, stop_token& __rhs) noexcept
95 { __lhs.swap(__rhs); }
98 friend class stop_source;
99 template<typename _Callback>
100 friend class stop_callback;
105 #if defined __i386__ || defined __x86_64__
106 __builtin_ia32_pause();
108 this_thread::yield();
111 #ifndef __cpp_lib_semaphore
112 struct binary_semaphore
114 explicit binary_semaphore(int __d) : _M_counter(__d > 0) { }
116 void release() { _M_counter.fetch_add(1, memory_order::release); }
121 while (!_M_counter.compare_exchange_weak(__old, 0,
122 memory_order::acquire,
123 memory_order::relaxed))
130 atomic<int> _M_counter;
136 using __cb_type = void(_Stop_cb*) noexcept;
137 __cb_type* _M_callback;
138 _Stop_cb* _M_prev = nullptr;
139 _Stop_cb* _M_next = nullptr;
140 bool* _M_destroyed = nullptr;
141 binary_semaphore _M_done{0};
143 [[__gnu__::__nonnull__]]
145 _Stop_cb(__cb_type* __cb)
149 void _M_run() noexcept { _M_callback(this); }
154 using value_type = uint32_t;
155 static constexpr value_type _S_stop_requested_bit = 1;
156 static constexpr value_type _S_locked_bit = 2;
157 static constexpr value_type _S_ssrc_counter_inc = 4;
159 std::atomic<value_type> _M_owners{1};
160 std::atomic<value_type> _M_value{_S_ssrc_counter_inc};
161 _Stop_cb* _M_head = nullptr;
162 std::thread::id _M_requester;
164 _Stop_state_t() = default;
167 _M_stop_possible() noexcept
169 // true if a stop request has already been made or there are still
170 // stop_source objects that would allow one to be made.
171 return _M_value.load(memory_order::acquire) & ~_S_locked_bit;
175 _M_stop_requested() noexcept
177 return _M_value.load(memory_order::acquire) & _S_stop_requested_bit;
181 _M_add_owner() noexcept
183 _M_owners.fetch_add(1, memory_order::relaxed);
187 _M_release_ownership() noexcept
189 if (_M_owners.fetch_sub(1, memory_order::acq_rel) == 1)
194 _M_add_ssrc() noexcept
196 _M_value.fetch_add(_S_ssrc_counter_inc, memory_order::relaxed);
200 _M_sub_ssrc() noexcept
202 _M_value.fetch_sub(_S_ssrc_counter_inc, memory_order::release);
209 // Can use relaxed loads to get the current value.
210 // The successful call to _M_try_lock is an acquire operation.
211 auto __old = _M_value.load(memory_order::relaxed);
212 while (!_M_try_lock(__old, memory_order::relaxed))
216 // Precondition: calling thread holds the lock.
220 _M_value.fetch_sub(_S_locked_bit, memory_order::release);
224 _M_request_stop() noexcept
226 // obtain lock and set stop_requested bit
227 auto __old = _M_value.load(memory_order::acquire);
230 if (__old & _S_stop_requested_bit) // stop request already made
233 while (!_M_try_lock_and_stop(__old));
235 _M_requester = this_thread::get_id();
240 _Stop_cb* __cb = _M_head;
241 _M_head = _M_head->_M_next;
244 _M_head->_M_prev = nullptr;
250 // Allow other callbacks to be unregistered while __cb runs.
253 bool __destroyed = false;
254 __cb->_M_destroyed = &__destroyed;
261 __cb->_M_destroyed = nullptr;
263 // synchronize with destructor of stop_callback that owns *__cb
264 if (!__gnu_cxx::__is_single_threaded())
265 __cb->_M_done.release();
268 // Avoid relocking if we already know there are no more callbacks.
279 [[__gnu__::__nonnull__]]
281 _M_register_callback(_Stop_cb* __cb) noexcept
283 auto __old = _M_value.load(memory_order::acquire);
286 if (__old & _S_stop_requested_bit) // stop request already made
288 __cb->_M_run(); // run synchronously
292 if (__old < _S_ssrc_counter_inc) // no stop_source owns *this
293 // No need to register callback if no stop request can be made.
294 // Returning false also means the stop_callback does not share
295 // ownership of this state, but that's not observable.
298 while (!_M_try_lock(__old));
300 __cb->_M_next = _M_head;
303 _M_head->_M_prev = __cb;
310 // Called by ~stop_callback just before destroying *__cb.
311 [[__gnu__::__nonnull__]]
313 _M_remove_callback(_Stop_cb* __cb)
319 _M_head = _M_head->_M_next;
321 _M_head->_M_prev = nullptr;
325 else if (__cb->_M_prev)
327 __cb->_M_prev->_M_next = __cb->_M_next;
329 __cb->_M_next->_M_prev = __cb->_M_prev;
336 // Callback is not in the list, so must have been removed by a call to
339 // Despite appearances there is no data race on _M_requester. The only
340 // write to it happens before the callback is removed from the list,
341 // and removing it from the list happens before this read.
342 if (!(_M_requester == this_thread::get_id()))
344 // Synchronize with completion of callback.
345 __cb->_M_done.acquire();
346 // Safe for ~stop_callback to destroy *__cb now.
350 if (__cb->_M_destroyed)
351 *__cb->_M_destroyed = true;
354 // Try to obtain the lock.
355 // Returns true if the lock is acquired (with memory order acquire).
356 // Otherwise, sets __curval = _M_value.load(__failure) and returns false.
357 // Might fail spuriously, so must be called in a loop.
359 _M_try_lock(value_type& __curval,
360 memory_order __failure = memory_order::acquire) noexcept
362 return _M_do_try_lock(__curval, 0, memory_order::acquire, __failure);
365 // Try to obtain the lock to make a stop request.
366 // Returns true if the lock is acquired and the _S_stop_requested_bit is
367 // set (with memory order acq_rel so that other threads see the request).
368 // Otherwise, sets __curval = _M_value.load(memory_order::acquire) and
370 // Might fail spuriously, so must be called in a loop.
372 _M_try_lock_and_stop(value_type& __curval) noexcept
374 return _M_do_try_lock(__curval, _S_stop_requested_bit,
375 memory_order::acq_rel, memory_order::acquire);
379 _M_do_try_lock(value_type& __curval, value_type __newbits,
380 memory_order __success, memory_order __failure) noexcept
382 if (__curval & _S_locked_bit)
385 __curval = _M_value.load(__failure);
388 __newbits |= _S_locked_bit;
389 return _M_value.compare_exchange_weak(__curval, __curval | __newbits,
390 __success, __failure);
394 struct _Stop_state_ref
396 _Stop_state_ref() = default;
399 _Stop_state_ref(const stop_source&)
400 : _M_ptr(new _Stop_state_t())
403 _Stop_state_ref(const _Stop_state_ref& __other) noexcept
404 : _M_ptr(__other._M_ptr)
407 _M_ptr->_M_add_owner();
410 _Stop_state_ref(_Stop_state_ref&& __other) noexcept
411 : _M_ptr(__other._M_ptr)
413 __other._M_ptr = nullptr;
417 operator=(const _Stop_state_ref& __other) noexcept
419 if (auto __ptr = __other._M_ptr; __ptr != _M_ptr)
422 __ptr->_M_add_owner();
424 _M_ptr->_M_release_ownership();
431 operator=(_Stop_state_ref&& __other) noexcept
433 _Stop_state_ref(std::move(__other)).swap(*this);
440 _M_ptr->_M_release_ownership();
444 swap(_Stop_state_ref& __other) noexcept
445 { std::swap(_M_ptr, __other._M_ptr); }
447 explicit operator bool() const noexcept { return _M_ptr != nullptr; }
449 _Stop_state_t* operator->() const noexcept { return _M_ptr; }
451 #if __cpp_impl_three_way_comparison >= 201907L
453 operator==(const _Stop_state_ref&, const _Stop_state_ref&) = default;
456 operator==(const _Stop_state_ref& __lhs, const _Stop_state_ref& __rhs)
458 { return __lhs._M_ptr == __rhs._M_ptr; }
461 operator!=(const _Stop_state_ref& __lhs, const _Stop_state_ref& __rhs)
463 { return __lhs._M_ptr != __rhs._M_ptr; }
467 _Stop_state_t* _M_ptr = nullptr;
470 _Stop_state_ref _M_state;
473 stop_token(const _Stop_state_ref& __state) noexcept
478 /// A type that allows a stop request to be made.
482 stop_source() : _M_state(*this)
485 explicit stop_source(std::nostopstate_t) noexcept
488 stop_source(const stop_source& __other) noexcept
489 : _M_state(__other._M_state)
492 _M_state->_M_add_ssrc();
495 stop_source(stop_source&&) noexcept = default;
498 operator=(const stop_source& __other) noexcept
500 if (_M_state != __other._M_state)
502 stop_source __sink(std::move(*this));
503 _M_state = __other._M_state;
505 _M_state->_M_add_ssrc();
511 operator=(stop_source&&) noexcept = default;
516 _M_state->_M_sub_ssrc();
521 stop_possible() const noexcept
523 return static_cast<bool>(_M_state);
528 stop_requested() const noexcept
530 return static_cast<bool>(_M_state) && _M_state->_M_stop_requested();
534 request_stop() const noexcept
537 return _M_state->_M_request_stop();
543 get_token() const noexcept
545 return stop_token{_M_state};
549 swap(stop_source& __other) noexcept
551 _M_state.swap(__other._M_state);
556 operator==(const stop_source& __a, const stop_source& __b) noexcept
558 return __a._M_state == __b._M_state;
562 swap(stop_source& __lhs, stop_source& __rhs) noexcept
568 stop_token::_Stop_state_ref _M_state;
571 /// A wrapper for callbacks to be run when a stop request is made.
572 template<typename _Callback>
573 class [[nodiscard]] stop_callback
575 static_assert(is_nothrow_destructible_v<_Callback>);
576 static_assert(is_invocable_v<_Callback>);
579 using callback_type = _Callback;
581 template<typename _Cb,
582 enable_if_t<is_constructible_v<_Callback, _Cb>, int> = 0>
584 stop_callback(const stop_token& __token, _Cb&& __cb)
585 noexcept(is_nothrow_constructible_v<_Callback, _Cb>)
586 : _M_cb(std::forward<_Cb>(__cb))
588 if (auto __state = __token._M_state)
590 if (__state->_M_register_callback(&_M_cb))
591 _M_state.swap(__state);
595 template<typename _Cb,
596 enable_if_t<is_constructible_v<_Callback, _Cb>, int> = 0>
598 stop_callback(stop_token&& __token, _Cb&& __cb)
599 noexcept(is_nothrow_constructible_v<_Callback, _Cb>)
600 : _M_cb(std::forward<_Cb>(__cb))
602 if (auto& __state = __token._M_state)
604 if (__state->_M_register_callback(&_M_cb))
605 _M_state.swap(__state);
613 _M_state->_M_remove_callback(&_M_cb);
617 stop_callback(const stop_callback&) = delete;
618 stop_callback& operator=(const stop_callback&) = delete;
619 stop_callback(stop_callback&&) = delete;
620 stop_callback& operator=(stop_callback&&) = delete;
623 struct _Cb_impl : stop_token::_Stop_cb
625 template<typename _Cb>
628 : _Stop_cb(&_S_execute),
629 _M_cb(std::forward<_Cb>(__cb))
634 [[__gnu__::__nonnull__]]
636 _S_execute(_Stop_cb* __that) noexcept
638 _Callback& __cb = static_cast<_Cb_impl*>(__that)->_M_cb;
639 std::forward<_Callback>(__cb)();
644 stop_token::_Stop_state_ref _M_state;
647 template<typename _Callback>
648 stop_callback(stop_token, _Callback) -> stop_callback<_Callback>;
650 _GLIBCXX_END_NAMESPACE_VERSION
652 #endif // __cplusplus > 201703L
653 #endif // _GLIBCXX_STOP_TOKEN