libstdc++
shared_mutex
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1// <shared_mutex> -*- C++ -*-
2
3// Copyright (C) 2013-2021 Free Software Foundation, Inc.
4//
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)
9// any later version.
10
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.
15
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.
19
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/>.
24
25/** @file include/shared_mutex
26 * This is a Standard C++ Library header.
27 */
28
29#ifndef _GLIBCXX_SHARED_MUTEX
30#define _GLIBCXX_SHARED_MUTEX 1
31
32#pragma GCC system_header
33
34#if __cplusplus >= 201402L
35
36#include <chrono>
37#include <bits/functexcept.h>
38#include <bits/move.h> // move, __exchange
39#include <bits/std_mutex.h> // defer_lock_t
40
41#if ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)
42# include <condition_variable>
43#endif
44
45namespace std _GLIBCXX_VISIBILITY(default)
46{
47_GLIBCXX_BEGIN_NAMESPACE_VERSION
48
49 /**
50 * @addtogroup mutexes
51 * @{
52 */
53
54#ifdef _GLIBCXX_HAS_GTHREADS
55
56#if __cplusplus >= 201703L
57#define __cpp_lib_shared_mutex 201505L
58 class shared_mutex;
59#endif
60
61#define __cpp_lib_shared_timed_mutex 201402L
62 class shared_timed_mutex;
63
64 /// @cond undocumented
65
66#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
67#ifdef __gthrw
68#define _GLIBCXX_GTHRW(name) \
69 __gthrw(pthread_ ## name); \
70 static inline int \
71 __glibcxx_ ## name (pthread_rwlock_t *__rwlock) \
72 { \
73 if (__gthread_active_p ()) \
74 return __gthrw_(pthread_ ## name) (__rwlock); \
75 else \
76 return 0; \
77 }
78 _GLIBCXX_GTHRW(rwlock_rdlock)
79 _GLIBCXX_GTHRW(rwlock_tryrdlock)
80 _GLIBCXX_GTHRW(rwlock_wrlock)
81 _GLIBCXX_GTHRW(rwlock_trywrlock)
82 _GLIBCXX_GTHRW(rwlock_unlock)
83# ifndef PTHREAD_RWLOCK_INITIALIZER
84 _GLIBCXX_GTHRW(rwlock_destroy)
85 __gthrw(pthread_rwlock_init);
86 static inline int
87 __glibcxx_rwlock_init (pthread_rwlock_t *__rwlock)
88 {
89 if (__gthread_active_p ())
90 return __gthrw_(pthread_rwlock_init) (__rwlock, NULL);
91 else
92 return 0;
93 }
94# endif
95# if _GTHREAD_USE_MUTEX_TIMEDLOCK
96 __gthrw(pthread_rwlock_timedrdlock);
97 static inline int
98 __glibcxx_rwlock_timedrdlock (pthread_rwlock_t *__rwlock,
99 const timespec *__ts)
100 {
101 if (__gthread_active_p ())
102 return __gthrw_(pthread_rwlock_timedrdlock) (__rwlock, __ts);
103 else
104 return 0;
105 }
106 __gthrw(pthread_rwlock_timedwrlock);
107 static inline int
108 __glibcxx_rwlock_timedwrlock (pthread_rwlock_t *__rwlock,
109 const timespec *__ts)
110 {
111 if (__gthread_active_p ())
112 return __gthrw_(pthread_rwlock_timedwrlock) (__rwlock, __ts);
113 else
114 return 0;
115 }
116# endif
117#else
118 static inline int
119 __glibcxx_rwlock_rdlock (pthread_rwlock_t *__rwlock)
120 { return pthread_rwlock_rdlock (__rwlock); }
121 static inline int
122 __glibcxx_rwlock_tryrdlock (pthread_rwlock_t *__rwlock)
123 { return pthread_rwlock_tryrdlock (__rwlock); }
124 static inline int
125 __glibcxx_rwlock_wrlock (pthread_rwlock_t *__rwlock)
126 { return pthread_rwlock_wrlock (__rwlock); }
127 static inline int
128 __glibcxx_rwlock_trywrlock (pthread_rwlock_t *__rwlock)
129 { return pthread_rwlock_trywrlock (__rwlock); }
130 static inline int
131 __glibcxx_rwlock_unlock (pthread_rwlock_t *__rwlock)
132 { return pthread_rwlock_unlock (__rwlock); }
133 static inline int
134 __glibcxx_rwlock_destroy(pthread_rwlock_t *__rwlock)
135 { return pthread_rwlock_destroy (__rwlock); }
136 static inline int
137 __glibcxx_rwlock_init(pthread_rwlock_t *__rwlock)
138 { return pthread_rwlock_init (__rwlock, NULL); }
139# if _GTHREAD_USE_MUTEX_TIMEDLOCK
140 static inline int
141 __glibcxx_rwlock_timedrdlock (pthread_rwlock_t *__rwlock,
142 const timespec *__ts)
143 { return pthread_rwlock_timedrdlock (__rwlock, __ts); }
144 static inline int
145 __glibcxx_rwlock_timedwrlock (pthread_rwlock_t *__rwlock,
146 const timespec *__ts)
147 { return pthread_rwlock_timedwrlock (__rwlock, __ts); }
148# endif
149#endif
150
151 /// A shared mutex type implemented using pthread_rwlock_t.
152 class __shared_mutex_pthread
153 {
154 friend class shared_timed_mutex;
155
156#ifdef PTHREAD_RWLOCK_INITIALIZER
157 pthread_rwlock_t _M_rwlock = PTHREAD_RWLOCK_INITIALIZER;
158
159 public:
160 __shared_mutex_pthread() = default;
161 ~__shared_mutex_pthread() = default;
162#else
163 pthread_rwlock_t _M_rwlock;
164
165 public:
166 __shared_mutex_pthread()
167 {
168 int __ret = __glibcxx_rwlock_init(&_M_rwlock);
169 if (__ret == ENOMEM)
170 __throw_bad_alloc();
171 else if (__ret == EAGAIN)
172 __throw_system_error(int(errc::resource_unavailable_try_again));
173 else if (__ret == EPERM)
174 __throw_system_error(int(errc::operation_not_permitted));
175 // Errors not handled: EBUSY, EINVAL
176 __glibcxx_assert(__ret == 0);
177 }
178
179 ~__shared_mutex_pthread()
180 {
181 int __ret __attribute((__unused__)) = __glibcxx_rwlock_destroy(&_M_rwlock);
182 // Errors not handled: EBUSY, EINVAL
183 __glibcxx_assert(__ret == 0);
184 }
185#endif
186
187 __shared_mutex_pthread(const __shared_mutex_pthread&) = delete;
188 __shared_mutex_pthread& operator=(const __shared_mutex_pthread&) = delete;
189
190 void
191 lock()
192 {
193 int __ret = __glibcxx_rwlock_wrlock(&_M_rwlock);
194 if (__ret == EDEADLK)
195 __throw_system_error(int(errc::resource_deadlock_would_occur));
196 // Errors not handled: EINVAL
197 __glibcxx_assert(__ret == 0);
198 }
199
200 bool
201 try_lock()
202 {
203 int __ret = __glibcxx_rwlock_trywrlock(&_M_rwlock);
204 if (__ret == EBUSY) return false;
205 // Errors not handled: EINVAL
206 __glibcxx_assert(__ret == 0);
207 return true;
208 }
209
210 void
211 unlock()
212 {
213 int __ret __attribute((__unused__)) = __glibcxx_rwlock_unlock(&_M_rwlock);
214 // Errors not handled: EPERM, EBUSY, EINVAL
215 __glibcxx_assert(__ret == 0);
216 }
217
218 // Shared ownership
219
220 void
221 lock_shared()
222 {
223 int __ret;
224 // We retry if we exceeded the maximum number of read locks supported by
225 // the POSIX implementation; this can result in busy-waiting, but this
226 // is okay based on the current specification of forward progress
227 // guarantees by the standard.
228 do
229 __ret = __glibcxx_rwlock_rdlock(&_M_rwlock);
230 while (__ret == EAGAIN);
231 if (__ret == EDEADLK)
232 __throw_system_error(int(errc::resource_deadlock_would_occur));
233 // Errors not handled: EINVAL
234 __glibcxx_assert(__ret == 0);
235 }
236
237 bool
238 try_lock_shared()
239 {
240 int __ret = __glibcxx_rwlock_tryrdlock(&_M_rwlock);
241 // If the maximum number of read locks has been exceeded, we just fail
242 // to acquire the lock. Unlike for lock(), we are not allowed to throw
243 // an exception.
244 if (__ret == EBUSY || __ret == EAGAIN) return false;
245 // Errors not handled: EINVAL
246 __glibcxx_assert(__ret == 0);
247 return true;
248 }
249
250 void
251 unlock_shared()
252 {
253 unlock();
254 }
255
256 void* native_handle() { return &_M_rwlock; }
257 };
258#endif
259
260#if ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)
261 /// A shared mutex type implemented using std::condition_variable.
262 class __shared_mutex_cv
263 {
264 friend class shared_timed_mutex;
265
266 // Based on Howard Hinnant's reference implementation from N2406.
267
268 // The high bit of _M_state is the write-entered flag which is set to
269 // indicate a writer has taken the lock or is queuing to take the lock.
270 // The remaining bits are the count of reader locks.
271 //
272 // To take a reader lock, block on gate1 while the write-entered flag is
273 // set or the maximum number of reader locks is held, then increment the
274 // reader lock count.
275 // To release, decrement the count, then if the write-entered flag is set
276 // and the count is zero then signal gate2 to wake a queued writer,
277 // otherwise if the maximum number of reader locks was held signal gate1
278 // to wake a reader.
279 //
280 // To take a writer lock, block on gate1 while the write-entered flag is
281 // set, then set the write-entered flag to start queueing, then block on
282 // gate2 while the number of reader locks is non-zero.
283 // To release, unset the write-entered flag and signal gate1 to wake all
284 // blocked readers and writers.
285 //
286 // This means that when no reader locks are held readers and writers get
287 // equal priority. When one or more reader locks is held a writer gets
288 // priority and no more reader locks can be taken while the writer is
289 // queued.
290
291 // Only locked when accessing _M_state or waiting on condition variables.
292 mutex _M_mut;
293 // Used to block while write-entered is set or reader count at maximum.
294 condition_variable _M_gate1;
295 // Used to block queued writers while reader count is non-zero.
296 condition_variable _M_gate2;
297 // The write-entered flag and reader count.
298 unsigned _M_state;
299
300 static constexpr unsigned _S_write_entered
301 = 1U << (sizeof(unsigned)*__CHAR_BIT__ - 1);
302 static constexpr unsigned _S_max_readers = ~_S_write_entered;
303
304 // Test whether the write-entered flag is set. _M_mut must be locked.
305 bool _M_write_entered() const { return _M_state & _S_write_entered; }
306
307 // The number of reader locks currently held. _M_mut must be locked.
308 unsigned _M_readers() const { return _M_state & _S_max_readers; }
309
310 public:
311 __shared_mutex_cv() : _M_state(0) {}
312
313 ~__shared_mutex_cv()
314 {
315 __glibcxx_assert( _M_state == 0 );
316 }
317
318 __shared_mutex_cv(const __shared_mutex_cv&) = delete;
319 __shared_mutex_cv& operator=(const __shared_mutex_cv&) = delete;
320
321 // Exclusive ownership
322
323 void
324 lock()
325 {
326 unique_lock<mutex> __lk(_M_mut);
327 // Wait until we can set the write-entered flag.
328 _M_gate1.wait(__lk, [=]{ return !_M_write_entered(); });
329 _M_state |= _S_write_entered;
330 // Then wait until there are no more readers.
331 _M_gate2.wait(__lk, [=]{ return _M_readers() == 0; });
332 }
333
334 bool
335 try_lock()
336 {
337 unique_lock<mutex> __lk(_M_mut, try_to_lock);
338 if (__lk.owns_lock() && _M_state == 0)
339 {
340 _M_state = _S_write_entered;
341 return true;
342 }
343 return false;
344 }
345
346 void
347 unlock()
348 {
349 lock_guard<mutex> __lk(_M_mut);
350 __glibcxx_assert( _M_write_entered() );
351 _M_state = 0;
352 // call notify_all() while mutex is held so that another thread can't
353 // lock and unlock the mutex then destroy *this before we make the call.
354 _M_gate1.notify_all();
355 }
356
357 // Shared ownership
358
359 void
360 lock_shared()
361 {
362 unique_lock<mutex> __lk(_M_mut);
363 _M_gate1.wait(__lk, [=]{ return _M_state < _S_max_readers; });
364 ++_M_state;
365 }
366
367 bool
368 try_lock_shared()
369 {
370 unique_lock<mutex> __lk(_M_mut, try_to_lock);
371 if (!__lk.owns_lock())
372 return false;
373 if (_M_state < _S_max_readers)
374 {
375 ++_M_state;
376 return true;
377 }
378 return false;
379 }
380
381 void
382 unlock_shared()
383 {
384 lock_guard<mutex> __lk(_M_mut);
385 __glibcxx_assert( _M_readers() > 0 );
386 auto __prev = _M_state--;
387 if (_M_write_entered())
388 {
389 // Wake the queued writer if there are no more readers.
390 if (_M_readers() == 0)
391 _M_gate2.notify_one();
392 // No need to notify gate1 because we give priority to the queued
393 // writer, and that writer will eventually notify gate1 after it
394 // clears the write-entered flag.
395 }
396 else
397 {
398 // Wake any thread that was blocked on reader overflow.
399 if (__prev == _S_max_readers)
400 _M_gate1.notify_one();
401 }
402 }
403 };
404#endif
405 /// @endcond
406
407#if __cplusplus >= 201703L
408 /// The standard shared mutex type.
409 class shared_mutex
410 {
411 public:
412 shared_mutex() = default;
413 ~shared_mutex() = default;
414
415 shared_mutex(const shared_mutex&) = delete;
416 shared_mutex& operator=(const shared_mutex&) = delete;
417
418 // Exclusive ownership
419
420 void lock() { _M_impl.lock(); }
421 bool try_lock() { return _M_impl.try_lock(); }
422 void unlock() { _M_impl.unlock(); }
423
424 // Shared ownership
425
426 void lock_shared() { _M_impl.lock_shared(); }
427 bool try_lock_shared() { return _M_impl.try_lock_shared(); }
428 void unlock_shared() { _M_impl.unlock_shared(); }
429
430#if _GLIBCXX_USE_PTHREAD_RWLOCK_T
431 typedef void* native_handle_type;
432 native_handle_type native_handle() { return _M_impl.native_handle(); }
433
434 private:
435 __shared_mutex_pthread _M_impl;
436#else
437 private:
438 __shared_mutex_cv _M_impl;
439#endif
440 };
441#endif // C++17
442
443 /// @cond undocumented
444#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
445 using __shared_timed_mutex_base = __shared_mutex_pthread;
446#else
447 using __shared_timed_mutex_base = __shared_mutex_cv;
448#endif
449 /// @endcond
450
451 /// The standard shared timed mutex type.
452 class shared_timed_mutex
453 : private __shared_timed_mutex_base
454 {
455 using _Base = __shared_timed_mutex_base;
456
457 // Must use the same clock as condition_variable for __shared_mutex_cv.
458#ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK
459 using __clock_t = chrono::steady_clock;
460#else
461 using __clock_t = chrono::system_clock;
462#endif
463
464 public:
465 shared_timed_mutex() = default;
466 ~shared_timed_mutex() = default;
467
468 shared_timed_mutex(const shared_timed_mutex&) = delete;
469 shared_timed_mutex& operator=(const shared_timed_mutex&) = delete;
470
471 // Exclusive ownership
472
473 void lock() { _Base::lock(); }
474 bool try_lock() { return _Base::try_lock(); }
475 void unlock() { _Base::unlock(); }
476
477 template<typename _Rep, typename _Period>
478 bool
479 try_lock_for(const chrono::duration<_Rep, _Period>& __rtime)
480 {
481 auto __rt = chrono::duration_cast<__clock_t::duration>(__rtime);
482 if (ratio_greater<__clock_t::period, _Period>())
483 ++__rt;
484 return try_lock_until(__clock_t::now() + __rt);
485 }
486
487 // Shared ownership
488
489 void lock_shared() { _Base::lock_shared(); }
490 bool try_lock_shared() { return _Base::try_lock_shared(); }
491 void unlock_shared() { _Base::unlock_shared(); }
492
493 template<typename _Rep, typename _Period>
494 bool
495 try_lock_shared_for(const chrono::duration<_Rep, _Period>& __rtime)
496 {
497 auto __rt = chrono::duration_cast<__clock_t::duration>(__rtime);
498 if (ratio_greater<__clock_t::period, _Period>())
499 ++__rt;
500 return try_lock_shared_until(__clock_t::now() + __rt);
501 }
502
503#if _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
504
505 // Exclusive ownership
506
507 template<typename _Duration>
508 bool
509 try_lock_until(const chrono::time_point<chrono::system_clock,
510 _Duration>& __atime)
511 {
512 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
513 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
514
515 __gthread_time_t __ts =
516 {
517 static_cast<std::time_t>(__s.time_since_epoch().count()),
518 static_cast<long>(__ns.count())
519 };
520
521 int __ret = __glibcxx_rwlock_timedwrlock(&_M_rwlock, &__ts);
522 // On self-deadlock, we just fail to acquire the lock. Technically,
523 // the program violated the precondition.
524 if (__ret == ETIMEDOUT || __ret == EDEADLK)
525 return false;
526 // Errors not handled: EINVAL
527 __glibcxx_assert(__ret == 0);
528 return true;
529 }
530
531#ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK
532 template<typename _Duration>
533 bool
534 try_lock_until(const chrono::time_point<chrono::steady_clock,
535 _Duration>& __atime)
536 {
537 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
538 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
539
540 __gthread_time_t __ts =
541 {
542 static_cast<std::time_t>(__s.time_since_epoch().count()),
543 static_cast<long>(__ns.count())
544 };
545
546 int __ret = pthread_rwlock_clockwrlock(&_M_rwlock, CLOCK_MONOTONIC,
547 &__ts);
548 // On self-deadlock, we just fail to acquire the lock. Technically,
549 // the program violated the precondition.
550 if (__ret == ETIMEDOUT || __ret == EDEADLK)
551 return false;
552 // Errors not handled: EINVAL
553 __glibcxx_assert(__ret == 0);
554 return true;
555 }
556#endif
557
558 template<typename _Clock, typename _Duration>
559 bool
560 try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
561 {
562#if __cplusplus > 201703L
563 static_assert(chrono::is_clock_v<_Clock>);
564#endif
565 // The user-supplied clock may not tick at the same rate as
566 // steady_clock, so we must loop in order to guarantee that
567 // the timeout has expired before returning false.
568 typename _Clock::time_point __now = _Clock::now();
569 do {
570 auto __rtime = __atime - __now;
571 if (try_lock_for(__rtime))
572 return true;
573 __now = _Clock::now();
574 } while (__atime > __now);
575 return false;
576 }
577
578 // Shared ownership
579
580 template<typename _Duration>
581 bool
582 try_lock_shared_until(const chrono::time_point<chrono::system_clock,
583 _Duration>& __atime)
584 {
585 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
586 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
587
588 __gthread_time_t __ts =
589 {
590 static_cast<std::time_t>(__s.time_since_epoch().count()),
591 static_cast<long>(__ns.count())
592 };
593
594 int __ret;
595 // Unlike for lock(), we are not allowed to throw an exception so if
596 // the maximum number of read locks has been exceeded, or we would
597 // deadlock, we just try to acquire the lock again (and will time out
598 // eventually).
599 // In cases where we would exceed the maximum number of read locks
600 // throughout the whole time until the timeout, we will fail to
601 // acquire the lock even if it would be logically free; however, this
602 // is allowed by the standard, and we made a "strong effort"
603 // (see C++14 30.4.1.4p26).
604 // For cases where the implementation detects a deadlock we
605 // intentionally block and timeout so that an early return isn't
606 // mistaken for a spurious failure, which might help users realise
607 // there is a deadlock.
608 do
609 __ret = __glibcxx_rwlock_timedrdlock(&_M_rwlock, &__ts);
610 while (__ret == EAGAIN || __ret == EDEADLK);
611 if (__ret == ETIMEDOUT)
612 return false;
613 // Errors not handled: EINVAL
614 __glibcxx_assert(__ret == 0);
615 return true;
616 }
617
618#ifdef _GLIBCXX_USE_PTHREAD_RWLOCK_CLOCKLOCK
619 template<typename _Duration>
620 bool
621 try_lock_shared_until(const chrono::time_point<chrono::steady_clock,
622 _Duration>& __atime)
623 {
624 auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
625 auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
626
627 __gthread_time_t __ts =
628 {
629 static_cast<std::time_t>(__s.time_since_epoch().count()),
630 static_cast<long>(__ns.count())
631 };
632
633 int __ret = pthread_rwlock_clockrdlock(&_M_rwlock, CLOCK_MONOTONIC,
634 &__ts);
635 // On self-deadlock, we just fail to acquire the lock. Technically,
636 // the program violated the precondition.
637 if (__ret == ETIMEDOUT || __ret == EDEADLK)
638 return false;
639 // Errors not handled: EINVAL
640 __glibcxx_assert(__ret == 0);
641 return true;
642 }
643#endif
644
645 template<typename _Clock, typename _Duration>
646 bool
647 try_lock_shared_until(const chrono::time_point<_Clock,
648 _Duration>& __atime)
649 {
650#if __cplusplus > 201703L
651 static_assert(chrono::is_clock_v<_Clock>);
652#endif
653 // The user-supplied clock may not tick at the same rate as
654 // steady_clock, so we must loop in order to guarantee that
655 // the timeout has expired before returning false.
656 typename _Clock::time_point __now = _Clock::now();
657 do {
658 auto __rtime = __atime - __now;
659 if (try_lock_shared_for(__rtime))
660 return true;
661 __now = _Clock::now();
662 } while (__atime > __now);
663 return false;
664 }
665
666#else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)
667
668 // Exclusive ownership
669
670 template<typename _Clock, typename _Duration>
671 bool
672 try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
673 {
674 unique_lock<mutex> __lk(_M_mut);
675 if (!_M_gate1.wait_until(__lk, __abs_time,
676 [=]{ return !_M_write_entered(); }))
677 {
678 return false;
679 }
680 _M_state |= _S_write_entered;
681 if (!_M_gate2.wait_until(__lk, __abs_time,
682 [=]{ return _M_readers() == 0; }))
683 {
684 _M_state ^= _S_write_entered;
685 // Wake all threads blocked while the write-entered flag was set.
686 _M_gate1.notify_all();
687 return false;
688 }
689 return true;
690 }
691
692 // Shared ownership
693
694 template <typename _Clock, typename _Duration>
695 bool
696 try_lock_shared_until(const chrono::time_point<_Clock,
697 _Duration>& __abs_time)
698 {
699 unique_lock<mutex> __lk(_M_mut);
700 if (!_M_gate1.wait_until(__lk, __abs_time,
701 [=]{ return _M_state < _S_max_readers; }))
702 {
703 return false;
704 }
705 ++_M_state;
706 return true;
707 }
708
709#endif // _GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK
710 };
711#endif // _GLIBCXX_HAS_GTHREADS
712
713 /// shared_lock
714 template<typename _Mutex>
715 class shared_lock
716 {
717 public:
718 typedef _Mutex mutex_type;
719
720 // Shared locking
721
722 shared_lock() noexcept : _M_pm(nullptr), _M_owns(false) { }
723
724 explicit
725 shared_lock(mutex_type& __m)
726 : _M_pm(std::__addressof(__m)), _M_owns(true)
727 { __m.lock_shared(); }
728
729 shared_lock(mutex_type& __m, defer_lock_t) noexcept
730 : _M_pm(std::__addressof(__m)), _M_owns(false) { }
731
732 shared_lock(mutex_type& __m, try_to_lock_t)
733 : _M_pm(std::__addressof(__m)), _M_owns(__m.try_lock_shared()) { }
734
735 shared_lock(mutex_type& __m, adopt_lock_t)
736 : _M_pm(std::__addressof(__m)), _M_owns(true) { }
737
738 template<typename _Clock, typename _Duration>
739 shared_lock(mutex_type& __m,
740 const chrono::time_point<_Clock, _Duration>& __abs_time)
741 : _M_pm(std::__addressof(__m)),
742 _M_owns(__m.try_lock_shared_until(__abs_time)) { }
743
744 template<typename _Rep, typename _Period>
745 shared_lock(mutex_type& __m,
746 const chrono::duration<_Rep, _Period>& __rel_time)
747 : _M_pm(std::__addressof(__m)),
748 _M_owns(__m.try_lock_shared_for(__rel_time)) { }
749
750 ~shared_lock()
751 {
752 if (_M_owns)
753 _M_pm->unlock_shared();
754 }
755
756 shared_lock(shared_lock const&) = delete;
757 shared_lock& operator=(shared_lock const&) = delete;
758
759 shared_lock(shared_lock&& __sl) noexcept : shared_lock()
760 { swap(__sl); }
761
762 shared_lock&
763 operator=(shared_lock&& __sl) noexcept
764 {
765 shared_lock(std::move(__sl)).swap(*this);
766 return *this;
767 }
768
769 void
770 lock()
771 {
772 _M_lockable();
773 _M_pm->lock_shared();
774 _M_owns = true;
775 }
776
777 bool
778 try_lock()
779 {
780 _M_lockable();
781 return _M_owns = _M_pm->try_lock_shared();
782 }
783
784 template<typename _Rep, typename _Period>
785 bool
786 try_lock_for(const chrono::duration<_Rep, _Period>& __rel_time)
787 {
788 _M_lockable();
789 return _M_owns = _M_pm->try_lock_shared_for(__rel_time);
790 }
791
792 template<typename _Clock, typename _Duration>
793 bool
794 try_lock_until(const chrono::time_point<_Clock, _Duration>& __abs_time)
795 {
796 _M_lockable();
797 return _M_owns = _M_pm->try_lock_shared_until(__abs_time);
798 }
799
800 void
801 unlock()
802 {
803 if (!_M_owns)
804 __throw_system_error(int(errc::resource_deadlock_would_occur));
805 _M_pm->unlock_shared();
806 _M_owns = false;
807 }
808
809 // Setters
810
811 void
812 swap(shared_lock& __u) noexcept
813 {
814 std::swap(_M_pm, __u._M_pm);
815 std::swap(_M_owns, __u._M_owns);
816 }
817
818 mutex_type*
819 release() noexcept
820 {
821 _M_owns = false;
822 return std::__exchange(_M_pm, nullptr);
823 }
824
825 // Getters
826
827 bool owns_lock() const noexcept { return _M_owns; }
828
829 explicit operator bool() const noexcept { return _M_owns; }
830
831 mutex_type* mutex() const noexcept { return _M_pm; }
832
833 private:
834 void
835 _M_lockable() const
836 {
837 if (_M_pm == nullptr)
838 __throw_system_error(int(errc::operation_not_permitted));
839 if (_M_owns)
840 __throw_system_error(int(errc::resource_deadlock_would_occur));
841 }
842
843 mutex_type* _M_pm;
844 bool _M_owns;
845 };
846
847 /// Swap specialization for shared_lock
848 /// @relates shared_mutex
849 template<typename _Mutex>
850 void
851 swap(shared_lock<_Mutex>& __x, shared_lock<_Mutex>& __y) noexcept
852 { __x.swap(__y); }
853
854 /// @} group mutexes
855_GLIBCXX_END_NAMESPACE_VERSION
856} // namespace
857
858#endif // C++14
859
860#endif // _GLIBCXX_SHARED_MUTEX