2 //===----------------------------------------------------------------------===//
4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // See https://llvm.org/LICENSE.txt for license information.
6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
8 //===----------------------------------------------------------------------===//
10 #ifndef _LIBCPP_SEMAPHORE
11 #define _LIBCPP_SEMAPHORE
18 template<ptrdiff_t least_max_value = implementation-defined>
19 class counting_semaphore
22 static constexpr ptrdiff_t max() noexcept;
24 constexpr explicit counting_semaphore(ptrdiff_t desired);
25 ~counting_semaphore();
27 counting_semaphore(const counting_semaphore&) = delete;
28 counting_semaphore& operator=(const counting_semaphore&) = delete;
30 void release(ptrdiff_t update = 1);
32 bool try_acquire() noexcept;
33 template<class Rep, class Period>
34 bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
35 template<class Clock, class Duration>
36 bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
39 ptrdiff_t counter; // exposition only
42 using binary_semaphore = counting_semaphore<1>;
48 #include <__cxx03/__config>
50 #if !defined(_LIBCPP_HAS_NO_THREADS)
52 # include <__cxx03/__assert>
53 # include <__cxx03/__atomic/atomic_base.h>
54 # include <__cxx03/__atomic/atomic_sync.h>
55 # include <__cxx03/__atomic/memory_order.h>
56 # include <__cxx03/__chrono/time_point.h>
57 # include <__cxx03/__thread/poll_with_backoff.h>
58 # include <__cxx03/__thread/support.h>
59 # include <__cxx03/__thread/timed_backoff_policy.h>
60 # include <__cxx03/cstddef>
61 # include <__cxx03/limits>
62 # include <__cxx03/version>
64 # if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
65 # pragma GCC system_header
69 # include <__cxx03/__undef_macros>
71 # if _LIBCPP_STD_VER >= 14
73 _LIBCPP_BEGIN_NAMESPACE_STD
77 __atomic_semaphore_base is the general-case implementation.
78 It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
79 functions. It avoids contention against users' own use of those facilities.
83 # define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
85 class __atomic_semaphore_base {
86 __atomic_base<ptrdiff_t> __a_;
89 _LIBCPP_HIDE_FROM_ABI constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count) {}
90 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
91 auto __old = __a_.fetch_add(__update, memory_order_release);
92 _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
93 __update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
98 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() {
99 std::__atomic_wait_unless(
100 __a_, [this](ptrdiff_t& __old) { return __try_acquire_impl(__old); }, memory_order_relaxed);
102 template <class _Rep, class _Period>
103 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
104 try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
105 if (__rel_time == chrono::duration<_Rep, _Period>::zero())
106 return try_acquire();
107 auto const __poll_fn = [this]() { return try_acquire(); };
108 return std::__libcpp_thread_poll_with_backoff(__poll_fn, __libcpp_timed_backoff_policy(), __rel_time);
110 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() {
111 auto __old = __a_.load(memory_order_relaxed);
112 return __try_acquire_impl(__old);
116 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __try_acquire_impl(ptrdiff_t& __old) {
120 if (__a_.compare_exchange_weak(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
126 template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
127 class _LIBCPP_DEPRECATED_ATOMIC_SYNC counting_semaphore {
128 __atomic_semaphore_base __semaphore_;
131 static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
133 static constexpr ptrdiff_t max() noexcept { return __least_max_value; }
135 _LIBCPP_HIDE_FROM_ABI constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) {
136 _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
138 "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
139 "initialized with a negative value");
140 _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
142 "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
143 "initialized with a value greater than max()");
145 ~counting_semaphore() = default;
147 counting_semaphore(const counting_semaphore&) = delete;
148 counting_semaphore& operator=(const counting_semaphore&) = delete;
150 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
151 _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "counting_semaphore:release called with a negative value");
152 __semaphore_.release(__update);
154 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() { __semaphore_.acquire(); }
155 template <class _Rep, class _Period>
156 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
157 try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
158 return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
160 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() { return __semaphore_.try_acquire(); }
161 template <class _Clock, class _Duration>
162 _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
163 try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time) {
164 auto const __current = _Clock::now();
165 if (__current >= __abs_time)
166 return try_acquire();
168 return try_acquire_for(__abs_time - __current);
172 _LIBCPP_SUPPRESS_DEPRECATED_PUSH
173 using binary_semaphore _LIBCPP_DEPRECATED_ATOMIC_SYNC = counting_semaphore<1>;
174 _LIBCPP_SUPPRESS_DEPRECATED_POP
176 _LIBCPP_END_NAMESPACE_STD
178 # endif // _LIBCPP_STD_VER >= 14
182 #endif // !defined(_LIBCPP_HAS_NO_THREADS)
184 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
185 # include <__cxx03/atomic>
188 #endif //_LIBCPP_SEMAPHORE