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1// -*- C++ -*-
2//===----------------------------------------------------------------------===//
3//
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
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef _LIBCPP_SEMAPHORE
11#define _LIBCPP_SEMAPHORE
12
13/*
14    semaphore synopsis
15
16namespace std {
17
18template<ptrdiff_t least_max_value = implementation-defined>
19class counting_semaphore
20{
21public:
22static constexpr ptrdiff_t max() noexcept;
23
24constexpr explicit counting_semaphore(ptrdiff_t desired);
25~counting_semaphore();
26
27counting_semaphore(const counting_semaphore&) = delete;
28counting_semaphore& operator=(const counting_semaphore&) = delete;
29
30void release(ptrdiff_t update = 1);
31void acquire();
32bool try_acquire() noexcept;
33template<class Rep, class Period>
34    bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
35template<class Clock, class Duration>
36    bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
37
38private:
39ptrdiff_t counter; // exposition only
40};
41
42using binary_semaphore = counting_semaphore<1>;
43
44}
45
46*/
47
48#include <__assert> // all public C++ headers provide the assertion handler
49#include <__atomic/atomic_base.h>
50#include <__atomic/atomic_sync.h>
51#include <__atomic/memory_order.h>
52#include <__availability>
53#include <__chrono/time_point.h>
54#include <__config>
55#include <__thread/timed_backoff_policy.h>
56#include <__threading_support>
57#include <cstddef>
58#include <limits>
59#include <version>
60
61#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
62#  pragma GCC system_header
63#endif
64
65#ifdef _LIBCPP_HAS_NO_THREADS
66# error "<semaphore> is not supported since libc++ has been configured without support for threads."
67#endif
68
69_LIBCPP_PUSH_MACROS
70#include <__undef_macros>
71
72#if _LIBCPP_STD_VER >= 14
73
74_LIBCPP_BEGIN_NAMESPACE_STD
75
76/*
77
78__atomic_semaphore_base is the general-case implementation.
79It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
80functions. It avoids contention against users' own use of those facilities.
81
82*/
83
84class __atomic_semaphore_base
85{
86    __atomic_base<ptrdiff_t> __a_;
87
88public:
89    _LIBCPP_INLINE_VISIBILITY
90    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count)
91    {
92    }
93    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
94    void release(ptrdiff_t __update = 1)
95    {
96        if(0 < __a_.fetch_add(__update, memory_order_release))
97            ;
98        else if(__update > 1)
99            __a_.notify_all();
100        else
101            __a_.notify_one();
102    }
103    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
104    void acquire()
105    {
106        auto const __test_fn = [this]() -> bool {
107            auto __old = __a_.load(memory_order_relaxed);
108            return (__old != 0) && __a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
109        };
110        __cxx_atomic_wait(&__a_.__a_, __test_fn);
111    }
112    template <class Rep, class Period>
113    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
114    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
115    {
116        if (__rel_time == chrono::duration<Rep, Period>::zero())
117            return try_acquire();
118        auto const __test_fn = [this]() { return try_acquire(); };
119        return std::__libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
120    }
121    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
122    bool try_acquire()
123    {
124        auto __old = __a_.load(memory_order_acquire);
125        while (true) {
126            if (__old == 0)
127                return false;
128            if (__a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
129                return true;
130        }
131    }
132};
133
134#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
135
136template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
137class counting_semaphore
138{
139    __atomic_semaphore_base __semaphore_;
140
141public:
142    static constexpr ptrdiff_t max() noexcept {
143        return __least_max_value;
144    }
145
146    _LIBCPP_INLINE_VISIBILITY
147    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) { }
148    ~counting_semaphore() = default;
149
150    counting_semaphore(const counting_semaphore&) = delete;
151    counting_semaphore& operator=(const counting_semaphore&) = delete;
152
153    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
154    void release(ptrdiff_t __update = 1)
155    {
156        __semaphore_.release(__update);
157    }
158    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
159    void acquire()
160    {
161        __semaphore_.acquire();
162    }
163    template<class Rep, class Period>
164    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
165    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
166    {
167        return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
168    }
169    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
170    bool try_acquire()
171    {
172        return __semaphore_.try_acquire();
173    }
174    template <class Clock, class Duration>
175    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
176    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
177    {
178        auto const __current = Clock::now();
179        if (__current >= __abs_time)
180            return try_acquire();
181        else
182            return try_acquire_for(__abs_time - __current);
183    }
184};
185
186using binary_semaphore = counting_semaphore<1>;
187
188_LIBCPP_END_NAMESPACE_STD
189
190#endif // _LIBCPP_STD_VER >= 14
191
192_LIBCPP_POP_MACROS
193
194#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
195#  include <atomic>
196#endif
197
198#endif //_LIBCPP_SEMAPHORE
199