xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision 5e801ac66d24704442eba426ed13c3effb8a34e7) 
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 <__availability>
49#include <__config>
50#include <__threading_support>
51#include <atomic>
52
53#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
54#pragma GCC system_header
55#endif
56
57#ifdef _LIBCPP_HAS_NO_THREADS
58# error <semaphore> is not supported on this single threaded system
59#endif
60
61_LIBCPP_PUSH_MACROS
62#include <__undef_macros>
63
64#if _LIBCPP_STD_VER >= 14
65
66_LIBCPP_BEGIN_NAMESPACE_STD
67
68/*
69
70__atomic_semaphore_base is the general-case implementation.
71It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
72functions. It avoids contention against users' own use of those facilities.
73
74*/
75
76class __atomic_semaphore_base
77{
78    __atomic_base<ptrdiff_t> __a;
79
80public:
81    _LIBCPP_INLINE_VISIBILITY
82    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
83    {
84    }
85    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
86    void release(ptrdiff_t __update = 1)
87    {
88        if(0 < __a.fetch_add(__update, memory_order_release))
89            ;
90        else if(__update > 1)
91            __a.notify_all();
92        else
93            __a.notify_one();
94    }
95    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
96    void acquire()
97    {
98        auto const __test_fn = [this]() -> bool {
99            auto __old = __a.load(memory_order_relaxed);
100            return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
101        };
102        __cxx_atomic_wait(&__a.__a_, __test_fn);
103    }
104    template <class Rep, class Period>
105    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
106    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
107    {
108        if (__rel_time == chrono::duration<Rep, Period>::zero())
109            return try_acquire();
110        auto const __test_fn = [this]() { return try_acquire(); };
111        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
112    }
113    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
114    bool try_acquire()
115    {
116        auto __old = __a.load(memory_order_acquire);
117        while (true) {
118            if (__old == 0)
119                return false;
120            if (__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
121                return true;
122        }
123    }
124};
125
126#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
127
128template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
129class counting_semaphore
130{
131    __atomic_semaphore_base __semaphore;
132
133public:
134    static constexpr ptrdiff_t max() noexcept {
135        return __least_max_value;
136    }
137
138    _LIBCPP_INLINE_VISIBILITY
139    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore(__count) { }
140    ~counting_semaphore() = default;
141
142    counting_semaphore(const counting_semaphore&) = delete;
143    counting_semaphore& operator=(const counting_semaphore&) = delete;
144
145    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
146    void release(ptrdiff_t __update = 1)
147    {
148        __semaphore.release(__update);
149    }
150    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
151    void acquire()
152    {
153        __semaphore.acquire();
154    }
155    template<class Rep, class Period>
156    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
157    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
158    {
159        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
160    }
161    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
162    bool try_acquire()
163    {
164        return __semaphore.try_acquire();
165    }
166    template <class Clock, class Duration>
167    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
168    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
169    {
170        auto const current = Clock::now();
171        if (current >= __abs_time)
172            return try_acquire();
173        else
174            return try_acquire_for(__abs_time - current);
175    }
176};
177
178using binary_semaphore = counting_semaphore<1>;
179
180_LIBCPP_END_NAMESPACE_STD
181
182#endif // _LIBCPP_STD_VER >= 14
183
184_LIBCPP_POP_MACROS
185
186#endif //_LIBCPP_SEMAPHORE
187