xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision 8ddb146abcdf061be9f2c0db7e391697dafad85c) 
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 <__thread/timed_backoff_policy.h>
51#include <__threading_support>
52#include <atomic>
53#include <version>
54
55#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
56#pragma GCC system_header
57#endif
58
59#ifdef _LIBCPP_HAS_NO_THREADS
60# error <semaphore> is not supported on this single threaded system
61#endif
62
63_LIBCPP_PUSH_MACROS
64#include <__undef_macros>
65
66#if _LIBCPP_STD_VER >= 14
67
68_LIBCPP_BEGIN_NAMESPACE_STD
69
70/*
71
72__atomic_semaphore_base is the general-case implementation.
73It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
74functions. It avoids contention against users' own use of those facilities.
75
76*/
77
78class __atomic_semaphore_base
79{
80    __atomic_base<ptrdiff_t> __a;
81
82public:
83    _LIBCPP_INLINE_VISIBILITY
84    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
85    {
86    }
87    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
88    void release(ptrdiff_t __update = 1)
89    {
90        if(0 < __a.fetch_add(__update, memory_order_release))
91            ;
92        else if(__update > 1)
93            __a.notify_all();
94        else
95            __a.notify_one();
96    }
97    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
98    void acquire()
99    {
100        auto const __test_fn = [this]() -> bool {
101            auto __old = __a.load(memory_order_relaxed);
102            return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
103        };
104        __cxx_atomic_wait(&__a.__a_, __test_fn);
105    }
106    template <class Rep, class Period>
107    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
108    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
109    {
110        if (__rel_time == chrono::duration<Rep, Period>::zero())
111            return try_acquire();
112        auto const __test_fn = [this]() { return try_acquire(); };
113        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
114    }
115    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
116    bool try_acquire()
117    {
118        auto __old = __a.load(memory_order_acquire);
119        while (true) {
120            if (__old == 0)
121                return false;
122            if (__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
123                return true;
124        }
125    }
126};
127
128#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
129
130template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
131class counting_semaphore
132{
133    __atomic_semaphore_base __semaphore;
134
135public:
136    static constexpr ptrdiff_t max() noexcept {
137        return __least_max_value;
138    }
139
140    _LIBCPP_INLINE_VISIBILITY
141    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore(__count) { }
142    ~counting_semaphore() = default;
143
144    counting_semaphore(const counting_semaphore&) = delete;
145    counting_semaphore& operator=(const counting_semaphore&) = delete;
146
147    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
148    void release(ptrdiff_t __update = 1)
149    {
150        __semaphore.release(__update);
151    }
152    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
153    void acquire()
154    {
155        __semaphore.acquire();
156    }
157    template<class Rep, class Period>
158    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
159    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
160    {
161        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
162    }
163    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
164    bool try_acquire()
165    {
166        return __semaphore.try_acquire();
167    }
168    template <class Clock, class Duration>
169    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
170    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
171    {
172        auto const current = Clock::now();
173        if (current >= __abs_time)
174            return try_acquire();
175        else
176            return try_acquire_for(__abs_time - current);
177    }
178};
179
180using binary_semaphore = counting_semaphore<1>;
181
182_LIBCPP_END_NAMESPACE_STD
183
184#endif // _LIBCPP_STD_VER >= 14
185
186_LIBCPP_POP_MACROS
187
188#endif //_LIBCPP_SEMAPHORE
189