xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision a50d73d5782a351ad83e8d1f84d11720a12e70d3) 
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 <__availability>
50#include <__chrono/time_point.h>
51#include <__config>
52#include <__thread/timed_backoff_policy.h>
53#include <__threading_support>
54#include <atomic>
55#include <limits>
56#include <version>
57
58#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
59#  pragma GCC system_header
60#endif
61
62#ifdef _LIBCPP_HAS_NO_THREADS
63# error "<semaphore> is not supported since libc++ has been configured without support for threads."
64#endif
65
66_LIBCPP_PUSH_MACROS
67#include <__undef_macros>
68
69#if _LIBCPP_STD_VER >= 14
70
71_LIBCPP_BEGIN_NAMESPACE_STD
72
73/*
74
75__atomic_semaphore_base is the general-case implementation.
76It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
77functions. It avoids contention against users' own use of those facilities.
78
79*/
80
81class __atomic_semaphore_base
82{
83    __atomic_base<ptrdiff_t> __a;
84
85public:
86    _LIBCPP_INLINE_VISIBILITY
87    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
88    {
89    }
90    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
91    void release(ptrdiff_t __update = 1)
92    {
93        if(0 < __a.fetch_add(__update, memory_order_release))
94            ;
95        else if(__update > 1)
96            __a.notify_all();
97        else
98            __a.notify_one();
99    }
100    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
101    void acquire()
102    {
103        auto const __test_fn = [this]() -> bool {
104            auto __old = __a.load(memory_order_relaxed);
105            return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
106        };
107        __cxx_atomic_wait(&__a.__a_, __test_fn);
108    }
109    template <class Rep, class Period>
110    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
111    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
112    {
113        if (__rel_time == chrono::duration<Rep, Period>::zero())
114            return try_acquire();
115        auto const __test_fn = [this]() { return try_acquire(); };
116        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
117    }
118    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
119    bool try_acquire()
120    {
121        auto __old = __a.load(memory_order_acquire);
122        while (true) {
123            if (__old == 0)
124                return false;
125            if (__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
126                return true;
127        }
128    }
129};
130
131#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
132
133template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
134class counting_semaphore
135{
136    __atomic_semaphore_base __semaphore;
137
138public:
139    static constexpr ptrdiff_t max() noexcept {
140        return __least_max_value;
141    }
142
143    _LIBCPP_INLINE_VISIBILITY
144    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore(__count) { }
145    ~counting_semaphore() = default;
146
147    counting_semaphore(const counting_semaphore&) = delete;
148    counting_semaphore& operator=(const counting_semaphore&) = delete;
149
150    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
151    void release(ptrdiff_t __update = 1)
152    {
153        __semaphore.release(__update);
154    }
155    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
156    void acquire()
157    {
158        __semaphore.acquire();
159    }
160    template<class Rep, class Period>
161    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
162    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
163    {
164        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
165    }
166    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
167    bool try_acquire()
168    {
169        return __semaphore.try_acquire();
170    }
171    template <class Clock, class Duration>
172    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
173    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
174    {
175        auto const current = Clock::now();
176        if (current >= __abs_time)
177            return try_acquire();
178        else
179            return try_acquire_for(__abs_time - current);
180    }
181};
182
183using binary_semaphore = counting_semaphore<1>;
184
185_LIBCPP_END_NAMESPACE_STD
186
187#endif // _LIBCPP_STD_VER >= 14
188
189_LIBCPP_POP_MACROS
190
191#endif //_LIBCPP_SEMAPHORE
192