xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision 5ca8e32633c4ffbbcd6762e5888b6a4ba0708c6c) 
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
84#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
85
86class __atomic_semaphore_base
87{
88    __atomic_base<ptrdiff_t> __a_;
89
90public:
91    _LIBCPP_INLINE_VISIBILITY
92    constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count)
93    {
94    }
95    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
96    void release(ptrdiff_t __update = 1)
97    {
98        auto __old = __a_.fetch_add(__update, memory_order_release);
99        _LIBCPP_ASSERT_UNCATEGORIZED(__update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
100
101        if (__old > 0)
102        {
103            // Nothing to do
104        }
105        else if (__update > 1)
106            __a_.notify_all();
107        else
108            __a_.notify_one();
109    }
110    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
111    void acquire()
112    {
113        auto const __test_fn = [this]() -> bool {
114            auto __old = __a_.load(memory_order_relaxed);
115            return (__old != 0) && __a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
116        };
117        __cxx_atomic_wait(&__a_.__a_, __test_fn);
118    }
119    template <class _Rep, class _Period>
120    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
121    bool try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time)
122    {
123        if (__rel_time == chrono::duration<_Rep, _Period>::zero())
124            return try_acquire();
125        auto const __test_fn = [this]() { return try_acquire(); };
126        return std::__libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
127    }
128    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
129    bool try_acquire()
130    {
131        auto __old = __a_.load(memory_order_acquire);
132        while (true) {
133            if (__old == 0)
134                return false;
135            if (__a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
136                return true;
137        }
138    }
139};
140
141template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
142class counting_semaphore
143{
144    __atomic_semaphore_base __semaphore_;
145
146public:
147    static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
148
149    static constexpr ptrdiff_t max() noexcept {
150        return __least_max_value;
151    }
152
153    _LIBCPP_INLINE_VISIBILITY
154    constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count)
155    {
156        _LIBCPP_ASSERT_UNCATEGORIZED(
157            __count >= 0,
158            "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
159            "initialized with a negative value");
160        _LIBCPP_ASSERT_UNCATEGORIZED(
161            __count <= max(),
162            "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
163            "initialized with a value greater than max()");
164    }
165    ~counting_semaphore() = default;
166
167    counting_semaphore(const counting_semaphore&) = delete;
168    counting_semaphore& operator=(const counting_semaphore&) = delete;
169
170    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
171    void release(ptrdiff_t __update = 1)
172    {
173        _LIBCPP_ASSERT_UNCATEGORIZED(__update >= 0, "counting_semaphore:release called with a negative value");
174        __semaphore_.release(__update);
175    }
176    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
177    void acquire()
178    {
179        __semaphore_.acquire();
180    }
181    template<class _Rep, class _Period>
182    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
183    bool try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time)
184    {
185        return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
186    }
187    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
188    bool try_acquire()
189    {
190        return __semaphore_.try_acquire();
191    }
192    template <class _Clock, class _Duration>
193    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
194    bool try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time)
195    {
196        auto const __current = _Clock::now();
197        if (__current >= __abs_time)
198            return try_acquire();
199        else
200            return try_acquire_for(__abs_time - __current);
201    }
202};
203
204using binary_semaphore = counting_semaphore<1>;
205
206_LIBCPP_END_NAMESPACE_STD
207
208#endif // _LIBCPP_STD_VER >= 14
209
210_LIBCPP_POP_MACROS
211
212#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
213#  include <atomic>
214#endif
215
216#endif //_LIBCPP_SEMAPHORE
217