xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision 6be3386466ab79a84b48429ae66244f21526d3df) 
1// -*- C++ -*-
2//===--------------------------- semaphore --------------------------------===//
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 <__config>
49#include <__threading_support>
50#include <atomic>
51#include <cassert>
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#if _LIBCPP_STD_VER >= 14
62
63_LIBCPP_BEGIN_NAMESPACE_STD
64
65/*
66
67__atomic_semaphore_base is the general-case implementation, to be used for
68user-requested least-max values that exceed the OS implementation support
69(incl. when the OS has no support of its own) and for binary semaphores.
70
71It is a typical Dijsktra 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    __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 = [=]() -> 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        auto const __test_fn = [=]() -> bool {
109            auto __old = __a.load(memory_order_acquire);
110            while(1) {
111                if (__old == 0)
112                    return false;
113                if(__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
114                    return true;
115            }
116        };
117        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
118    }
119};
120
121#ifndef _LIBCPP_NO_NATIVE_SEMAPHORES
122
123/*
124
125__platform_semaphore_base a simple wrapper for the OS semaphore type. That
126is, every call is routed to the OS in the most direct manner possible.
127
128*/
129
130class __platform_semaphore_base
131{
132    __libcpp_semaphore_t __semaphore;
133
134public:
135    _LIBCPP_INLINE_VISIBILITY
136    __platform_semaphore_base(ptrdiff_t __count) :
137        __semaphore()
138    {
139        __libcpp_semaphore_init(&__semaphore, __count);
140    }
141    _LIBCPP_INLINE_VISIBILITY
142    ~__platform_semaphore_base() {
143        __libcpp_semaphore_destroy(&__semaphore);
144    }
145    _LIBCPP_INLINE_VISIBILITY
146    void release(ptrdiff_t __update)
147    {
148        for(; __update; --__update)
149            __libcpp_semaphore_post(&__semaphore);
150    }
151    _LIBCPP_INLINE_VISIBILITY
152    void acquire()
153    {
154        __libcpp_semaphore_wait(&__semaphore);
155    }
156    _LIBCPP_INLINE_VISIBILITY
157    bool try_acquire_for(chrono::nanoseconds __rel_time)
158    {
159        return __libcpp_semaphore_wait_timed(&__semaphore, __rel_time);
160    }
161};
162
163template<ptrdiff_t __least_max_value>
164using __semaphore_base =
165    typename conditional<(__least_max_value > 1 && __least_max_value <= _LIBCPP_SEMAPHORE_MAX),
166                         __platform_semaphore_base,
167                         __atomic_semaphore_base>::type;
168
169#else
170
171template<ptrdiff_t __least_max_value>
172using __semaphore_base =
173    __atomic_semaphore_base;
174
175#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
176
177#endif //_LIBCPP_NO_NATIVE_SEMAPHORES
178
179template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
180class counting_semaphore
181{
182    __semaphore_base<__least_max_value> __semaphore;
183
184public:
185    static constexpr ptrdiff_t max() noexcept {
186        return __least_max_value;
187    }
188
189    _LIBCPP_INLINE_VISIBILITY
190    counting_semaphore(ptrdiff_t __count = 0) : __semaphore(__count) { }
191    ~counting_semaphore() = default;
192
193    counting_semaphore(const counting_semaphore&) = delete;
194    counting_semaphore& operator=(const counting_semaphore&) = delete;
195
196    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
197    void release(ptrdiff_t __update = 1)
198    {
199        __semaphore.release(__update);
200    }
201    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
202    void acquire()
203    {
204        __semaphore.acquire();
205    }
206    template<class Rep, class Period>
207    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
208    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
209    {
210        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
211    }
212    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
213    bool try_acquire()
214    {
215        return try_acquire_for(chrono::nanoseconds::zero());
216    }
217    template <class Clock, class Duration>
218    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
219    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
220    {
221        auto const current = Clock::now();
222        if(current >= __abs_time)
223            return try_acquire();
224        else
225            return try_acquire_for(__abs_time - current);
226    }
227};
228
229using binary_semaphore = counting_semaphore<1>;
230
231_LIBCPP_END_NAMESPACE_STD
232
233#endif // _LIBCPP_STD_VER >= 14
234
235#endif //_LIBCPP_SEMAPHORE
236