xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision e9e8876a4d6afc1ad5315faaa191b25121a813d7) 
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 <__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, to be used for
71user-requested least-max values that exceed the OS implementation support
72(incl. when the OS has no support of its own) and for binary semaphores.
73
74It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
75functions. It avoids contention against users' own use of those facilities.
76
77*/
78
79class __atomic_semaphore_base
80{
81    __atomic_base<ptrdiff_t> __a;
82
83public:
84    _LIBCPP_INLINE_VISIBILITY
85    __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
86    {
87    }
88    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
89    void release(ptrdiff_t __update = 1)
90    {
91        if(0 < __a.fetch_add(__update, memory_order_release))
92            ;
93        else if(__update > 1)
94            __a.notify_all();
95        else
96            __a.notify_one();
97    }
98    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
99    void acquire()
100    {
101        auto const __test_fn = [this]() -> bool {
102            auto __old = __a.load(memory_order_relaxed);
103            return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
104        };
105        __cxx_atomic_wait(&__a.__a_, __test_fn);
106    }
107    template <class Rep, class Period>
108    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
109    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
110    {
111        auto const __test_fn = [this]() -> bool {
112            auto __old = __a.load(memory_order_acquire);
113            while(1) {
114                if (__old == 0)
115                    return false;
116                if(__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
117                    return true;
118            }
119        };
120        return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
121    }
122};
123
124#ifndef _LIBCPP_NO_NATIVE_SEMAPHORES
125
126/*
127
128__platform_semaphore_base a simple wrapper for the OS semaphore type. That
129is, every call is routed to the OS in the most direct manner possible.
130
131*/
132
133class __platform_semaphore_base
134{
135    __libcpp_semaphore_t __semaphore;
136
137public:
138    _LIBCPP_INLINE_VISIBILITY
139    __platform_semaphore_base(ptrdiff_t __count) :
140        __semaphore()
141    {
142        __libcpp_semaphore_init(&__semaphore, __count);
143    }
144    _LIBCPP_INLINE_VISIBILITY
145    ~__platform_semaphore_base() {
146        __libcpp_semaphore_destroy(&__semaphore);
147    }
148    _LIBCPP_INLINE_VISIBILITY
149    void release(ptrdiff_t __update)
150    {
151        for(; __update; --__update)
152            __libcpp_semaphore_post(&__semaphore);
153    }
154    _LIBCPP_INLINE_VISIBILITY
155    void acquire()
156    {
157        __libcpp_semaphore_wait(&__semaphore);
158    }
159    _LIBCPP_INLINE_VISIBILITY
160    bool try_acquire_for(chrono::nanoseconds __rel_time)
161    {
162        return __libcpp_semaphore_wait_timed(&__semaphore, __rel_time);
163    }
164};
165
166template<ptrdiff_t __least_max_value>
167using __semaphore_base =
168    typename conditional<(__least_max_value > 1 && __least_max_value <= _LIBCPP_SEMAPHORE_MAX),
169                         __platform_semaphore_base,
170                         __atomic_semaphore_base>::type;
171
172#else
173
174template<ptrdiff_t __least_max_value>
175using __semaphore_base =
176    __atomic_semaphore_base;
177
178#define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
179
180#endif //_LIBCPP_NO_NATIVE_SEMAPHORES
181
182template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
183class counting_semaphore
184{
185    __semaphore_base<__least_max_value> __semaphore;
186
187public:
188    static constexpr ptrdiff_t max() noexcept {
189        return __least_max_value;
190    }
191
192    _LIBCPP_INLINE_VISIBILITY
193    counting_semaphore(ptrdiff_t __count = 0) : __semaphore(__count) { }
194    ~counting_semaphore() = default;
195
196    counting_semaphore(const counting_semaphore&) = delete;
197    counting_semaphore& operator=(const counting_semaphore&) = delete;
198
199    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
200    void release(ptrdiff_t __update = 1)
201    {
202        __semaphore.release(__update);
203    }
204    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
205    void acquire()
206    {
207        __semaphore.acquire();
208    }
209    template<class Rep, class Period>
210    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
211    bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
212    {
213        return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
214    }
215    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
216    bool try_acquire()
217    {
218        return try_acquire_for(chrono::nanoseconds::zero());
219    }
220    template <class Clock, class Duration>
221    _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
222    bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
223    {
224        auto const current = Clock::now();
225        if(current >= __abs_time)
226            return try_acquire();
227        else
228            return try_acquire_for(__abs_time - current);
229    }
230};
231
232using binary_semaphore = counting_semaphore<1>;
233
234_LIBCPP_END_NAMESPACE_STD
235
236#endif // _LIBCPP_STD_VER >= 14
237
238_LIBCPP_POP_MACROS
239
240#endif //_LIBCPP_SEMAPHORE
241