xref: /freebsd/contrib/llvm-project/libcxx/include/semaphore (revision 95eb4b873b6a8b527c5bd78d7191975dfca38998) 
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 <__config>
49
50#ifdef _LIBCPP_HAS_NO_THREADS
51#  error "<semaphore> is not supported since libc++ has been configured without support for threads."
52#endif
53
54#include <__assert> // all public C++ headers provide the assertion handler
55#include <__atomic/atomic_base.h>
56#include <__atomic/atomic_sync.h>
57#include <__atomic/memory_order.h>
58#include <__availability>
59#include <__chrono/time_point.h>
60#include <__thread/poll_with_backoff.h>
61#include <__thread/timed_backoff_policy.h>
62#include <__threading_support>
63#include <cstddef>
64#include <limits>
65#include <version>
66
67#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
68#  pragma GCC system_header
69#endif
70
71_LIBCPP_PUSH_MACROS
72#include <__undef_macros>
73
74#if _LIBCPP_STD_VER >= 14
75
76_LIBCPP_BEGIN_NAMESPACE_STD
77
78/*
79
80__atomic_semaphore_base is the general-case implementation.
81It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
82functions. It avoids contention against users' own use of those facilities.
83
84*/
85
86#  define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
87
88class __atomic_semaphore_base {
89  __atomic_base<ptrdiff_t> __a_;
90
91public:
92  _LIBCPP_HIDE_FROM_ABI constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count) {}
93  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
94    auto __old = __a_.fetch_add(__update, memory_order_release);
95    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
96        __update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
97
98    if (__old > 0) {
99      // Nothing to do
100    } else if (__update > 1)
101      __a_.notify_all();
102    else
103      __a_.notify_one();
104  }
105  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() {
106    auto const __test_fn = [this]() -> bool {
107      auto __old = __a_.load(memory_order_relaxed);
108      return (__old != 0) && __a_.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
109    };
110    __cxx_atomic_wait(&__a_.__a_, __test_fn);
111  }
112  template <class _Rep, class _Period>
113  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
114  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
115    if (__rel_time == chrono::duration<_Rep, _Period>::zero())
116      return try_acquire();
117    auto const __test_fn = [this]() { return try_acquire(); };
118    return std::__libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
119  }
120  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() {
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
131template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
132class counting_semaphore {
133  __atomic_semaphore_base __semaphore_;
134
135public:
136  static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
137
138  static constexpr ptrdiff_t max() noexcept { return __least_max_value; }
139
140  _LIBCPP_HIDE_FROM_ABI constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) {
141    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
142        __count >= 0,
143        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
144        "initialized with a negative value");
145    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
146        __count <= max(),
147        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
148        "initialized with a value greater than max()");
149  }
150  ~counting_semaphore() = default;
151
152  counting_semaphore(const counting_semaphore&)            = delete;
153  counting_semaphore& operator=(const counting_semaphore&) = delete;
154
155  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
156    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "counting_semaphore:release called with a negative value");
157    __semaphore_.release(__update);
158  }
159  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() { __semaphore_.acquire(); }
160  template <class _Rep, class _Period>
161  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
162  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
163    return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
164  }
165  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() { return __semaphore_.try_acquire(); }
166  template <class _Clock, class _Duration>
167  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
168  try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time) {
169    auto const __current = _Clock::now();
170    if (__current >= __abs_time)
171      return try_acquire();
172    else
173      return try_acquire_for(__abs_time - __current);
174  }
175};
176
177using binary_semaphore = counting_semaphore<1>;
178
179_LIBCPP_END_NAMESPACE_STD
180
181#endif // _LIBCPP_STD_VER >= 14
182
183_LIBCPP_POP_MACROS
184
185#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
186#  include <atomic>
187#endif
188
189#endif //_LIBCPP_SEMAPHORE
190