xref: /aosp_15_r20/external/cronet/third_party/libc++/src/include/semaphore (revision 6777b5387eb2ff775bb5750e3f5d96f37fb7352b) 
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>
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/support.h>
62#include <__thread/timed_backoff_policy.h>
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    if (__old == 0) {
98      __a_.notify_all();
99    }
100  }
101  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() {
102    std::__atomic_wait_unless(
103        __a_, [this](ptrdiff_t& __old) { return __try_acquire_impl(__old); }, memory_order_relaxed);
104  }
105  template <class _Rep, class _Period>
106  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
107  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
108    if (__rel_time == chrono::duration<_Rep, _Period>::zero())
109      return try_acquire();
110    auto const __poll_fn = [this]() { return try_acquire(); };
111    return std::__libcpp_thread_poll_with_backoff(__poll_fn, __libcpp_timed_backoff_policy(), __rel_time);
112  }
113  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() {
114    auto __old = __a_.load(memory_order_relaxed);
115    return __try_acquire_impl(__old);
116  }
117
118private:
119  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __try_acquire_impl(ptrdiff_t& __old) {
120    while (true) {
121      if (__old == 0)
122        return false;
123      if (__a_.compare_exchange_weak(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
124        return true;
125    }
126  }
127};
128
129template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
130class counting_semaphore {
131  __atomic_semaphore_base __semaphore_;
132
133public:
134  static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");
135
136  static constexpr ptrdiff_t max() noexcept { return __least_max_value; }
137
138  _LIBCPP_HIDE_FROM_ABI constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) {
139    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
140        __count >= 0,
141        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
142        "initialized with a negative value");
143    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
144        __count <= max(),
145        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
146        "initialized with a value greater than max()");
147  }
148  ~counting_semaphore() = default;
149
150  counting_semaphore(const counting_semaphore&)            = delete;
151  counting_semaphore& operator=(const counting_semaphore&) = delete;
152
153  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
154    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "counting_semaphore:release called with a negative value");
155    __semaphore_.release(__update);
156  }
157  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() { __semaphore_.acquire(); }
158  template <class _Rep, class _Period>
159  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
160  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
161    return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
162  }
163  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() { return __semaphore_.try_acquire(); }
164  template <class _Clock, class _Duration>
165  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
166  try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time) {
167    auto const __current = _Clock::now();
168    if (__current >= __abs_time)
169      return try_acquire();
170    else
171      return try_acquire_for(__abs_time - __current);
172  }
173};
174
175using binary_semaphore = counting_semaphore<1>;
176
177_LIBCPP_END_NAMESPACE_STD
178
179#endif // _LIBCPP_STD_VER >= 14
180
181_LIBCPP_POP_MACROS
182
183#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
184#  include <atomic>
185#endif
186
187#endif //_LIBCPP_SEMAPHORE
188