libstdc++
latch
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1 // <latch> -*- C++ -*-
2 
3 // Copyright (C) 2020-2024 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the
7 // terms of the GNU General Public License as published by the
8 // Free Software Foundation; either version 3, or (at your option)
9 // any later version.
10 
11 // This library is distributed in the hope that it will be useful,
12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 // GNU General Public License for more details.
15 
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19 
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24 
25 /** @file include/latch
26  * This is a Standard C++ Library header.
27  */
28 
29 #ifndef _GLIBCXX_LATCH
30 #define _GLIBCXX_LATCH 1
31 
32 #pragma GCC system_header
33 
34 #include <bits/requires_hosted.h> // concurrency
35 
36 #define __glibcxx_want_latch
37 #include <bits/version.h>
38 
39 #ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
40 #include <bits/atomic_base.h>
41 #include <ext/numeric_traits.h>
42 
43 namespace std _GLIBCXX_VISIBILITY(default)
44 {
45 _GLIBCXX_BEGIN_NAMESPACE_VERSION
46 
47  class latch
48  {
49  public:
50  static constexpr ptrdiff_t
51  max() noexcept
52  { return __gnu_cxx::__int_traits<__detail::__platform_wait_t>::__max; }
53 
54  constexpr explicit latch(ptrdiff_t __expected) noexcept
55  : _M_a(__expected) { }
56 
57  ~latch() = default;
58  latch(const latch&) = delete;
59  latch& operator=(const latch&) = delete;
60 
61  _GLIBCXX_ALWAYS_INLINE void
62  count_down(ptrdiff_t __update = 1)
63  {
64  auto const __old = __atomic_impl::fetch_sub(&_M_a,
65  __update, memory_order::release);
66  if (__old == __update)
67  __atomic_impl::notify_all(&_M_a);
68  }
69 
70  _GLIBCXX_ALWAYS_INLINE bool
71  try_wait() const noexcept
72  { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
73 
74  _GLIBCXX_ALWAYS_INLINE void
75  wait() const noexcept
76  {
77  auto const __pred = [this] { return this->try_wait(); };
78  std::__atomic_wait_address(&_M_a, __pred);
79  }
80 
81  _GLIBCXX_ALWAYS_INLINE void
82  arrive_and_wait(ptrdiff_t __update = 1) noexcept
83  {
84  count_down(__update);
85  wait();
86  }
87 
88  private:
89  alignas(__alignof__(__detail::__platform_wait_t)) __detail::__platform_wait_t _M_a;
90  };
91 _GLIBCXX_END_NAMESPACE_VERSION
92 } // namespace
93 #endif // __cpp_lib_latch
94 #endif // _GLIBCXX_LATCH