libstdc++
latch
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1// <latch> -*- C++ -*-
2
3// Copyright (C) 2020-2025 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#ifdef _GLIBCXX_SYSHDR
33#pragma GCC system_header
34#endif
35
36#include <bits/requires_hosted.h> // concurrency
37
38#define __glibcxx_want_latch
39#include <bits/version.h>
40
41#ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
42#include <bits/atomic_base.h>
43#include <ext/numeric_traits.h>
44
45namespace std _GLIBCXX_VISIBILITY(default)
46{
47_GLIBCXX_BEGIN_NAMESPACE_VERSION
48
49 class latch
50 {
51 public:
52 static constexpr ptrdiff_t
53 max() noexcept
55
56 constexpr explicit latch(ptrdiff_t __expected) noexcept
57 : _M_a(__expected) { }
58
59 ~latch() = default;
60 latch(const latch&) = delete;
61 latch& operator=(const latch&) = delete;
62
63 _GLIBCXX_ALWAYS_INLINE void
64 count_down(ptrdiff_t __update = 1)
65 {
66 __glibcxx_assert(__update >= 0);
67 auto const __old = __atomic_impl::fetch_sub(&_M_a,
68 __update, memory_order::release);
69 __glibcxx_assert(__update >= 0);
70 if (__old == static_cast<__detail::__platform_wait_t>(__update))
71 __atomic_impl::notify_all(&_M_a);
72 }
73
74 _GLIBCXX_ALWAYS_INLINE bool
75 try_wait() const noexcept
76 { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
77
78 _GLIBCXX_ALWAYS_INLINE void
79 wait() const noexcept
80 {
81 auto const __pred = [this] { return this->try_wait(); };
82 std::__atomic_wait_address(&_M_a, __pred);
83 }
84
85 _GLIBCXX_ALWAYS_INLINE void
86 arrive_and_wait(ptrdiff_t __update = 1) noexcept
87 {
88 count_down(__update);
89 wait();
90 }
91
92 private:
93 // This alignas is not redundant, it increases the alignment for
94 // long long on x86.
95 alignas(__alignof__(__detail::__platform_wait_t)) __detail::__platform_wait_t _M_a;
96 };
97_GLIBCXX_END_NAMESPACE_VERSION
98} // namespace
99#endif // __cpp_lib_latch
100#endif // _GLIBCXX_LATCH
ISO C++ entities toplevel namespace is std.
__numeric_traits_integer< _Tp > __int_traits
Convenience alias for __numeric_traits<integer-type>.