libstdc++: latch Source File

libstdc++

latch
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1// <latch> -*- C++ -*-
2
3// Copyright (C) 2020-2026 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#include <bits/intcmp.h> // cmp_equal, cmp_less_equal, etc.
45
46namespace std _GLIBCXX_VISIBILITY(default)
47{
48_GLIBCXX_BEGIN_NAMESPACE_VERSION
49
50 class latch
51 {
52 public:
53 static constexpr ptrdiff_t
54 max() noexcept
55 {
56 using __gnu_cxx::__int_traits;
57 constexpr auto __max = __int_traits<__detail::__platform_wait_t>::__max;
58 if constexpr (std::cmp_less(__max, __PTRDIFF_MAX__))
59 return __max;
60 return __PTRDIFF_MAX__;
61 }
62
63 constexpr explicit
64 latch(ptrdiff_t __expected) noexcept
65 : _M_counter(__expected)
66 { __glibcxx_assert(__expected >= 0 && __expected <= max()); }
67
68 ~latch() = default;
69
70 latch(const latch&) = delete;
71 latch& operator=(const latch&) = delete;
72
73 _GLIBCXX_ALWAYS_INLINE void
74 count_down(ptrdiff_t __update = 1)
75 {
76 __glibcxx_assert(__update >= 0 && __update <= max());
77 auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
78 memory_order::release);
79 if (std::cmp_equal(__old, __update))
80 __atomic_impl::notify_all(&_M_counter);
81 else
82 __glibcxx_assert(std::cmp_less(__update, __old));
83 }
84
85 _GLIBCXX_ALWAYS_INLINE bool
86 try_wait() const noexcept
87 { return __atomic_impl::load(&_M_counter, memory_order::acquire) == 0; }
88
89 _GLIBCXX_ALWAYS_INLINE void
90 wait() const noexcept
91 {
92 auto const __vfn = [this] {
93 return __atomic_impl::load(&_M_counter, memory_order::acquire);
94 };
95 auto const __pred = [](__detail::__platform_wait_t __v) {
96 return __v == 0;
97 };
98 std::__atomic_wait_address(&_M_counter, __pred, __vfn);
99 }
100
101 _GLIBCXX_ALWAYS_INLINE void
102 arrive_and_wait(ptrdiff_t __update = 1) noexcept
103 {
104 // The standard specifies this functions as count_down(update); wait();
105 // but we combine those two calls into one and avoid the wait() if we
106 // know the counter reached zero.
107
108 __glibcxx_assert(__update >= 0 && __update <= max());
109 // Use acq_rel here because an omitted wait() would have used acquire:
110 auto const __old = __atomic_impl::fetch_sub(&_M_counter, __update,
111 memory_order::acq_rel);
112 if (std::cmp_equal(__old, __update))
113 __atomic_impl::notify_all(&_M_counter);
114 else
115 {
116 __glibcxx_assert(std::cmp_less(__update, __old));
117 wait();
118 }
119 }
120
121 private:
122 alignas(__detail::__platform_wait_alignment)
123 __detail::__platform_wait_t _M_counter;
124 };
125_GLIBCXX_END_NAMESPACE_VERSION
126} // namespace
127#endif // __cpp_lib_latch
128#endif // _GLIBCXX_LATCH
atomic_base.h
intcmp.h
version.h
requires_hosted.h
std
ISO C++ entities toplevel namespace is std.
__gnu_cxx::__int_traits
__numeric_traits_integer< _Tp > __int_traits
Convenience alias for __numeric_traits<integer-type>.
Definition ext/numeric_traits.h:134
numeric_traits.h