3 // Copyright (C) 2020-2025 Free Software Foundation, Inc.
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)
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.
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.
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/>.
25 /** @file include/latch
26 * This is a Standard C++ Library header.
29 #ifndef _GLIBCXX_LATCH
30 #define _GLIBCXX_LATCH 1
32 #ifdef _GLIBCXX_SYSHDR
33 #pragma GCC system_header
36 #include <bits/requires_hosted.h> // concurrency
38 #define __glibcxx_want_latch
39 #include <bits/version.h>
41 #ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
42 #include <bits/atomic_base.h>
43 #include <ext/numeric_traits.h>
44 #include <utility> // cmp_equal, cmp_less_equal, etc.
46 namespace std _GLIBCXX_VISIBILITY(default)
48 _GLIBCXX_BEGIN_NAMESPACE_VERSION
53 static constexpr ptrdiff_t
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__))
60 return __PTRDIFF_MAX__;
64 latch(ptrdiff_t __expected) noexcept
66 { __glibcxx_assert(__expected >= 0 && __expected <= max()); }
70 latch(const latch&) = delete;
71 latch& operator=(const latch&) = delete;
73 _GLIBCXX_ALWAYS_INLINE void
74 count_down(ptrdiff_t __update = 1)
76 __glibcxx_assert(__update >= 0 && __update <= max());
77 auto const __old = __atomic_impl::fetch_sub(&_M_a, __update,
78 memory_order::release);
79 if (std::cmp_equal(__old, __update))
80 __atomic_impl::notify_all(&_M_a);
82 __glibcxx_assert(std::cmp_less(__update, __old));
85 _GLIBCXX_ALWAYS_INLINE bool
86 try_wait() const noexcept
87 { return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
89 _GLIBCXX_ALWAYS_INLINE void
92 auto const __pred = [this] { return this->try_wait(); };
93 std::__atomic_wait_address(&_M_a, __pred);
96 _GLIBCXX_ALWAYS_INLINE void
97 arrive_and_wait(ptrdiff_t __update = 1) noexcept
104 alignas(__detail::__platform_wait_alignment)
105 __detail::__platform_wait_t _M_a;
107 _GLIBCXX_END_NAMESPACE_VERSION
109 #endif // __cpp_lib_latch
110 #endif // _GLIBCXX_LATCH