latch
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// <latch> -*- C++ -*-
// Copyright (C) 2020-2025 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
/** @file include/latch
* This is a Standard C++ Library header.
*/
#ifndef _GLIBCXX_LATCH
#define _GLIBCXX_LATCH 1
#ifdef _GLIBCXX_SYSHDR
#pragma GCC system_header
#endif
#include <bits/requires_hosted.h> // concurrency
#define __glibcxx_want_latch
#include <bits/version.h>
#ifdef __cpp_lib_latch // C++ >= 20 && atomic_wait
#include <bits/atomic_base.h>
#include <ext/numeric_traits.h>
#include <utility> // cmp_equal, cmp_less_equal, etc.
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
class latch
{
public:
static constexpr ptrdiff_t
max() noexcept
{
using __gnu_cxx::__int_traits;
constexpr auto __max = __int_traits<__detail::__platform_wait_t>::__max;
if constexpr (std::cmp_less(__max, __PTRDIFF_MAX__))
return __max;
return __PTRDIFF_MAX__;
}
constexpr explicit
latch(ptrdiff_t __expected) noexcept
: _M_a(__expected)
{ __glibcxx_assert(__expected >= 0 && __expected <= max()); }
~latch() = default;
latch(const latch&) = delete;
latch& operator=(const latch&) = delete;
_GLIBCXX_ALWAYS_INLINE void
count_down(ptrdiff_t __update = 1)
{
__glibcxx_assert(__update >= 0 && __update <= max());
auto const __old = __atomic_impl::fetch_sub(&_M_a, __update,
memory_order::release);
if (std::cmp_equal(__old, __update))
__atomic_impl::notify_all(&_M_a);
else
__glibcxx_assert(std::cmp_less(__update, __old));
}
_GLIBCXX_ALWAYS_INLINE bool
try_wait() const noexcept
{ return __atomic_impl::load(&_M_a, memory_order::acquire) == 0; }
_GLIBCXX_ALWAYS_INLINE void
wait() const noexcept
{
auto const __pred = [this] { return this->try_wait(); };
std::__atomic_wait_address(&_M_a, __pred);
}
_GLIBCXX_ALWAYS_INLINE void
arrive_and_wait(ptrdiff_t __update = 1) noexcept
{
count_down(__update);
wait();
}
private:
alignas(__detail::__platform_wait_alignment)
__detail::__platform_wait_t _M_a;
};
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // __cpp_lib_latch
#endif // _GLIBCXX_LATCH
utility
atomic_base.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:140
numeric_traits.h