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The C and C++ Include Header Files

cat -n /usr/include/c++/15/latch

     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	#include <utility> // cmp_equal, cmp_less_equal, etc.
    45	
    46	namespace 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_a(__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_a, __update,
    78							  memory_order::release);
    79	      if (std::cmp_equal(__old, __update))
    80		__atomic_impl::notify_all(&_M_a);
    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_a, memory_order::acquire) == 0; }
    88	
    89	    _GLIBCXX_ALWAYS_INLINE void
    90	    wait() const noexcept
    91	    {
    92	      auto const __pred = [this] { return this->try_wait(); };
    93	      std::__atomic_wait_address(&_M_a, __pred);
    94	    }
    95	
    96	    _GLIBCXX_ALWAYS_INLINE void
    97	    arrive_and_wait(ptrdiff_t __update = 1) noexcept
    98	    {
    99	      count_down(__update);
   100	      wait();
   101	    }
   102	
   103	  private:
   104	    alignas(__detail::__platform_wait_alignment)
   105	      __detail::__platform_wait_t _M_a;
   106	  };
   107	_GLIBCXX_END_NAMESPACE_VERSION
   108	} // namespace
   109	#endif // __cpp_lib_latch
   110	#endif // _GLIBCXX_LATCH