libcxx/include/semaphore

   1 // -*- C++ -*-
   2 //===----------------------------------------------------------------------===//
   3 //
   4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   5 // See https://llvm.org/LICENSE.txt for license information.
   6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #ifndef _LIBCPP_SEMAPHORE
  11 #define _LIBCPP_SEMAPHORE
  12
  13 /*
  14     semaphore synopsis
  15
  16 namespace std {
  17
  18 template<ptrdiff_t least_max_value = implementation-defined>
  19 class counting_semaphore
  20 {
  21 public:
  22 static constexpr ptrdiff_t max() noexcept;
  23
  24 constexpr explicit counting_semaphore(ptrdiff_t desired);
  25 ~counting_semaphore();
  26
  27 counting_semaphore(const counting_semaphore&) = delete;
  28 counting_semaphore& operator=(const counting_semaphore&) = delete;
  29
  30 void release(ptrdiff_t update = 1);
  31 void acquire();
  32 bool try_acquire() noexcept;
  33 template<class Rep, class Period>
  34     bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
  35 template<class Clock, class Duration>
  36     bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);
  37
  38 private:
  39 ptrdiff_t counter; // exposition only
  40 };
  41
  42 using binary_semaphore = counting_semaphore<1>;
  43
  44 }
  45
  46 */
  47
  48 #include <__availability>
  49 #include <__config>
  50 #include <__thread/timed_backoff_policy.h>
  51 #include <__threading_support>
  52 #include <atomic>
  53 #include <version>
  54
  55 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  56 #pragma GCC system_header
  57 #endif
  58
  59 #ifdef _LIBCPP_HAS_NO_THREADS
  60 # error <semaphore> is not supported on this single threaded system
  61 #endif
  62
  63 _LIBCPP_PUSH_MACROS
  64 #include <__undef_macros>
  65
  66 #if _LIBCPP_STD_VER >= 14
  67
  68 _LIBCPP_BEGIN_NAMESPACE_STD
  69
  70 /*
  71
  72 __atomic_semaphore_base is the general-case implementation.
  73 It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
  74 functions. It avoids contention against users' own use of those facilities.
  75
  76 */
  77
  78 class __atomic_semaphore_base
  79 {
  80     __atomic_base<ptrdiff_t> __a;
  81
  82 public:
  83     _LIBCPP_INLINE_VISIBILITY
  84     constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a(__count)
  85     {
  86     }
  87     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
  88     void release(ptrdiff_t __update = 1)
  89     {
  90         if(0 < __a.fetch_add(__update, memory_order_release))
  91             ;
  92         else if(__update > 1)
  93             __a.notify_all();
  94         else
  95             __a.notify_one();
  96     }
  97     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
  98     void acquire()
  99     {
 100         auto const __test_fn = [this]() -> bool {
 101             auto __old = __a.load(memory_order_relaxed);
 102             return (__old != 0) && __a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed);
 103         };
 104         __cxx_atomic_wait(&__a.__a_, __test_fn);
 105     }
 106     template <class Rep, class Period>
 107     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 108     bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
 109     {
 110         if (__rel_time == chrono::duration<Rep, Period>::zero())
 111             return try_acquire();
 112         auto const __test_fn = [this]() { return try_acquire(); };
 113         return __libcpp_thread_poll_with_backoff(__test_fn, __libcpp_timed_backoff_policy(), __rel_time);
 114     }
 115     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 116     bool try_acquire()
 117     {
 118         auto __old = __a.load(memory_order_acquire);
 119         while (true) {
 120             if (__old == 0)
 121                 return false;
 122             if (__a.compare_exchange_strong(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
 123                 return true;
 124         }
 125     }
 126 };
 127
 128 #define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())
 129
 130 template<ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
 131 class counting_semaphore
 132 {
 133     __atomic_semaphore_base __semaphore;
 134
 135 public:
 136     static constexpr ptrdiff_t max() noexcept {
 137         return __least_max_value;
 138     }
 139
 140     _LIBCPP_INLINE_VISIBILITY
 141     constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore(__count) { }
 142     ~counting_semaphore() = default;
 143
 144     counting_semaphore(const counting_semaphore&) = delete;
 145     counting_semaphore& operator=(const counting_semaphore&) = delete;
 146
 147     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 148     void release(ptrdiff_t __update = 1)
 149     {
 150         __semaphore.release(__update);
 151     }
 152     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 153     void acquire()
 154     {
 155         __semaphore.acquire();
 156     }
 157     template<class Rep, class Period>
 158     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 159     bool try_acquire_for(chrono::duration<Rep, Period> const& __rel_time)
 160     {
 161         return __semaphore.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
 162     }
 163     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 164     bool try_acquire()
 165     {
 166         return __semaphore.try_acquire();
 167     }
 168     template <class Clock, class Duration>
 169     _LIBCPP_AVAILABILITY_SYNC _LIBCPP_INLINE_VISIBILITY
 170     bool try_acquire_until(chrono::time_point<Clock, Duration> const& __abs_time)
 171     {
 172         auto const current = Clock::now();
 173         if (current >= __abs_time)
 174             return try_acquire();
 175         else
 176             return try_acquire_for(__abs_time - current);
 177     }
 178 };
 179
 180 using binary_semaphore = counting_semaphore<1>;
 181
 182 _LIBCPP_END_NAMESPACE_STD
 183
 184 #endif // _LIBCPP_STD_VER >= 14
 185
 186 _LIBCPP_POP_MACROS
 187
 188 #endif //_LIBCPP_SEMAPHORE