xref: /llvm-project/libcxx/include/semaphore (revision b9a2658a3e8bd13b0f9e7a8a440832a95b377216)

// -*- C++ -*-
//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP_SEMAPHORE
#define _LIBCPP_SEMAPHORE

/*
    semaphore synopsis

namespace std {

template<ptrdiff_t least_max_value = implementation-defined>
class counting_semaphore                          // since C++20
{
public:
static constexpr ptrdiff_t max() noexcept;

constexpr explicit counting_semaphore(ptrdiff_t desired);
~counting_semaphore();

counting_semaphore(const counting_semaphore&) = delete;
counting_semaphore& operator=(const counting_semaphore&) = delete;

void release(ptrdiff_t update = 1);
void acquire();
bool try_acquire() noexcept;
template<class Rep, class Period>
    bool try_acquire_for(const chrono::duration<Rep, Period>& rel_time);
template<class Clock, class Duration>
    bool try_acquire_until(const chrono::time_point<Clock, Duration>& abs_time);

private:
ptrdiff_t counter; // exposition only
};

using binary_semaphore = counting_semaphore<1>; // since C++20

}

*/

#if __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)
#  include <__cxx03/semaphore>
#else
#  include <__config>

#  if _LIBCPP_HAS_THREADS

#    include <__assert>
#    include <__atomic/atomic.h>
#    include <__atomic/atomic_sync.h>
#    include <__atomic/memory_order.h>
#    include <__chrono/time_point.h>
#    include <__cstddef/ptrdiff_t.h>
#    include <__thread/poll_with_backoff.h>
#    include <__thread/support.h>
#    include <__thread/timed_backoff_policy.h>
#    include <limits>
#    include <version>

#    if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#      pragma GCC system_header
#    endif

_LIBCPP_PUSH_MACROS
#    include <__undef_macros>

#    if _LIBCPP_STD_VER >= 20

_LIBCPP_BEGIN_NAMESPACE_STD

/*

__atomic_semaphore_base is the general-case implementation.
It is a typical Dijkstra semaphore algorithm over atomics, wait and notify
functions. It avoids contention against users' own use of those facilities.

*/

#      define _LIBCPP_SEMAPHORE_MAX (numeric_limits<ptrdiff_t>::max())

class __atomic_semaphore_base {
  atomic<ptrdiff_t> __a_;

public:
  _LIBCPP_HIDE_FROM_ABI constexpr explicit __atomic_semaphore_base(ptrdiff_t __count) : __a_(__count) {}
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
    auto __old = __a_.fetch_add(__update, memory_order_release);
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
        __update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
    if (__old == 0) {
      __a_.notify_all();
    }
  }
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() {
    std::__atomic_wait_unless(__a_, memory_order_relaxed, [this](ptrdiff_t& __old) {
      return __try_acquire_impl(__old);
    });
  }
  template <class _Rep, class _Period>
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
    if (__rel_time == chrono::duration<_Rep, _Period>::zero())
      return try_acquire();
    auto const __poll_fn = [this]() { return try_acquire(); };
    return std::__libcpp_thread_poll_with_backoff(__poll_fn, __libcpp_timed_backoff_policy(), __rel_time);
  }
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() {
    auto __old = __a_.load(memory_order_relaxed);
    return __try_acquire_impl(__old);
  }

private:
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool __try_acquire_impl(ptrdiff_t& __old) {
    while (true) {
      if (__old == 0)
        return false;
      if (__a_.compare_exchange_weak(__old, __old - 1, memory_order_acquire, memory_order_relaxed))
        return true;
    }
  }
};

template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
class counting_semaphore {
  __atomic_semaphore_base __semaphore_;

public:
  static_assert(__least_max_value >= 0, "The least maximum value must be a positive number");

  static constexpr ptrdiff_t max() noexcept { return __least_max_value; }

  _LIBCPP_HIDE_FROM_ABI constexpr explicit counting_semaphore(ptrdiff_t __count) : __semaphore_(__count) {
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
        __count >= 0,
        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
        "initialized with a negative value");
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
        __count <= max(),
        "counting_semaphore::counting_semaphore(ptrdiff_t): counting_semaphore cannot be "
        "initialized with a value greater than max()");
  }
  ~counting_semaphore() = default;

  counting_semaphore(const counting_semaphore&)            = delete;
  counting_semaphore& operator=(const counting_semaphore&) = delete;

  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void release(ptrdiff_t __update = 1) {
    _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "counting_semaphore:release called with a negative value");
    __semaphore_.release(__update);
  }
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void acquire() { __semaphore_.acquire(); }
  template <class _Rep, class _Period>
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
  try_acquire_for(chrono::duration<_Rep, _Period> const& __rel_time) {
    return __semaphore_.try_acquire_for(chrono::duration_cast<chrono::nanoseconds>(__rel_time));
  }
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool try_acquire() { return __semaphore_.try_acquire(); }
  template <class _Clock, class _Duration>
  _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI bool
  try_acquire_until(chrono::time_point<_Clock, _Duration> const& __abs_time) {
    auto const __current = _Clock::now();
    if (__current >= __abs_time)
      return try_acquire();
    else
      return try_acquire_for(__abs_time - __current);
  }
};

using binary_semaphore = counting_semaphore<1>;

_LIBCPP_END_NAMESPACE_STD

#    endif // _LIBCPP_STD_VER >= 20

_LIBCPP_POP_MACROS

#  endif // _LIBCPP_HAS_THREADS

#  if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
#    include <atomic>
#    include <cstddef>
#  endif
#endif // __cplusplus < 201103L && defined(_LIBCPP_USE_FROZEN_CXX03_HEADERS)

#endif // _LIBCPP_SEMAPHORE