text: // Copyright 2017 The Abseil Authors.
  //
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  //
  //      https://www.apache.org/licenses/LICENSE-2.0
  //
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  //
  
  #ifndef ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
  #define ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
  
  #include "absl/base/config.h"
  
  #ifdef _WIN32
  #include <windows.h>
  #else
  #include <pthread.h>
  #endif
  
  #ifdef ABSL_HAVE_SEMAPHORE_H
  #include <semaphore.h>
  #endif
  
  #include <atomic>
  #include <cstdint>
  
  #include "absl/base/internal/thread_identity.h"
  #include "absl/synchronization/internal/kernel_timeout.h"
  
  // May be chosen at compile time via -DABSL_FORCE_WAITER_MODE=<index>
  #define ABSL_WAITER_MODE_FUTEX 0
  #define ABSL_WAITER_MODE_SEM 1
  #define ABSL_WAITER_MODE_CONDVAR 2
  #define ABSL_WAITER_MODE_WIN32 3
  
  #if defined(ABSL_FORCE_WAITER_MODE)
  #define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
  #elif defined(_WIN32)
  #define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
  #elif defined(__linux__)
  #define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
  #elif defined(ABSL_HAVE_SEMAPHORE_H)
  #define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
  #else
  #define ABSL_WAITER_MODE ABSL_WAITER_MODE_CONDVAR
  #endif
  
  namespace absl {
  namespace synchronization_internal {
  
  // Waiter is an OS-specific semaphore.
  class Waiter {
   public:
    // No constructor, instances use the reserved space in ThreadIdentity.
    // All initialization logic belongs in `Init()`.
    Waiter() = delete;
    Waiter(const Waiter&) = delete;
    Waiter& operator=(const Waiter&) = delete;
  
    // Prepare any data to track waits.
    void Init();
  
    // Blocks the calling thread until a matching call to `Post()` or
    // `t` has passed. Returns `true` if woken (`Post()` called),
    // `false` on timeout.
    bool Wait(KernelTimeout t);
  
    // Restart the caller of `Wait()` as with a normal semaphore.
    void Post();
  
    // If anyone is waiting, wake them up temporarily and cause them to
    // call `MaybeBecomeIdle()`. They will then return to waiting for a
    // `Post()` or timeout.
    void Poke();
  
    // Returns the Waiter associated with the identity.
    static Waiter* GetWaiter(base_internal::ThreadIdentity* identity) {
      static_assert(
          sizeof(Waiter) <= sizeof(base_internal::ThreadIdentity::WaiterState),
          "Insufficient space for Waiter");
      return reinterpret_cast<Waiter*>(identity->waiter_state.data);
    }
  
    // How many periods to remain idle before releasing resources
  #ifndef THREAD_SANITIZER
    static const int kIdlePeriods = 60;
  #else
    // Memory consumption under ThreadSanitizer is a serious concern,
    // so we release resources sooner. The value of 1 leads to 1 to 2 second
    // delay before marking a thread as idle.
    static const int kIdlePeriods = 1;
  #endif
  
   private:
  #if ABSL_WAITER_MODE == ABSL_WAITER_MODE_FUTEX
    // Futexes are defined by specification to be 32-bits.
    // Thus std::atomic<int32_t> must be just an int32_t with lockfree methods.
    std::atomic<int32_t> futex_;
    static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
  
  #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
    pthread_mutex_t mu_;
    pthread_cond_t cv_;
    std::atomic<int> waiter_count_;
    std::atomic<int> wakeup_count_;  // Unclaimed wakeups, written under lock.
  
  #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
    sem_t sem_;
    // This seems superfluous, but for Poke() we need to cause spurious
    // wakeups on the semaphore. Hence we can't actually use the
    // semaphore's count.
    std::atomic<int> wakeups_;
  
  #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
    // The Windows API has lots of choices for synchronization
    // primivitives.  We are using SRWLOCK and CONDITION_VARIABLE
    // because they don't require a destructor to release system
    // resources.
    SRWLOCK mu_;
    CONDITION_VARIABLE cv_;
    std::atomic<int> waiter_count_;
    std::atomic<int> wakeup_count_;
  
  #else
    #error Unknown ABSL_WAITER_MODE
  #endif
  };
  
  }  // namespace synchronization_internal
  }  // namespace absl
  
  #endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
  