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// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// -----------------------------------------------------------------------------
// notification.h
// -----------------------------------------------------------------------------
// This header file defines a `Notification` abstraction, which allows threads
// to receive notification of a single occurrence of a single event.
// The `Notification` object maintains a private boolean "notified" state that
// transitions to `true` at most once. The `Notification` class provides the
// following primary member functions:
// * `HasBeenNotified()` to query its state
// * `WaitForNotification*()` to have threads wait until the "notified" state
// is `true`.
// * `Notify()` to set the notification's "notified" state to `true` and
// notify all waiting threads that the event has occurred.
// This method may only be called once.
// Note that while `Notify()` may only be called once, it is perfectly valid to
// call any of the `WaitForNotification*()` methods multiple times, from
// multiple threads -- even after the notification's "notified" state has been
// set -- in which case those methods will immediately return.
// Note that the lifetime of a `Notification` requires careful consideration;
// it might not be safe to destroy a notification after calling `Notify()` since
// it is still legal for other threads to call `WaitForNotification*()` methods
// on the notification. However, observers responding to a "notified" state of
// `true` can safely delete the notification without interfering with the call
// to `Notify()` in the other thread.
// Memory ordering: For any threads X and Y, if X calls `Notify()`, then any
// action taken by X before it calls `Notify()` is visible to thread Y after:
// * Y returns from `WaitForNotification()`, or
// * Y receives a `true` return value from either `HasBeenNotified()` or
// `WaitForNotificationWithTimeout()`.
#include <atomic>
#include "absl/base/attributes.h"
#include "absl/synchronization/mutex.h"
#include "absl/time/time.h"
namespace absl {
// -----------------------------------------------------------------------------
// Notification
// -----------------------------------------------------------------------------
class Notification {
// Initializes the "notified" state to unnotified.
Notification() : notified_yet_(false) {}
explicit Notification(bool prenotify) : notified_yet_(prenotify) {}
Notification(const Notification&) = delete;
Notification& operator=(const Notification&) = delete;
// Notification::HasBeenNotified()
// Returns the value of the notification's internal "notified" state.
ABSL_MUST_USE_RESULT bool HasBeenNotified() const {
return HasBeenNotifiedInternal(&this->notified_yet_);
// Notification::WaitForNotification()
// Blocks the calling thread until the notification's "notified" state is
// `true`. Note that if `Notify()` has been previously called on this
// notification, this function will immediately return.
void WaitForNotification() const;
// Notification::WaitForNotificationWithTimeout()
// Blocks until either the notification's "notified" state is `true` (which
// may occur immediately) or the timeout has elapsed, returning the value of
// its "notified" state in either case.
bool WaitForNotificationWithTimeout(absl::Duration timeout) const;
// Notification::WaitForNotificationWithDeadline()
// Blocks until either the notification's "notified" state is `true` (which
// may occur immediately) or the deadline has expired, returning the value of
// its "notified" state in either case.
bool WaitForNotificationWithDeadline(absl::Time deadline) const;
// Notification::Notify()
// Sets the "notified" state of this notification to `true` and wakes waiting
// threads. Note: do not call `Notify()` multiple times on the same
// `Notification`; calling `Notify()` more than once on the same notification
// results in undefined behavior.
void Notify();
static inline bool HasBeenNotifiedInternal(
const std::atomic<bool>* notified_yet) {
return notified_yet->load(std::memory_order_acquire);
mutable Mutex mutex_;
std::atomic<bool> notified_yet_; // written under mutex_
} // namespace absl