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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
//
// 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.
//
// -----------------------------------------------------------------------------
// barrier.h
// -----------------------------------------------------------------------------
#ifndef ABSL_SYNCHRONIZATION_BARRIER_H_
#define ABSL_SYNCHRONIZATION_BARRIER_H_
#include "absl/base/thread_annotations.h"
#include "absl/synchronization/mutex.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
// Barrier
//
// This class creates a barrier which blocks threads until a prespecified
// threshold of threads (`num_threads`) utilizes the barrier. A thread utilizes
// the `Barrier` by calling `Block()` on the barrier, which will block that
// thread; no call to `Block()` will return until `num_threads` threads have
// called it.
//
// Exactly one call to `Block()` will return `true`, which is then responsible
// for destroying the barrier; because stack allocation will cause the barrier
// to be deleted when it is out of scope, barriers should not be stack
// allocated.
//
// Example:
//
// // Main thread creates a `Barrier`:
// barrier = new Barrier(num_threads);
//
// // Each participating thread could then call:
// if (barrier->Block()) delete barrier; // Exactly one call to `Block()`
// // returns `true`; that call
// // deletes the barrier.
class Barrier {
public:
// `num_threads` is the number of threads that will participate in the barrier
explicit Barrier(int num_threads)
: num_to_block_(num_threads), num_to_exit_(num_threads) {}
Barrier(const Barrier&) = delete;
Barrier& operator=(const Barrier&) = delete;
// Barrier::Block()
//
// Blocks the current thread, and returns only when the `num_threads`
// threshold of threads utilizing this barrier has been reached. `Block()`
// returns `true` for precisely one caller, which may then destroy the
// barrier.
//
// Memory ordering: For any threads X and Y, any action taken by X
// before X calls `Block()` will be visible to Y after Y returns from
// `Block()`.
bool Block();
private:
Mutex lock_;
int num_to_block_ ABSL_GUARDED_BY(lock_);
int num_to_exit_ ABSL_GUARDED_BY(lock_);
};
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_SYNCHRONIZATION_BARRIER_H_
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