THUAI6/CAPI/cpp/grpc/include/absl/synchronization/barrier.h

// 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_