// 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_