C++并发之条件变量(std::condition_variable)

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1 概述

  条件变量是一个能够阻塞调用线程直到被通知恢复的对象。
  当调用其中一个等待函数时，它使用unique_lock(通过互斥锁)来锁定线程。线程保持阻塞状态，直到被另一个调用同一condition_variable对象上的通知函数的线程唤醒。
  条件变量类型的对象总是使用unique_lock来等待.
其类图如下:

2 使用实例

struct Function4NotiryAll
{bool is_ready = false;std::mutex mutex;std::condition_variable cv;int counter = 0;void print_id(int id){std::unique_lock<std::mutex> lock(mutex);while(!is_ready)cv.wait(lock);std::cerr << "id:" << id << std::endl;counter++;}void go(){std::unique_lock<std::mutex> lock(mutex);is_ready = true;cv.notify_all();}
};void ConditionVariableSuite::notiry_all()
{std::thread threads[10];Function4NotiryAll function;for(int i = 0; i < 10; ++i)threads[i] = std::thread(&Function4NotiryAll::print_id, std::ref(function), i);function.go();for(auto & thread : threads)thread.join();TEST_ASSERT_EQUALS(true, function.counter == 10)
}

3 接口使用

3.1 wait

struct Function4Wait
{volatile int cargo = 0;int counter = 0;std::mutex mutex;std::condition_variable cv;inline bool have_cargo() { return cargo != 0; }inline void consume_cargo() { cargo = 0; }void consume(int n){for(int i = 0; i < n; i++){std::unique_lock<std::mutex> lock(mutex);while(!have_cargo())cv.wait(lock);std::cerr << "cargo: " << cargo << std::endl;counter++;consume_cargo();}}void consume_with_predicate(int n){for(int i = 0; i < n; i++){std::unique_lock<std::mutex> lock(mutex);cv.wait(lock, std::bind(&Function4Wait::have_cargo, this));std::cerr << "cargo: " << cargo << std::endl;counter++;consume_cargo();}}inline void product(int n){std::unique_lock<std::mutex> lock(mutex);cargo = n;cv.notify_one();}
};void ConditionVariableSuite::wait()
{Function4Wait function;  std::thread thread[2];int n = 10;thread[0] = std::thread(&Function4Wait::consume, std::ref(function), 10);for(int i = 0; i < n; i++){while(function.have_cargo())std::this_thread::yield();function.product(i + 1);}thread[0].join();TEST_ASSERT_EQUALS(true, function.counter == 10)function.counter = 0;thread[1] = std::thread(&Function4Wait::consume_with_predicate, std::ref(function), 10);for(int i = 0; i < n; i++){while(function.have_cargo())std::this_thread::yield();function.product(i + 1);}thread[1].join();TEST_ASSERT_EQUALS(true, function.counter == 10)
}

3.2 wait_for

struct Function4WaitFor
{volatile int cargo = 0;int counter = 0;std::mutex mutex;std::condition_variable cv;inline bool have_cargo() { return cargo != 0; }inline void consume_cargo() { cargo = 0; }void consume(int n){for(int i = 0; i < n; i++){std::unique_lock<std::mutex> lock(mutex);while(!have_cargo() && cv.wait_for(lock, std::chrono::seconds(1)) == std::cv_status::timeout);std::cerr << "cargo: " << cargo << std::endl;counter++;consume_cargo();}}void consume_with_predicate(int n){for(int i = 0; i < n; i++){std::unique_lock<std::mutex> lock(mutex);while(!cv.wait_for(lock, std::chrono::seconds(1), std::bind(&Function4WaitFor::have_cargo, this)));std::cerr << "cargo: " << cargo << std::endl;counter++;consume_cargo();}}inline void product(int n){std::unique_lock<std::mutex> lock(mutex);cargo = n;cv.notify_one();}
};void ConditionVariableSuite::wait_for()
{Function4WaitFor function;  std::thread thread[2];int n = 10;thread[0] = std::thread(&Function4WaitFor::consume, std::ref(function), 10);for(int i = 0; i < n; i++){while(function.have_cargo())std::this_thread::yield();function.product(i + 1);}thread[0].join();TEST_ASSERT_EQUALS(true, function.counter == 10)function.counter = 0;thread[1] = std::thread(&Function4WaitFor::consume_with_predicate, std::ref(function), 10);for(int i = 0; i < n; i++){while(function.have_cargo())std::this_thread::yield();function.product(i + 1);}thread[1].join();TEST_ASSERT_EQUALS(true, function.counter == 10)
}

3.3 wait_until

struct Function4WaitUntil
{volatile int cargo = 0;int counter = 0;std::mutex mutex;std::condition_variable cv;inline bool have_cargo() { return cargo != 0; }inline void consume_cargo() { cargo = 0; }void consume(int n){for(int i = 0; i < n; i++){std::unique_lock<std::mutex> lock(mutex);std::chrono::time_point<std::chrono::system_clock> timePoint = std::chrono::system_clock::now() + std::chrono::seconds(1);while(!have_cargo() && cv.wait_until(lock, timePoint) == std::cv_status::timeout);std::cerr << "cargo: " << cargo << std::endl;counter++;consume_cargo();}}void consume_with_predicate(int n){for(int i = 0; i < n; i++){std::unique_lock<std::mutex> lock(mutex);std::chrono::time_point<std::chrono::system_clock> timePoint = std::chrono::system_clock::now() + std::chrono::seconds(1);while(!cv.wait_until(lock, timePoint, std::bind(&Function4WaitUntil::have_cargo, this)));std::cerr << "cargo: " << cargo << std::endl;counter++;consume_cargo();}}inline void product(int n){std::unique_lock<std::mutex> lock(mutex);cargo = n;cv.notify_one();}
};void ConditionVariableSuite::wait_until()
{Function4WaitUntil function;  std::thread thread[2];int n = 10;thread[0] = std::thread(&Function4WaitUntil::consume, std::ref(function), 10);for(int i = 0; i < n; i++){while(function.have_cargo())std::this_thread::yield();function.product(i + 1);}thread[0].join();TEST_ASSERT_EQUALS(true, function.counter == 10)function.counter = 0;thread[1] = std::thread(&Function4WaitUntil::consume_with_predicate, std::ref(function), 10);for(int i = 0; i < n; i++){while(function.have_cargo())std::this_thread::yield();function.product(i + 1);}thread[1].join();TEST_ASSERT_EQUALS(true, function.counter == 10)
}

3.4 notify_one

struct Function4NotityOne
{int cargo = 0;int counter = 0;std::mutex mutex;std::condition_variable produce;std::condition_variable consume;void consumer(){std::unique_lock<std::mutex> lock(mutex);while(cargo == 0)consume.wait(lock);std::cerr << "cargo: " << cargo << std::endl;cargo = 0;counter++;produce.notify_one();}void producer(int id){std::unique_lock<std::mutex> lock(mutex);while(cargo != 0)produce.wait(lock);cargo = id;consume.notify_one();}
};
void ConditionVariableSuite::notify_one()
{std::thread consumers[10];std::thread producers[10];Function4NotityOne function;for(int i = 0; i < 10; ++i){consumers[i] = std::thread(&Function4NotityOne::consumer, std::ref(function));producers[i] = std::thread(&Function4NotityOne::producer, std::ref(function), i + 1);}for(int i = 0; i < 10; ++i){consumers[i].join();producers[i].join();}TEST_ASSERT_EQUALS(true, function.counter == 10)
}

3.5 notiry_all

struct Function4NotiryAll
{bool is_ready = false;std::mutex mutex;std::condition_variable cv;int counter = 0;void print_id(int id){std::unique_lock<std::mutex> lock(mutex);while(!is_ready)cv.wait(lock);std::cerr << "id:" << id << std::endl;counter++;}void go(){std::unique_lock<std::mutex> lock(mutex);is_ready = true;cv.notify_all();}void allgo(){std::unique_lock<std::mutex> lock(mutex);is_ready = true;std::notify_all_at_thread_exit(cv, std::move(lock));}
};void ConditionVariableSuite::notiry_all()
{std::thread threads[10];Function4NotiryAll function;for(int i = 0; i < 10; ++i)threads[i] = std::thread(&Function4NotiryAll::print_id, std::ref(function), i);function.go();for(auto & thread : threads)thread.join();TEST_ASSERT_EQUALS(true, function.counter == 10)
}

3.5 notify_all_at_thread_exit

void ConditionVariableSuite::notify_all_at_thread_exit()
{std::thread threads[10];Function4NotiryAll function;for(int i = 0; i < 10; ++i)threads[i] = std::thread(&Function4NotiryAll::print_id, std::ref(function), i);std::thread(&Function4NotiryAll::allgo, std::ref(function)).detach();for(auto & thread : threads)thread.join();TEST_ASSERT_EQUALS(true, function.counter == 10)   
}

说明:

• notify_all_at_thread_exit 只能在线程中调用，在进程中调用将不起作用。

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