C++ 多线程之互斥量(mutex)详解

mutex();
mutex(const mutex&) = delete;

~mutex();

void lock();

bool try_lock();

void unlock();

#include <iostream>
#include <chrono>  // std::chrono
#include <thread>  // std::thread
#include <mutex>  // std::mutex
int g_num = 0;  // 为 g_num_mutex 所保护
std::mutex g_num_mutex;
void slow_increment(int id) 
{
    for (int i = 0; i < 3; ++i) {
        g_num_mutex.lock();
        ++g_num;
        std::cout << "th" << id << " => " << g_num << '\n';
        g_num_mutex.unlock();
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }
}  
int main()
{
    std::thread t1(slow_increment, 0);
    std::thread t2(slow_increment, 1);
    t1.join();
    t2.join();
}

#include <iostream>
#include <thread>
#include <mutex>
class X {
    std::recursive_mutex m;
    std::string shared;
  public:
    void fun1() {
      m.lock();
      shared = "fun1";
      std::cout << "in fun1, shared variable is now " << shared << '\n';
      m.unlock();
    }
    void fun2() {
      m.lock();
      shared = "fun2";
      std::cout << "in fun2, shared variable is now " << shared << '\n';
      fun3(); // 递归锁在此处变得有用
      std::cout << "back in fun2, shared variable is " << shared << '\n';
      m.unlock();
    }
    void fun3() {
      m.lock();
      shared = "fun3";
      std::cout << "in fun3, shared variable is now " << shared << '\n';
      m.unlock();
    }
};
int main() 
{
    X x;
    std::thread t1(&X::fun1, &x);
    std::thread t2(&X::fun2, &x);
    t1.join();
    t2.join();
}

template< class Rep, class Period >
bool try_lock_for( const std::chrono::duration<Rep,Period>& timeout_duration );

#include <iostream>
#include <sstream>
#include <thread>
#include <chrono>
#include <vector>
#include <mutex>
std::timed_mutex mutex;
using namespace std::chrono_literals;
void do_work(int id) {
  std::ostringstream stream;
  for (int i = 0; i < 3; ++i) {
    if (mutex.try_lock_for(100ms)) {
      stream << "success ";
      std::this_thread::sleep_for(100ms);
      mutex.unlock();
    } else {
      stream << "failed ";
    }
    std::this_thread::sleep_for(100ms);
  }
  std::cout << "[" << id << "] " << stream.str() << std::endl;
}
int main() {
  // try_lock_for
  std::vector<std::thread> threads;
  for (int i = 0; i < 4; ++i) {
    threads.emplace_back(do_work, i);
  }
  for (auto& t : threads) {
    t.join();
  }
}

template< class Clock, class Duration >
bool try_lock_until( const std::chrono::time_point<Clock,Duration>& timeout_time);

#include <iostream>
#include <sstream>
#include <thread>
#include <chrono>
#include <vector>
#include <mutex>
std::timed_mutex mutex;
using namespace std::chrono;
void do_work() {
    mutex.lock();
    std::cout << "thread 1, sleeping..." << std::endl;
    std::this_thread::sleep_for(std::chrono::seconds(4));
    mutex.unlock();
}
void do_work2() {
    auto now = std::chrono::steady_clock::now();
    if (mutex.try_lock_until(now + 5s)) {
        auto end = steady_clock::now();
        std::cout << "try_lock_until success, ";
        std::cout << "time use: " << duration_cast<milliseconds>(end-now).count() 
            << "ms." << std::endl;
        mutex.unlock();
    } else {
        auto end = steady_clock::now();
        std::cout << "try_lock_until failed, ";
        std::cout << "time use: " << duration_cast<milliseconds>(end-now).count() 
            << "ms." << std::endl;
    }
}
int main() {
  // try_lock_until
  std::thread t1(do_work);
  std::thread t2(do_work2);
  t1.join();
  t2.join();
}

void lock_shared();
bool try_lock_shared();
void unlock_shared();

#include <iostream>
#include <mutex>  // 对于 std::unique_lock
#include <shared_mutex>
#include <thread>
class ThreadSafeCounter {
 public:
  ThreadSafeCounter() = default;
  // 多个线程/读者能同时读计数器的值。
  unsigned int get() const {
    std::shared_lock<std::shared_mutex> lock(mutex_);
    return value_;
  }
  // 只有一个线程/写者能增加/写线程的值。
  void increment() {
    std::unique_lock<std::shared_mutex> lock(mutex_);
    value_++;
  }
  // 只有一个线程/写者能重置/写线程的值。
  void reset() {
    std::unique_lock<std::shared_mutex> lock(mutex_);
    value_ = 0;
  }
 private:
  mutable std::shared_mutex mutex_;
  unsigned int value_ = 0;
};
int main() {
  ThreadSafeCounter counter;
  auto increment_and_print = [&counter]() {
    for (int i = 0; i < 3; i++) {
      counter.increment();
      std::cout << std::this_thread::get_id() << ' ' << counter.get() << '\n';
      // 注意：写入 std::cout 实际上也要由另一互斥同步。省略它以保持示例简洁。
    }
  };
  std::thread thread1(increment_and_print);
  std::thread thread2(increment_and_print);
  thread1.join();
  thread2.join();
}
// 解释：下列输出在单核机器上生成。 thread1 开始时，它首次进入循环并调用 increment() ，
// 随后调用 get() 。然而，在它能打印返回值到 std::cout 前，调度器将 thread1 置于休眠
// 并唤醒 thread2 ，它显然有足够时间一次运行全部三个循环迭代。再回到 thread1 ，它仍在首个
// 循环迭代中，它最终打印其局部的计数器副本的值，即 1 到 std::cout ，再运行剩下二个循环。
// 多核机器上，没有线程被置于休眠，且输出更可能为递增顺序。

template< class Rep, class Period >
bool try_lock_shared_for( const std::chrono::duration<Rep,Period>& timeout_duration );
template< class Clock, class Duration >
bool try_lock_shared_until( const std::chrono::time_point<Clock,Duration>& timeout_time );