C++ mutex

mutex

• <mutex> 라이브러리

• Mutual Exclusion의 약자이다.

---

예제

• mutex를 적용하지 않은 예제

  • 출력이 섞여서 나오는 것을 볼 수 있다.

  #include <iostream>
  #include <thread>
  #include <chrono>
  
  int         main()
  {
      using namespace std;
  
      const int num_logical_processors = std::thread::hardware_concurrency();
  
      cout << "Number of processors : " << num_logical_processors << endl;
      cout << "ID of this thread : " << std::this_thread::get_id() << endl;
  
      auto work_func = [](const string& name)
      {
          for (int i = 0; i < 5; ++i)
          {
              std::this_thread::sleep_for(std::chrono::milliseconds(100));
              cout << name << ' ' << std::this_thread::get_id() << " is working " << i << endl;
          }
      };
  
      std::thread t1 = std::thread(work_func, "Jack Jack");
      std::thread t2 = std::thread(work_func, "Dash");
  
      t1.join();
      t2.join();
  }
  
  /* stdout stderr
  Number of processors : 12
  ID of this thread : 16040
  Jack Jack 5336 is working 0
  Dash 8368 is working 0
  Jack Jack 5336 is working 1Dash 8368 is working 1
  
  Dash 8368 is working 2
  Jack Jack 5336 is working 2
  Dash 8368 is working 3
  Jack Jack 5336 is working 3
  Jack Jack 5336 is working 4
  Dash 8368 is working 4
  */

  ---

• mutex를 사용한 예제

  #include <iostream>
  #include <thread>
  #include <chrono>
  #include <mutex>
  
  int         main()
  {
      using namespace std;
  
      mutex mtx;
  
      const int num_logical_processors = std::thread::hardware_concurrency();
  
      cout << "Number of processors : " << num_logical_processors << endl;
      cout << "ID of this thread : " << std::this_thread::get_id() << endl;
  
      auto work_func = [&](const string& name)
      {
          for (int i = 0; i < 5; ++i)
          {
              std::this_thread::sleep_for(std::chrono::milliseconds(100));
  
              mtx.lock();
              cout << name << ' ' << std::this_thread::get_id() << " is working " << i << endl;
              mtx.unlock();
          }
      };
  
      std::thread t1 = std::thread(work_func, "Jack Jack");
      std::thread t2 = std::thread(work_func, "Dash");
  
      t1.join();
      t2.join();
  }
  
  /* stdout stderr
  Number of processors : 12
  ID of this thread : 1588
  Jack Jack 17596 is working 0
  Dash 14068 is working 0
  Jack Jack 17596 is working 1
  Dash 14068 is working 1
  Dash 14068 is working 2
  Jack Jack 17596 is working 2
  Jack Jack 17596 is working 3
  Dash 14068 is working 3
  Dash 14068 is working 4
  Jack Jack 17596 is working 4
  */

  • 만약 mutex.unlock()을 하지 않으면 다른 쓰레드에서 해당 자원을 사용할 수 없다.

---

C++11

lock_guard

• mutex.lock(), mutex.unlock()을 사용하지 않고, 정의하면 락이 걸리고 영역을 벗어나면 언락을 해주는 클래스이다.

• C++17부터는 템플릿타입 자리에 아무것도 안넣어도 된다. (lock_guard lock(mtx) 이런 식으로)

  #include <iostream>
  #include <thread>
  #include <chrono>
  #include <mutex>
  
  int         main()
  {
      using namespace std;
  
      int shared_memory(0);
      mutex mtx;
  
      auto count_func = [&]() {
          for (int i = 0; i < 1000; ++i)
          {
              this_thread::sleep_for(chrono::milliseconds(1));
  
              lock_guard<mutex> lock(mtx);
  
              shared_memory++;
          }
      };
  
      thread t1 = thread(count_func);
      thread t2 = thread(count_func);
  
      t1.join();
      t2.join();
  
      cout << "Value is " << shared_memory << endl;
  }
  
  /* stdout stderr
  Value is 2000
  */

---

C++17

scoped_lock

• 뮤텍스 락을 자주 걸면 오버헤드가 커지므로 성능 저하가 발생한다.

• lock_guard에서 여러 mutex를 한번에 lock할 수 있는 기능을 추가한 클래스라고 한다. 즉 상위 호환이다.

  • https://stackoverflow.com/questions/43019598/stdlock-guard-or-stdscoped-lock

  • https://umbum.dev/766

  #include <iostream>
  #include <thread>
  #include <chrono>
  #include <mutex>
  
  int         main()
  {
      using namespace std;
  
      int shared_memory(0);
      mutex mtx;
  
      auto count_func = [&]() {
          for (int i = 0; i < 1000; ++i)
          {
              this_thread::sleep_for(chrono::milliseconds(1));
  
              scoped_lock<mutex> lock(mtx);
  
              shared_memory++;
          }
      };
  
      thread t1 = thread(count_func);
      thread t2 = thread(count_func);
  
      t1.join();
      t2.join();
  
      cout << "Value is " << shared_memory << endl;
  }
  
  /* stdout stderr
  Value is 2000
  */