std::mutex std::unique_lock std::lock_guard std::recursive_mutex的理解

C++11并发控制

最新推荐文章于 2025-07-16 12:08:19 发布

置顶 jack_20

最新推荐文章于 2025-07-16 12:08:19 发布

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分类专栏： c++ 文章标签： C++ 对象

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本文介绍了C++11中的互斥量std::mutex及其高级用法，包括std::unique_lock、std::lock_guard和std::recursive_mutex。通过示例展示了如何使用这些工具来保护临界区，并确保线程安全。

std::mutex 称为互斥量；

C++11中，std::unique_lock, std::lock_guard, std::recursive_mutex可以简单理解为对std::mutex的封装，且对互斥量的unlock是在对象（比如std::unique_lock对象）销毁时执行。

区域锁lock_guard使用起来比较简单，除了构造函数外没有其他member function，在整个区域都有效。
区域锁unique_lock除了lock_guard的功能外，提供了更多的member_function，相对来说更灵活一些。
unique_lock的最有用的一组函数为：

通过上面的函数，可以通过lock/unlock可以比较灵活的控制锁的范围，减小锁的粒度。
通过try_lock_for/try_lock_until则可以控制加锁的等待时间，此时这种锁为乐观锁。

下面举两个例子，注意红色标注的部分：

Example 1

// mutex example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex

std::mutex mtx;           // mutex for critical section

void print_block (int n, char c) {
  // critical section (exclusive access to std::cout signaled by locking mtx):
  mtx.lock();
  for (int i=0; i<n; ++i) { std::cout << c; }
  std::cout << '\n';
  mtx.unlock();
}

int main ()
{
  std::thread th1 (print_block,50,'*');
  std::thread th2 (print_block,50,'$');

  th1.join();
  th2.join();

  return 0;
}

Possible output (order of lines may vary, but characters are never mixed):


**************************************************
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Example 2

// unique_lock example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::unique_lock

std::mutex mtx;           // mutex for critical section

void print_block (int n, char c) {
  // critical section (exclusive access to std::cout signaled by lifetime of lck):
  std::unique_lock<std::mutex> lck (mtx);
  for (int i=0; i<n; ++i) { std::cout << c; }
  std::cout << '\n';
}

int main ()
{
  std::thread th1 (print_block,50,'*');
  std::thread th2 (print_block,50,'$');

  th1.join();
  th2.join();

  return 0;
}

Edit & Run

Possible output (order of lines may vary, but characters are never mixed):


**************************************************
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Example 3

// lock_guard example
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex, std::lock_guard
#include <stdexcept>      // std::logic_error

std::mutex mtx;

void print_even (int x) {
  if (x%2==0) std::cout << x << " is even\n";
  else throw (std::logic_error("not even"));
}

void print_thread_id (int id) {
  try {
    // using a local lock_guard to lock mtx guarantees unlocking on destruction / exception:
    std::lock_guard<std::mutex> lck (mtx);
    print_even(id);
  }
  catch (std::logic_error&) {
    std::cout << "[exception caught]\n";
  }
}

int main ()
{
  std::thread threads[10];
  // spawn 10 threads:
  for (int i=0; i<10; ++i)
    threads[i] = std::thread(print_thread_id,i+1);

  for (auto& th : threads) th.join();

  return 0;
}

Edit & Run

Possible output:

[exception caught]
2 is even
[exception caught]
4 is even
[exception caught]
6 is even
[exception caught]
8 is even
[exception caught]

10 is even