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C++11线程库 (四) 互斥量

我什么都布吉岛 发布时间：2021-02-28 00:49:28 ，浏览量：3

互斥锁的出现其实就是为了完成线程之间数据的同步和有序访问，防止由于多个线程同时进行写，或者一个读一个写导致的数据脏读的情况。有一种锁叫做自旋锁，互斥锁属于sleep-waiting类型的锁，自旋锁属于busy-waiting类型的锁，后者适合占用时间非常短的临界区。

C++标准库支持的几种互斥锁类型如下：

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Mutex
mutex
timed_mutex
recursive_mutex
recursive_timed_mutex
shared_mutex C++14
shared_timed_mutex C++17

除了最后两个互斥锁是定义在shared_mutex外，其他都定义在头文件mutex中。所有互斥锁共有的操作是lock unlock和try_lock；为了满足不同场景的需求，还可能有：

  • recursive属性 允许同个线程多次加锁不阻塞
  • timed属性 指定时限进行加锁操作
  • shared属性 多线程共享互斥锁，但是只能由一个线程独有
一、什么是互斥锁？

互斥锁算法防止了多线程同时获取共享资源，能够防止数据争抢并为多个线程提供同步支持。

1.1 std::mutex

std::mutex类是一个同步原语，其目的是为了防止多个线程同时访问数据误读的问题，达到保护共享数据的作用。他提供了一个互斥的、非递归的所有权语义：

  • 当一个线程成功调用lock或try_lock意味着其拥有了互斥锁所有权，直至其调用unlock;
  • 当一个线程拥有了互斥锁，其他线程将会阻塞(lock)或者立即返回（try_lock)；
  • 一个线程在调用lock和try_lock之前不能已经拥有互斥锁所有权；

注意：std::mutex既不可以拷贝，也不可以移动。

互斥量大多数的操作无外乎加锁lock()、解锁unlock()及尝试加锁try_lock()；当然根据应用场景不同，有些互斥量可能有特有操作：try_lock_for() try_lock_until() lock_shared() try_lock_shared() unlock_shared() try_lock_shared_for() try_lock_shared_until()。

二、互斥锁操作

互斥锁有些操作是共有的，如加锁、解锁和尝试加锁：

互斥锁操作含义lock()锁定互斥，若互斥不可用则阻塞unlock()解锁互斥try_lock()尝试锁定互斥，若互斥不可用则返回false

如果互斥锁拥有了时间属性将会除了try_lock外，还有与时间相关的定制try_lock：

  • try_lock_for 尝试锁定互斥，给定时间若仍不可用则返回
  • try_lock_until 尝试锁定互斥，直至某个时刻仍不可用则返回

如果互斥锁拥有了共享属性，也就是shared_mutex将会增加：

  • lock_shared
  • try_lock_shared
  • unlock_shared

如果一个互斥锁同时拥有时间和共享属性，则会增加：

  • try_lock_shared_for
  • try_lock_shared_until

小结：互斥锁都支持lock、unlock和try_lock三种方法。当一个互斥锁有了时限属性，将会增加两个额外的方法for和until；当一个互斥锁有了共享属性，会增加三个额外方法lock_shared、try_lock_shared和unlock_shared。

构造函数都是采用的无参构造，且无拷贝、无赋值；析构时任何线程不应该持有锁资源，行为是未定义的。下面是一个例子：

mutex lo1;//无参构造mutex
time_mutex time_lo1;//无参构造time_mutex，其他的也一样
mutex lo2(lo1);//error,deleted
mutex lo2;
mutex lo2=lo1;//error,deleted
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