std::unique_lock与std::lock_guard分析

背景

C++多线程编程中通常会对共享的数据进行写保护，以防止多线程在对共享数据成员进行读写时造成资源争抢，导致程序出现未定义或异常行为。通常的做法是在修改共享数据成员时进行加锁（mutex）。在使用锁时通常是在对共享数据进行修改之前进行lock操作，在写完之后再进行unlock操作，但经常会出现lock之后离开共享成员操作区域时忘记unlock导致死锁的现象。针对以上的问题，C++11中引入了std::unique_lock与std::lock_guard两种数据结构。通过对lock和unlock进行一次封装，实现自动unlock的功能。

std::lock_guard

std::lock_guard是典型的RAII实现，功能相对简单。在构造函数中进行加锁，析构函数中进行解锁。下面是std::lock_guard的源码，也非常容易看出是RAII的设计。

template <typename _Mutex>
class lock_guard
{
public:
    typedef _Mutex mutex_type;

    explicit lock_guard(mutex_type &__m) : _M_device(__m)
    {
        _M_device.lock();  // 构造加锁
    }

    lock_guard(mutex_type &__m, adopt_lock_t) noexcept : _M_device(__m)
    {
    }

    ~lock_guard()
    {
        _M_device.unlock();  //析构解锁
    }

    lock_guard(const lock_guard &) = delete;
    lock_guard &operator=(const lock_guard &) = delete;

private:
    mutex_type &_M_device;
};

std::unique_lock

std::unique_lock同样能够实现自动解锁的功能，但比std::lock_guard提供了更多的成员方法，更加灵活一点，相对来说占用空也间更大并且相对较慢，即需要付出更多的时间、性能成本。下面是其源码：

template <typename _Mutex>
class unique_lock
{
public:
    typedef _Mutex mutex_type;

    unique_lock() noexcept
        : _M_device(0), _M_owns(false)
    {
    }

    explicit unique_lock(mutex_type &__m)
        : _M_device(std::__addressof(__m)), _M_owns(false)
    {
        lock();
        _M_owns = true;
    }

    unique_lock(mutex_type &__m, defer_lock_t) noexcept
        : _M_device(std::__addressof(__m)), _M_owns(false)
    {
    }

    unique_lock(mutex_type &__m, try_to_lock_t)
        : _M_device(std::__addressof(__m)), _M_owns(_M_device->try_lock())
    {
    }

    unique_lock(mutex_type &__m, adopt_lock_t) noexcept
        : _M_device(std::__addressof(__m)), _M_owns(true)
    {
        // XXX calling thread owns mutex
    }

    template <typename _Clock, typename _Duration>
    unique_lock(mutex_type &__m,
                const chrono::time_point<_Clock, _Duration> &__atime)
        : _M_device(std::__addressof(__m)),
          _M_owns(_M_device->try_lock_until(__atime))
    {
    }

    template <typename _Rep, typename _Period>
    unique_lock(mutex_type &__m,
                const chrono::duration<_Rep, _Period> &__rtime)
        : _M_device(std::__addressof(__m)),
          _M_owns(_M_device->try_lock_for(__rtime))
    {
    }

    ~unique_lock()
    {
        if (_M_owns)
            unlock();
    }

    unique_lock(const unique_lock &) = delete;
    unique_lock &operator=(const unique_lock &) = delete;

    unique_lock(unique_lock &&__u) noexcept
        : _M_device(__u._M_device), _M_owns(__u._M_owns)
    {
        __u._M_device = 0;
        __u._M_owns = false;
    }

    unique_lock &operator=(unique_lock &&__u) noexcept
    {
        if (_M_owns)
            unlock();

        unique_lock(std::move(__u)).swap(*this);

        __u._M_device = 0;
        __u._M_owns = false;

        return *this;
    }

    void
    lock()
    {
        if (!_M_device)
            __throw_system_error(int(errc::operation_not_permitted));
        else if (_M_owns)
            __throw_system_error(int(errc::resource_deadlock_would_occur));
        else
        {
            _M_device->lock();
            _M_owns = true;
        }
    }

    bool
    try_lock()
    {
        if (!_M_device)
            __throw_system_error(int(errc::operation_not_permitted));
        else if (_M_owns)
            __throw_system_error(int(errc::resource_deadlock_would_occur));
        else
        {
            _M_owns = _M_device->try_lock();
            return _M_owns;
        }
    }

    template <typename _Clock, typename _Duration>
    bool
    try_lock_until(const chrono::time_point<_Clock, _Duration> &__atime)
    {
        if (!_M_device)
            __throw_system_error(int(errc::operation_not_permitted));
        else if (_M_owns)
            __throw_system_error(int(errc::resource_deadlock_would_occur));
        else
        {
            _M_owns = _M_device->try_lock_until(__atime);
            return _M_owns;
        }
    }

    template <typename _Rep, typename _Period>
    bool
    try_lock_for(const chrono::duration<_Rep, _Period> &__rtime)
    {
        if (!_M_device)
            __throw_system_error(int(errc::operation_not_permitted));
        else if (_M_owns)
            __throw_system_error(int(errc::resource_deadlock_would_occur));
        else
        {
            _M_owns = _M_device->try_lock_for(__rtime);
            return _M_owns;
        }
    }

    void
    unlock()
    {
        if (!_M_owns)
            __throw_system_error(int(errc::operation_not_permitted));
        else if (_M_device)
        {
            _M_device->unlock();
            _M_owns = false;
        }
    }

    void
    swap(unique_lock &__u) noexcept
    {
        std::swap(_M_device, __u._M_device);
        std::swap(_M_owns, __u._M_owns);
    }

    mutex_type *
    release() noexcept
    {
        mutex_type *__ret = _M_device;
        _M_device = 0;
        _M_owns = false;
        return __ret;
    }

    bool
    owns_lock() const noexcept
    {
        return _M_owns;
    }

    explicit operator bool() const noexcept
    {
        return owns_lock();
    }

    mutex_type *
    mutex() const noexcept
    {
        return _M_device;
    }

private:
    mutex_type *_M_device;
    bool _M_owns; // XXX use atomic_bool
};

template <typename _Mutex>
inline void
swap(unique_lock<_Mutex> &__x, unique_lock<_Mutex> &__y) noexcept
{
    __x.swap(__y);
}

从上面的源码对比非常容易看出std::unique_lock的实现比std::lock_guard复杂多了，提供了几个方法使编程更灵活，具体如下：

lock	locks the associated mutex
try_lock	tries to lock the associated mutex, returns if the mutex is not available
try_lock_for	attempts to lock the associated TimedLockable mutex, returns if the mutex has been unavailable for the specified time duration
try_lock_until	tries to lock the associated TimedLockable mutex, returns if the mutex has been unavailable until specified time point has been reached
unlock	unlocks the associated mutex

以上方法，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。

std::unique_lock与条件变量

这里举个并发消息队列的简单例子，是std::unique_lock与条件变量配合使用经典场景，并发消费共享成员变量m_queue的内容，且保证线程安全。

#include <queue>
#include <mutex>
#include <thread>
#include <chrono>
#include <memory>
#include <condition_variable>

typedef struct task_tag
{
    int data;
    task_tag( int i ) : data(i) { }
} Task, *PTask;

class MessageQueue
{
public:
    MessageQueue(){}
    ~MessageQueue()
    {
        if ( !m_queue.empty() )
        {
            PTask pRtn = m_queue.front();
            delete pRtn;
        }
        
    }

    void PushTask( PTask pTask )
    {
        std::unique_lock<std::mutex> lock( m_queueMutex );
        m_queue.push( pTask );
        m_cond.notify_one();
    }

    PTask PopTask()
    {
        PTask pRtn = NULL;
        std::unique_lock<std::mutex> lock( m_queueMutex );
        while ( m_queue.empty() )
        {
            m_cond.wait_for( lock, std::chrono::seconds(1) );
        }

        if ( !m_queue.empty() )
        {
            pRtn = m_queue.front();
            if ( pRtn->data != 0 )
                m_queue.pop();
        }

        return pRtn;
    }

private:
    std::mutex m_queueMutex;
    std::condition_variable m_cond; 
    std::queue<PTask> m_queue;
};

void thread_fun( MessageQueue *arguments )
{
    while ( true )
    {
        PTask data = arguments->PopTask();

        if (data != NULL)
        {
            printf( "Thread is: %d
", std::this_thread::get_id() );
            printf("   %d
", data->data );
            if ( 0 == data->data ) //Thread end.
                break;
            else
                delete data;
        }
    }
}

 int main( int argc, char *argv[] )
{
    MessageQueue cq;

    #define THREAD_NUM 3
    std::thread threads[THREAD_NUM];

    for ( int i=0; i<THREAD_NUM; ++i )
        threads[i] = std::thread( thread_fun, &cq );

    int i = 10;
    while( i > 0 )
    {
        Task *pTask = new Task( --i );
        cq.PushTask( pTask );
    }

    for ( int i=0; i<THREAD_NUM; ++i) 
        threads[i].join();

    system( "pause" );
    return 0;
}

在示例代码中，我们使主线程向公共队列cq中Push任务，而其他的线程则负责取出任务并打印任务，由于std::cout并不支持并发线程安全，所以在打印任务时使用printf。主线程new出的任务，在其他线程中使用并销毁，当主线程发送data为0的任务时，则规定任务发送完毕，而其他的线程获取到data为0的任务后退出线程，data为0的任务则有消息队列负责销毁。整个消息队列使用标准模板库实现，现实跨平台。

std::unique_lock与std::lock_guard区别

上述例子中，std::unique_lock在线程等待期间解锁mutex，并在唤醒时重新将其锁定，而std::lock_guard却不具备这样的功能。所以std::unique_lock和std::lock_guard在编程应用中的主要区别总结如下：

• 如果只为保证数据同步，那么std::lock_guard完全够用；
• 如果除了同步还需要实现条件阻塞时，那么就需要用std::unique_lock。