C++多线程-读写锁

こ雲淡風輕ζ 提交于 2020-01-21 05:38:18

在编写多线程的时候,有一种情况是十分常见的。那就是,有些公共数据修改的机会比较少。相比较改写,它们读的机会反而高的多。通常而言,在读的过程中,往往伴随着查找的操作,中间耗时很长。给这种代码段加锁,会极大地降低我们程序的效率。那么有没有一种方法,可以专门处理这种多读少写的情况呢?
有,那就是读写锁。

(1)首先,我们定义一下基本的数据结构。

typedef struct _RWLock  
{  
    int count;  
    int state;  
    HANDLE hRead;  
    HANDLE hWrite;  
}RWLock;  

同时,为了判断当前的锁是处于读状态,还是写状态,我们要定义一个枚举量,

typedef enum  
{  
    STATE_EMPTY = 0,  
    STATE_READ,  
    STATE_WRITE  
};  

(2)初始化数据结构

RWLock* create_read_write_lock(HANDLE hRead, HANDLE hWrite)  
{  
    RWLock* pRwLock = NULL;  
  
    assert(NULL != hRead && NULL != hWrite);  
    pRwLock = (RWLock*)malloc(sizeof(RWLock));  
    
    pRwLock->hRead = hRead;  
    pRwLock->hWrite = hWrite;  
    pRwLock->count = 0;  
    pRwLock->state = STATE_EMPTY;  
    return pRwLock;  
} 

(3)获取读锁

void read_lock(RWLock* pRwLock)  
{  
    assert(NULL != pRwLock);  
      
    WaitForSingleObject(pRwLock->hRead, INFINITE);  
    pRwLock->counnt ++;  
    if(1 == pRwLock->count){  
        WaitForSingleObject(pRwLock->hWrite, INFINITE);  
        pRwLock->state = STATE_READ;  
    }  
    ReleaseMutex(pRwLock->hRead);  
}  

(4)获取写锁

void write_lock(RWLock* pRwLock)  
{  
    assert(NULL != pRwLock);  
  
    WaitForSingleObject(pRwLock->hWrite, INFINITE);  
    pRwLock->state = STATE_WRITE;  
}  

(5)释放读写锁

void read_write_unlock(RWLock* pRwLock)  
{  
    assert(NULL != pRwLock);  
  
    if(STATE_READ == pRwLock->state){  
        WaitForSingleObject(pRwLock->hRead, INFINITE);  
        pRwLock->count --;  
        if(0 == pRwLock->count){  
            pRwLock->state = STATE_EMPTY;  
            ReleaseMutex(pRwLock->hWrite);  
        }  
        ReleaseMutex(pRwLock->hRead);  
    }else{  
        pRwLock->state = STATE_EMPTY;  
        ReleaseMutex(pRwLock->hWrite);  
    }  
      
    return;  
}  

文章总结:
(1)读写锁的优势只有在多读少写、代码段运行时间长这两个条件下才会效率达到最大化;
(2)任何公共数据的修改都必须在锁里面完成;
(3)读写锁有自己的应用场所,选择合适的应用环境十分重要;
(4)编写读写锁很容易出错,朋友们应该多加练习;
(5)读锁和写锁一定要分开使用,否则达不到效果。

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