linux之读写锁技术

    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。

[cpp] view plain copy print ?

1. typedef struct _RWLock  
2. {  
3.     int count;  
4.     int state;  
5.     HANDLE hRead;  
6.     HANDLE hWrite;  
7. }RWLock;     

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，

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1. typedef enum  
2. {  
3.     STATE_EMPTY = 0,  
4.     STATE_READ,  
5.     STATE_WRITE  
6. };  

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

     （2）初始化数据结构

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1. RWLock* create_read_write_lock(HANDLE hRead, HANDLE hWrite)  
2. {  
3.     RWLock* pRwLock = NULL;  
4.   
5.     assert(NULL != hRead && NULL != hWrite);  
6.     pRwLock = (RWLock*)malloc(sizeof(RWLock));  
7.     
8.     pRwLock->hRead = hRead;  
9.     pRwLock->hWrite = hWrite;  
10.     pRwLock->count = 0;  
11.     pRwLock->state = STATE_EMPTY;  
12.     return pRwLock;  
13. }  

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

[cpp] view plain copy print ?

1. void read_lock(RWLock* pRwLock)  
2. {  
3.     assert(NULL != pRwLock);  
4.       
5.     WaitForSingleObject(pRwLock->hRead, INFINITE);  
6.     pRwLock->counnt ++;  
7.     if(1 == pRwLock->count){  
8.         WaitForSingleObject(pRwLock->hWrite, INFINITE);  
9.         pRwLock->state = STATE_READ;  
10.     }  
11.     ReleaseMutex(pRwLock->hRead);  
12. }  

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

[cpp] view plain copy print ?

1. void write_lock(RWLock* pRwLock)  
2. {  
3.     assert(NULL != pRwLock);  
4.   
5.     WaitForSingleObject(pRwLock->hWrite, INFINITE);  
6.     pRwLock->state = STATE_WRITE;  
7. }  

void write_lock(RWLock* pRwLock)
{
    assert(NULL != pRwLock);

    WaitForSingleObject(pRwLock->hWrite, INFINITE);
    pRwLock->state = STATE_WRITE;
}

     （5）释放读写锁

[cpp] view plain copy print ?

1. void read_write_unlock(RWLock* pRwLock)  
2. {  
3.     assert(NULL != pRwLock);  
4.   
5.     if(STATE_READ == pRwLock->state){  
6.         WaitForSingleObject(pRwLock->hRead, INFINITE);  
7.         pRwLock->count --;  
8.         if(0 == pRwLock->count){  
9.             pRwLock->state = STATE_EMPTY;  
10.             ReleaseMutex(pRwLock->hWrite);  
11.         }  
12.         ReleaseMutex(pRwLock->hRead);  
13.     }else{  
14.         pRwLock->state = STATE_EMPTY;  
15.         ReleaseMutex(pRwLock->hWrite);  
16.     }  
17.       
18.     return;  
19. }  

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。