多线程的那点儿事09(之读写锁)(转)
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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。