多线程的那点儿事（之顺序锁）
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在互斥数据访问中有一种多读少写的情况。正对这么一种情形，我们也提出了读写锁的方案。但是呢，这个锁有些缺陷。什么缺陷呢？那就是，这个写锁需要在所有的读锁完成之后才能写。否则的话，写锁需要这么一直等下去。
那么，有没有什么办法能使得写操作快速一点进行呢？那就是顺序锁。
copy to clipboardprint?typedef struct _SEQUENCE_LOCK
{
unsigned int sequence;
HANDLE hLock;
}SEQUENCE_LOCK;
typedef struct _SEQUENCE_LOCK
{
unsigned int sequence;
HANDLE hLock;
}SEQUENCE_LOCK; 有了这么一个数据结构之后。那么读锁怎么开始呢，
copy to clipboardprint?unsigned int get_lock_begin(SEQUENCE_LOCK* hSeqLock)
{
assert(NULL != hSeqLock);
return hSeqLock->sequence;
}
int get_lock_retry(SEQUENCE_LOCK* hSeqLock, unsigned int value)
{
unsigned int new_value;
assert(NULL != hSeqLock);
new_value = hSeqLock->sequence;
return (new_value & 0x1) || (new_value ^ value);
}
unsigned int get_lock_begin(SEQUENCE_LOCK* hSeqLock)
{
assert(NULL != hSeqLock);
return hSeqLock->sequence;
}
int get_lock_retry(SEQUENCE_LOCK* hSeqLock, unsigned int value)
{
unsigned int new_value;
assert(NULL != hSeqLock);
new_value = hSeqLock->sequence;
return (new_value & 0x1) || (new_value ^ value);
} 自然写锁也需要修改了，
copy to clipboardprint?void get_write_lock(SEQUENCE_LOCK* hSeqLock)
{
assert(NULL != hSeqLock);
WaitForSingleObject(hSeqLock->hLock);
hSeqLock->sequence ++;
}
void release_write_lock(SEQUENCE_LOCK* hSeqLock)
{
assert(NULL != hSeqLock);
hSeqLock->sequence ++;
ReleaseMutex(hSeqLock->hLock);
}
void get_write_lock(SEQUENCE_LOCK* hSeqLock)
{
assert(NULL != hSeqLock);
WaitForSingleObject(hSeqLock->hLock);
hSeqLock->sequence ++;
}
void release_write_lock(SEQUENCE_LOCK* hSeqLock)
{
assert(NULL != hSeqLock);
hSeqLock->sequence ++;
ReleaseMutex(hSeqLock->hLock);
} 如果应用呢，其实也不难，
copy to clipboardprint?void read_process(SEQUENCE_LOCK* hSeqLock)
{
unsigned int sequence;
do{
sequence = get_lock_begin(hSeqLock);
/* read operation */
}while(get_lock_retry(hSeqLock, sequence));
}
void write_process(SEQUENCCE_LOCK* hSeqLock)
{
get_write_lock(hSeqLock);
/* write operation */
release_write_lock(hSeqLock);
}
void read_process(SEQUENCE_LOCK* hSeqLock)
{
unsigned int sequence;
do{
sequence = get_lock_begin(hSeqLock);
/* read operation */
}while(get_lock_retry(hSeqLock, sequence));
}
void write_process(SEQUENCCE_LOCK* hSeqLock)
{
get_write_lock(hSeqLock);
/* write operation */
release_write_lock(hSeqLock);
}
总结：
（1）读锁退出有两个条件，要么写操作正在进行呢，要么没有写锁
（2）写锁之间需要互斥操作
（3）互斥操作的数据不能是指针，否则有可能在访问的时候会造成异常，因为有可能边写边读
（4）顺序锁代替不了读写锁，因为读写锁可以保证所有的数据操作，而顺序锁不行
补充：软件开发 , 其他 ,
