多线程编程——读写锁

一、什么是读写锁

读写锁（也叫共享-独占锁）实际是一种特殊的自旋锁，它把对共享资源的访问者划分成读者和写者，读者只对共享资源进行读访问，写者则需要对共享资源进行写操作。这种锁相对于自旋锁而言，能提高并发性，因为在多处理器系统中，它允许同时有多个读者来访问共享资源，最大可能的读者数为实际的逻辑CPU数。写者是排他性的，一个读写锁同时只能有一个写者或多个读者（与CPU数相关），但不能同时既有读者又有写者。

如果读写锁当前没有读者，也没有写者，那么写者可以立刻获得读写锁，否则它必须“自旋”在那里，直到没有任何写者或读者。如果读写锁没有写者，那么读者可以立即获得该读写锁，否则读者必须“自旋”在那里，直到写者释放该读写锁。读写锁适合于对数据结构的读次数比写次数多很多的场合。
二、示例代码

#include 
#include 
#include 
/* 线程控制块 */
static pthread_t reader1;
static pthread_t reader2;
static pthread_t writer1;
/* 共享数据book */
static int book = 0;
/* 读写锁 */
static pthread_rwlock_t rwlock;
/* 函数结果检查 */
static void check_result(char* str,int result)
{
	if (0 == result)
	{
		printf("%s successfully!\n",str);
	}
	else
	{
		printf("%s failed! error code is %d\n",str,result);
	}
}
/*线程入口*/
static void* reader1_entry(void* parameter)
{
	int i = 0;
	//pthread_detach(pthread_self());
	while (1)
	{
		if(i++%5<2)//读2秒，停3秒。让写可获得锁，否则读线程一直锁着，将无法写
		{
			pthread_rwlock_rdlock(&rwlock); /* 尝试读锁定该读写锁 */
			printf("reader1 read book value is %d\n",book);
			sleep(1); /* 线程休眠2秒，切换到其他线程运行 */
			pthread_rwlock_unlock(&rwlock); /* 线程运行后对读写锁解锁 */
		}else
		{
			sleep(1);
		}
	}
}
static void* reader2_entry(void* parameter)
{
	int i = 0;
	//pthread_detach(pthread_self());
	while (1)
	{
		if(i++%5<2)
		{
			pthread_rwlock_rdlock(&rwlock); /* 尝试读锁定该读写锁 */
			printf("reader2 read book value is %d\n",book);
			sleep(1); /* 线程休眠2秒，切换到其他线程运行 */
			pthread_rwlock_unlock(&rwlock); /* 线程运行后对读写锁解锁 */
		}else
		{
			sleep(1); /* 线程休眠2秒，切换到其他线程运行 */
		}
	}
}
static void* writer1_entry(void* parameter)
{
	//pthread_detach(pthread_self());
	while (1)
	{
		//printf("writer1 \r\n");
		if(!pthread_rwlock_trywrlock(&rwlock)) /* 尝试写锁定该读写锁 */
		{
			book++;
			printf("writer1 write book value is %d\n",book);
			pthread_rwlock_unlock(&rwlock); /* 对读写锁解锁 */
			sleep(1); /* 线程休眠2秒，切换到其他线程运行 */
		}else
		{
			printf("Be BUSY ,can`t write now\r\n");
			sleep(1);
		}
	}
}
/* 用户应用入口 */
int application_init()
{
	int result;
	/* 默认属性初始化读写锁 */
	pthread_rwlock_init(&rwlock,NULL);
	/*创建writer1线程,线程入口是writer1_entry, 线程属性为，入口参数为NULL*/
	result = pthread_create(&writer1,NULL,writer1_entry,NULL);
	check_result("writer1 created",result);
	/*创建reader1线程,线程入口是reader1_entry, 线程属性为默认值，入口参数为NULL*/
	result = pthread_create(&reader1,NULL,reader1_entry,NULL);
	check_result("reader1 created",result);
	/*创建reader2线程,线程入口是reader2_entry, 线程属性为默认值，入口参数为NULL*/
	result = pthread_create(&reader2,NULL,reader2_entry,NULL);
	check_result("reader2 created",result);
	
	
	return 0;
}
int main()
{
	int i ;
	application_init();
	i=100;
	do{
		sleep(1);
	}while(i--);
}

运行结果：

-bash-3.2$ !g
gcc -pthread rwlock.c -o app
-bash-3.2$ ./app
writer1 created successfully!
reader1 created successfully!
reader2 created successfully!
writer1 write book value is 1
reader1 read book value is 1
reader2 read book value is 1
Be BUSY ,cant write now
reader1 read book value is 1
reader2 read book value is 1
Be BUSY ,cant write now
writer1 write book value is 2
writer1 write book value is 3
reader2 read book value is 3
reader1 read book value is 3
Be BUSY ,cant write now
reader2 read book value is 3
reader1 read book value is 3
Be BUSY ,cant write now
writer1 write book value is 4
writer1 write book value is 5
writer1 write book value is 6
reader2 read book value is 6
reader1 read book value is 6
Be BUSY ,cant write now
reader2 read book value is 6
reader1 read book value is 6
Be BUSY ,cant write now
writer1 write book value is 7
writer1 write book value is 8
writer1 write book value is 9
reader2 read book value is 9
Be BUSY ,can`t write now
reader1 read book value is 9