Linux 读写锁

一、读写锁的概念
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得多。通常而言，在读的过程中，往往伴随着查找的操作，中间耗时比较长。给这种代码段加锁，会极大地降低程序的效率。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？那就是读写锁。读写锁的本质是一种自旋锁。

注意：写独占，读共享，写锁优先级高。

二、读写锁接口
1、初始化

int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr);

2、销毁

int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);

3、加锁和解锁

int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_rdlock(pthread_rwlock_t *rdlock);
int pthread_rwlock_unlock(pthread_rwlock_t rwlock);

【例】

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<pthread.h>

int counter;
pthread_rwlock_t rwlock;

void* route_write(void* arg)
{
		int t;
		int i = *(int*)arg;
		free(arg);
		
		while(1){
				t = counter;
				usleep(1000);
				
				pthread_rwlock_wrlock(&rwlock);
				printf("write:%d:%#X：counter=%d ++counter=%d\n", i, pthread_self(), t, ++counter);
				pthread_rwlock_unlock(&rwlock);
				
				sleep(2);
		}
}

void *route_read(void* arg)
{
		int t;
		int i = *(int*)arg;
		free(arg);
		
		while(1){
				pthread_rwlock_rdlock(&rwlock);
				printf("read: %d:%#X: counter=%d\n", i, pthread_self(), counter);
				pthread_rwlock_unlock(&rwlock);
				sleep(5);
		}
}

int main()
{
		int i;
		pthread_t tid[8];
		pthread_rwlock_init(&rwlock, NULL);
		
		for(i = 0; i < 3; i++)
		{
				int *p = (int*)malloc(sizeof(int));
				*p = i;
				pthread_create(&tid[i], NULL, route_write, (void*)p);
		}
		
		for(i = 0; i < 5; i++)
		{
				int *p = (int*)malloc(sizeof(int));
				*p = i;
				pthread_create(&tid[i+3], NULL, route_read, (void*)p);
		}
		
		for(i = 0; i < 8; i++)
		{
				pthread_join(tid[i], NULL);
		}		
				
		pthread_rwlock_destroy(&rwlock);
}

运行结果：