读写者问题------读写锁

读写者问题是一类比较特殊的问题。可以允许多个读者同时读，但不允许多个写者或者写者和读者同时操作。所以需要保证读者和写者之间的同步与互斥关系，以及多个写者之间的互斥关系。
读写锁是一种自旋锁：自旋锁是当一个线程没有申请资源成功，不会等待，而是一直在进行循环判断是否锁被释放。

1 #include<stdio.h>
2 #include<pthread.h>
3
4 pthread_rwlock_t rwlock;
5
6 int buf;
7 void *reader(void *arg)
8 {
9     while(1)
10     {
11         pthread_rwlock_rdlock(&rwlock);
12         printf("reader%d is reading...:%d\n",(int)arg,buf);
13         pthread_rwlock_unlock(&rwlock);
14         sleep(1);
15
16     }
17 }
18 void *writer(void *arg)
19 {
20     int i = 0;
21     while(1)
22     {
23         pthread_rwlock_wrlock(&rwlock);
24         buf = i++;
25         printf("writer:%d\n",buf);
26         pthread_rwlock_unlock(&rwlock);
27         sleep(1);
28     }
29 }
30
31 int main()
32 {
33     pthread_t tid1,tid2,tid3,tid4;
34     pthread_create(&tid1,NULL,reader,(void*)1);
35     pthread_create(&tid3,NULL,reader,(void*)2);
36     pthread_create(&tid2,NULL,writer,NULL);
37
38     pthread_rwlock_init(&rwlock,NULL);
39     pthread_join(tid1,NULL);
40     pthread_join(tid3,NULL);
41     pthread_join(tid2,NULL);
42     pthread_rwlock_destroy(&rwlock);
43     return 0;
44 }

运行结果
[fbl@localhost rw_lock]$ ./rwlock
writer:0
reader2 is reading...:0
reader1 is reading...:0
writer:1
reader2 is reading...:1
reader1 is reading...:1
writer:2
reader2 is reading...:2
reader1 is reading...:2
writer:3
reader2 is reading...:3
reader1 is reading...:3
writer:4
reader2 is reading...:4
reader1 is reading...:4
writer:5
reader2 is reading...:5
reader1 is reading...:5
writer:6
reader2 is reading...:6
reader1 is reading...:6