讀寫者問題是一類比較特殊的問題。可以允許多個讀者同時讀，但不允許多個寫者或者寫者和讀者同時操作。所以需要保證讀者和寫者之間的同步與互斥關係，以及多個寫者之間的互斥關係。
讀寫鎖是一種自旋鎖：自旋鎖是當一個線程沒有申請資源成功，不會等待，而是一直在進行循環判斷是否鎖被釋放。
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 }
運行結果
[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