做网站的公司怎么发展业务,环保局 网站建设,网站备案是怎么回事,视频直播怎么做http://www.cnblogs.com/yuuyuu/p/5143881.html 一.概述 读写锁与互斥量的功能类似#xff0c;对临界区的共享资源进行保护#xff01;互斥量一次只让一个线程进入临界区#xff0c;读写锁比它有更高的并行性。读写锁有…http://www.cnblogs.com/yuuyuu/p/5143881.html 一.概述 读写锁与互斥量的功能类似对临界区的共享资源进行保护互斥量一次只让一个线程进入临界区读写锁比它有更高的并行性。读写锁有以下特点 1.如果一个线程用读锁锁定了临界区那么其他线程也可以用读锁来进入临界区这样就可以多个线程并行操作。但这个时候如果再进行写锁加锁就会发生阻塞写锁请求阻塞后后面如果继续有读锁来请求这些后来的读锁都会被阻塞这样避免了读锁长期占用资源防止写锁饥饿 2.如果一个线程用写锁锁住了临界区那么其他线程不管是读锁还是写锁都会发生阻塞 二.函数接口 1.创建读写锁 1.1宏常量初始化 1 pthread_rwlock_t rwlock PTHREAD_RWLOCK_INITIALIZER; 1.2函数初始化 1 #include pthread.h
2
3 int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr); rwlock读写锁的pthread_rwlock_t结构指针 attr读写锁的属性结构指针。不需要别的属性默认为NULL。 2.读写锁加锁与解锁 1 #include pthread.h
2
3 int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
4 int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
5 int pthread_rwlock_unlock(pthread_rwlock_t *rwlock); rwlock创建的读写锁指针 3.其他类型的加锁 1 #include pthread.h
2 #include time.h
3
4
5 int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
6 int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);
7
8 int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abs_timeout);
9 int pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abs_timeout); try类函数加锁如果获取不到锁会立即返回错误EBUSY timed类函数加锁如果规定的时间内获取不到锁会返回ETIMEDOUT错误 4.销毁读写锁 1 #include pthread.h
2
3 int pthread_rwlock_destroy(pthread_rwlock_t *rwlock); 三.简单的例子 创建4个线程2个线程读锁2个线程写锁观察4个线程进入临界区的顺序 1 /**2 * file pthread_rwlock.c3 */4 5 #include stdio.h6 #include stdlib.h7 #include string.h8 #include unistd.h9 #include pthread.h
10
11 /* 初始化读写锁 */
12 pthread_rwlock_t rwlock PTHREAD_RWLOCK_INITIALIZER;
13 /* 全局资源 */
14 int global_num 10;
15
16 void err_exit(const char *err_msg)
17 {
18 printf(error:%s\n, err_msg);
19 exit(1);
20 }
21
22 /* 读锁线程函数 */
23 void *thread_read_lock(void *arg)
24 {
25 char *pthr_name (char *)arg;
26
27 while (1)
28 {
29 /* 读加锁 */
30 pthread_rwlock_rdlock(rwlock);
31
32 printf(线程%s进入临界区global_num %d\n, pthr_name, global_num);
33 sleep(1);
34 printf(线程%s离开临界区...\n, pthr_name);
35
36 /* 读解锁 */
37 pthread_rwlock_unlock(rwlock);
38
39 sleep(1);
40 }
41
42 return NULL;
43 }
44
45 /* 写锁线程函数 */
46 void *thread_write_lock(void *arg)
47 {
48 char *pthr_name (char *)arg;
49
50 while (1)
51 {
52 /* 写加锁 */
53 pthread_rwlock_wrlock(rwlock);
54
55 /* 写操作 */
56 global_num;
57 printf(线程%s进入临界区global_num %d\n, pthr_name, global_num);
58 sleep(1);
59 printf(线程%s离开临界区...\n, pthr_name);
60
61 /* 写解锁 */
62 pthread_rwlock_unlock(rwlock);
63
64 sleep(2);
65 }
66
67 return NULL;
68 }
69
70 int main(void)
71 {
72 pthread_t tid_read_1, tid_read_2, tid_write_1, tid_write_2;
73
74 /* 创建4个线程2个读2个写 */
75 if (pthread_create(tid_read_1, NULL, thread_read_lock, read_1) ! 0)
76 err_exit(create tid_read_1);
77
78 if (pthread_create(tid_read_2, NULL, thread_read_lock, read_2) ! 0)
79 err_exit(create tid_read_2);
80
81 if (pthread_create(tid_write_1, NULL, thread_write_lock, write_1) ! 0)
82 err_exit(create tid_write_1);
83
84 if (pthread_create(tid_write_2, NULL, thread_write_lock, write_2) ! 0)
85 err_exit(create tid_write_2);
86
87 /* 随便等待一个线程防止main结束 */
88 if (pthread_join(tid_read_1, NULL) ! 0)
89 err_exit(pthread_join());
90
91 return 0;
92 } 2个线程函数的临界区里面都sleep(1)测试给足够的时间看其他线程能不能进来。64行写锁函数里面sleep(2)因为写锁请求会阻塞后面的读锁2个写锁一起请求会让读锁饥饿所以比39行的sleep(1)多一秒 编译运行 可以看到读锁可以一起进入临界区而写锁在临界区里面等1秒都不会有其他线程能进来
