linux线程同步(3)-读写锁

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.c
 3  */
 4 
 5 #include <stdio.h>
 6 #include <stdlib.h>
 7 #include <string.h>
 8 #include <unistd.h>
 9 #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 }