linux多线程之读写锁

基本概念：

读写锁也叫做共享互斥锁。

当读写锁是写加锁状态时，在这个锁被解锁之前，所有试图对这个锁加锁的线程都会被阻塞。当读写锁在读加锁状态，所有试图以读模式对它进行加锁的线程都可以得到访问权。

与互斥量相比，读写锁在使用之前必须初始化，在释放它们底层的内存之前必须销毁。

一、锁的初始化与销毁

1. PTHREAD_RWLOCK_DESTROY(P) POSIX Programmer' s Manual PTHREAD_RWLOCK_DESTROY(P)
3. NAME
4. pthread_rwlock_destroy, pthread_rwlock_init - destroy and initialize a
5. read-write lock object
7. SYNOPSIS
8. #include
10. int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
11. int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock,
12. const pthread_rwlockattr_t * restrict attr);

两个函数的返回值：若成功，返回0；否则，返回错误编号

二、读锁

1. PTHREAD_RWLOCK_RDLOCK(P) POSIX Programmer' s Manual PTHREAD_RWLOCK_RDLOCK(P)
3. NAME
4. pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write
5. lock object for reading
7. SYNOPSIS
8. #include
10. int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
11. int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

两个函数的返回值：若成功，返回0；否则，返回错误编号
pthread_rwlock_tryrdlock函数可以获取锁时，返回0.否则，返回错误EBUSY

三、写锁

1. PTHREAD_RWLOCK_TRYWRLOCK(P)POSIX Programmer' s ManuaPTHREAD_RWLOCK_TRYWRLOCK(P)
3. NAME
4. pthread_rwlock_trywrlock, pthread_rwlock_wrlock - lock a read-write
5. lock object for writing
7. SYNOPSIS
8. #include
10. int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);
11. int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);

两个函数的返回值：若成功，返回0；否则，返回错误编号

pthread_rwlock_trywrlock函数可以获取锁时，返回0.否则，返回错误EBUSY

四、带有超时的读写锁

1. PTHREAD_RWLOCK_TIMEDWRLOCK(POSIX Programmer's ManPTHREAD_RWLOCK_TIMEDWRLOCK(P)
3. NAME
4. pthread_rwlock_timedrdlock - lock a read-write lock for reading
5. pthread_rwlock_timedwrlock - lock a read-write lock for writing
7. SYNOPSIS
8. #include
9. #include
11. int pthread_rwlock_timedrdlock( pthread_rwlock_t * restrict rwlock,
12. const struct timespec * restrict abs_timeout);
13. int pthread_rwlock_timedwrlock( pthread_rwlock_t * restrict rwlock,
14. const struct timespec * restrict abs_timeout);

两个函数的返回值：若成功，返回0；否则，返回错误编号
如果它们不能获取锁，那么超时到期时，这两个函数将返回ETIMEDOUT

五、解锁

1. PTHREAD_RWLOCK_UNLOCK(P) POSIX Programmer' s Manual PTHREAD_RWLOCK_UNLOCK(P)
3. NAME
4. pthread_rwlock_unlock - unlock a read-write lock object
6. SYNOPSIS
7. #include
9. int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

返回值：若成功，返回0；否则，返回错误编号

读写锁属性：

读写锁支持的唯一属性是进程共享属性。它与互斥量的进程共享属性是相同的，这里不展开讨论。

例子：gcc pthread_rwlock.c -pthread

1. #include
2. #include
3. #include
4. #include
5. #include
7. static int num = 0;
8. static int count = 100000;
9. static pthread_rwlock_t rwlock;
11. void Perror(const char *s)
12. {
13. perror(s);
14. exit(EXIT_FAILURE);
15. }
17. void* fun2(void *arg)
18. {
19. pthread_t thread_id = pthread_self();
20. printf( "the thread2 id is %ld\n", ( long)thread_id);
21. int i = 1;
22. for (; i<=count; ++i) {
23. pthread_rwlock_wrlock(&rwlock);
24. num += 1;
25. pthread_rwlock_unlock(&rwlock);
26. }
27. }
29. void* fun3(void *arg)
30. {
31. pthread_t thread_id = pthread_self();
32. printf( "the thread3 id is %ld\n", ( long)thread_id);
33. int i = 1;
34. for (; i<=count; ++i) {
35. pthread_rwlock_wrlock(&rwlock);
36. num += 1;
37. pthread_rwlock_unlock(&rwlock);
38. }
39. }
41. int main()
42. {
43. int err;
44. pthread_t thread1;
45. pthread_t thread2;
46. pthread_t thread3;
48. // init
49. pthread_rwlock_init(&rwlock, NULL);
51. thread1 = pthread_self();
52. printf( "the thread1 id is %ld\n", ( long)thread1);
54. // Create thread
55. err = pthread_create(&thread2, NULL, fun2, NULL);
56. if (err != 0) {
57. Perror( "can't create thread2\n");
58. }
59. err = pthread_create(&thread3, NULL, fun3, NULL);
60. if (err != 0) {
61. Perror( "can't create thread3\n");
62. }
64. // detach thread3
65. err = pthread_detach(thread2);
66. if (err != 0) {
67. Perror( "can't detach thread2\n");
68. }
69. err = pthread_detach(thread3);
70. if (err != 0) {
71. Perror( "can't detach thread3\n");
72. }
74. int i = 1;
75. for (; i<=count; ++i) {
76. pthread_rwlock_rdlock(&rwlock);
77. int temp = num;
78. pthread_rwlock_unlock(&rwlock);
79. }
81. sleep( 10);
82. printf( "The num is %d\n", num);
84. pthread_rwlock_destroy(&rwlock);
85. return 0;
86. }

参考：《unix环境高级编程》·第三版

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