读写锁

读写锁

与互斥量类似，但读写锁允许更高的并行性。其特性为：写独占，读共享。

读写锁状态：

一把读写锁具备三种状态：

       1. 读模式下加锁状态 (读锁)

       2. 写模式下加锁状态 (写锁)

       3. 不加锁状态

读写锁特性： 

1. 读写锁是“写模式加锁”时， 解锁前，所有对该锁加锁的线程都会被阻塞。
2. 读写锁是“读模式加锁”时， 如果线程以读模式对其加锁会成功；如果线程以写模式加锁会阻塞。
3. 读写锁是“读模式加锁”时， 既有试图以写模式加锁的线程，也有试图以读模式加锁的线程。那么读写锁会阻塞随后的读模式锁请求。优先满足写模式锁。读锁、写锁并行阻塞，写锁优先级高

       读写锁也叫共享-独占锁。当读写锁以读模式锁住时，它是以共享模式锁住的；当它以写模式锁住时，它是以独占模式锁住的。写独占、读共享。

       读写锁非常适合于对数据结构读的次数远大于写的情况。

主要应用函数：

       pthread_rwlock_init函数

       pthread_rwlock_destroy函数

       pthread_rwlock_rdlock函数 

       pthread_rwlock_wrlock函数

       pthread_rwlock_tryrdlock函数

       pthread_rwlock_trywrlock函数

       pthread_rwlock_unlock函数

以上7 个函数的返回值都是：成功返回0， 失败直接返回错误号。

       pthread_rwlock_t类型 用于定义一个读写锁变量。

       pthread_rwlock_t rwlock;

pthread_rwlock_init函数

初始化一把读写锁

       int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr);

       参2：attr表读写锁属性，通常使用默认属性，传NULL即可。

pthread_rwlock_destroy函数

销毁一把读写锁

       int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);

pthread_rwlock_rdlock函数

以读方式请求读写锁。（常简称为：请求读锁）

    int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

pthread_rwlock_wrlock函数

以写方式请求读写锁。（常简称为：请求写锁）

    int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);

pthread_rwlock_unlock函数

解锁

       int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

pthread_rwlock_tryrdlock函数

非阻塞以读方式请求读写锁（非阻塞请求读锁）

int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

pthread_rwlock_trywrlock函数

非阻塞以写方式请求读写锁（非阻塞请求写锁）

       int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);

读写锁示例

看如下示例，同时有多个线程对同一全局数据读、写操作。

#include <stdio.h>
#include <unistd.h>
#include <pthread.h>

int counter;
pthread_rwlock_t rwlock;

/* 3个线程不定时写同一全局资源，5个线程不定时读同一全局资源 */
void *th_write(void *arg)
{
    int t, i = (int)arg;
    while (1) {
        pthread_rwlock_wrlock(&rwlock);
        t = counter;
        usleep(1000);
        printf("=======write %d: %lu: counter=%d ++counter=%d\n", i, pthread_self(), t, ++counter);
        pthread_rwlock_unlock(&rwlock);
        usleep(10000);
    }
    return NULL;
}
void *th_read(void *arg)
{
    int i = (int)arg;

    while (1) {
        pthread_rwlock_rdlock(&rwlock);
        printf("----------------------------read %d: %lu: %d\n", i, pthread_self(), counter);
        pthread_rwlock_unlock(&rwlock);
        usleep(2000);
    }
    return NULL;
}
int main(void)
{
    int i;
    pthread_t tid[8];
    pthread_rwlock_init(&rwlock, NULL);

    for (i = 0; i < 3; i++)
        pthread_create(&tid[i], NULL, th_write, (void *)i);
    for (i = 0; i < 5; i++)
        pthread_create(&tid[i+3], NULL, th_read, (void *)i);
    for (i = 0; i < 8; i++)
        pthread_join(tid[i], NULL);

    pthread_rwlock_destroy(&rwlock);
    return 0;
}        

运行结果：

ubuntu1604@ubuntu:~/wangqinghe/linux/20190821$ ./rwlock
----------------------------read 1: 140692331108096: 0
----------------------------read 2: 140692322715392: 0
----------------------------read 0: 140692339500800: 0
----------------------------read 3: 140692314322688: 0
----------------------------read 4: 140692305929984: 0
=======write 2: 140692347893504: counter=0 ++counter=1
=======write 1: 140692356286208: counter=1 ++counter=2
----------------------------read 2: 140692322715392: 2
----------------------------read 3: 140692314322688: 2
----------------------------read 0: 140692339500800: 2
----------------------------read 4: 140692305929984: 2
----------------------------read 1: 140692331108096: 2
=======write 0: 140692364678912: counter=2 ++counter=3
----------------------------read 3: 140692314322688: 3
----------------------------read 0: 140692339500800: 3
----------------------------read 4: 140692305929984: 3
----------------------------read 1: 140692331108096: 3
----------------------------read 2: 140692322715392: 3
----------------------------read 3: 140692314322688: 3
----------------------------read 0: 140692339500800: 3
----------------------------read 4: 140692305929984: 3
----------------------------read 1: 140692331108096: 3
----------------------------read 2: 140692322715392: 3
----------------------------read 4: 140692305929984: 3
----------------------------read 0: 140692339500800: 3
----------------------------read 3: 140692314322688: 3
----------------------------read 1: 140692331108096: 3
----------------------------read 2: 140692322715392: 3
----------------------------read 0: 140692339500800: 3
----------------------------read 3: 140692314322688: 3
----------------------------read 4: 140692305929984: 3
----------------------------read 1: 140692331108096: 3
----------------------------read 2: 140692322715392: 3
=======write 2: 140692347893504: counter=3 ++counter=4
----------------------------read 4: 140692305929984: 4
----------------------------read 3: 140692314322688: 4
=======write 1: 140692356286208: counter=4 ++counter=5
----------------------------read 1: 140692331108096: 5
----------------------------read 0: 140692339500800: 5
----------------------------read 2: 140692322715392: 5
=======write 0: 140692364678912: counter=5 ++counter=6
----------------------------read 4: 140692305929984: 6
----------------------------read 3: 140692314322688: 6
----------------------------read 0: 140692339500800: 6
----------------------------read 1: 140692331108096: 6
----------------------------read 2: 140692322715392: 6
----------------------------read 4: 140692305929984: 6
----------------------------read 3: 140692314322688: 6
----------------------------read 0: 140692339500800: 6
----------------------------read 1: 140692331108096: 6
----------------------------read 2: 140692322715392: 6
----------------------------read 3: 140692314322688: 6
----------------------------read 0: 140692339500800: 6
----------------------------read 1: 140692331108096: 6
----------------------------read 2: 140692322715392: 6