Unix IPC之读写锁

linux中读写锁的rwlock介绍

读写锁比mutex有更高的适用性，可以多个线程同时占用读模式的读写锁，但是只能一个线程占用写模式的读写锁；

1，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.

2，当读写锁在读加锁状态时, 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，但是以写模式对它进行枷锁的线程将阻塞；

3，当读写锁在读模式锁状态时, 如果有另外线程试图以写模式加锁, 读写锁通常会阻塞随后的读模式锁请求, 这样可以避免读模式锁长期占用, 而等待的写模式锁请求长期阻塞;

这种锁适用对数据结构进行读的次数比写的次数多的情况下，因为可以进行读锁共享

API接口说明：

1)初始化和销毁

#include <pthread.h>

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

成功则返回0, 出错则返回错误编号.

2)读加锁和写加锁
获取锁的两个函数是阻塞操作

#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
成功则返回0, 出错则返回错误编号.

3)非阻塞获得读锁和写锁
非阻塞的获取锁操作, 如果可以获取则返回0, 否则返回错误的EBUSY.

#include <pthread.h>

int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);

成功则返回0, 出错则返回错误编号.

[b]Unix网络编程-卷2：用互斥锁和信号量实现写者优先的读写锁[/b]

typedef struct
{
pthread_mutex_t   rw_mutex;        /* basic lock on this struct */
pthread_cond_t    rw_condreaders;  /* for reader threads waiting */
pthread_cond_t    rw_condwriters;  /* for writer threads waiting */
int               rw_magic;        /* for error checking */
int               rw_nwaitreaders; /* the number waiting */
int               rw_nwaitwriters; /* the number waiting */
int               rw_refcount;     /* -1:one w_mutex; 0:mutex free; >0:num of r_mutex */
/* 4-1 if writer has the lock, else # readers holding the lock */
} pthread_rwlock_t;

int
pthread_rwlock_unlock(pthread_rwlock_t *rw)
{
int     result;

if (rw->rw_magic != RW_MAGIC)
return(EINVAL);

if ( (result = pthread_mutex_lock(&rw->rw_mutex)) != 0)
return(result);

if (rw->rw_refcount > 0)
rw->rw_refcount--;          /* releasing a reader */
else if (rw->rw_refcount == -1)
rw->rw_refcount = 0;        /* releasing a writer */
else
err_dump("rw_refcount = %d", rw->rw_refcount);

/* 4give preference to waiting writers over waiting readers */
if (rw->rw_nwaitwriters > 0) // writers first-------------
{
if (rw->rw_refcount == 0)
result = pthread_cond_signal(&rw->rw_condwriters);
}
else if (rw->rw_nwaitreaders > 0)
result = pthread_cond_broadcast(&rw->rw_condreaders);

pthread_mutex_unlock(&rw->rw_mutex);
return(result);
}
/* end unlock */

pthread_rwlock_unlock