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.
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
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_rdlock(pthread_rwlock_t *rw) { int result; if (rw->rw_magic != RW_MAGIC) // 判断是否初始化了 return(EINVAL); if ( (result = pthread_mutex_lock(&rw->rw_mutex)) != 0) return(result); /* 4give preference to waiting writers */ /** * rw->rw_refcount < 0 * 写者持有读写锁 * * rw->rw_nwaitwriters > 0 (rw->rw_refcount >= 0) * 有写者等待 */ while (rw->rw_refcount < 0 || rw->rw_nwaitwriters > 0) { rw->rw_nwaitreaders++; result = pthread_cond_wait(&rw->rw_condreaders, &rw->rw_mutex); rw->rw_nwaitreaders--; if (result != 0) break; } if (result == 0) rw->rw_refcount++; /* another reader has a read lock */ pthread_mutex_unlock(&rw->rw_mutex); return (result); } /* end rdlock */
int pthread_rwlock_wrlock(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); /* 读写锁不可用 */ while (rw->rw_refcount != 0) { rw->rw_nwaitwriters++; result = pthread_cond_wait(&rw->rw_condwriters, &rw->rw_mutex); rw->rw_nwaitwriters--; if (result != 0) break; } if (result == 0) rw->rw_refcount = -1; pthread_mutex_unlock(&rw->rw_mutex); return(result); } /* end wrlock */
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 */