linux读写锁

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！

读写锁的优势往往展现在读操作很频繁，而写操作较少的情况下

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

#include <pthread.h>
int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);

应用实例：

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：

 /**
  *  * @file pthread_rwlock.c
  *   */

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 #include <pthread.h>
 using namespace std;
 /* 初始化读写锁 */
 pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
 /* 全局资源 */
 class CSingle{
 public:
   static CSingle& instance(){
     static CSingle single;
     return single;
   }
   void setX(int y){
     this->x = y;
   }
   int getX(){
     return this->x;
   }
   int x;
 };
 int global_num = ;

 void err_exit(const char *err_msg)
 {
     printf("error:%s\n", err_msg);
     exit();
 }

 /* 读锁线程函数 */
 void *thread_read_lock(void *arg)
 {
     char *pthr_name = (char *)arg;

     while (global_num)
     {
         /* 读加锁 */
         pthread_rwlock_rdlock(&rwlock);

         printf("线程%s进入临界区，global_num = %d, X:%d\n", pthr_name, global_num, CSingle::instance().getX());
         sleep();
         printf("线程%s离开临界区...\n", pthr_name);

         /* 读解锁 */
         pthread_rwlock_unlock(&rwlock);

         sleep();
     }

     return NULL;
 }

 /* 写锁线程函数 */
 void *thread_write_lock(void *arg)
 {
     char *pthr_name = (char *)arg;

     while (global_num)
     {
         /* 写加锁 */
         pthread_rwlock_wrlock(&rwlock);

         /* 写操作 */
         --global_num;
     CSingle::instance().setX(global_num);
         printf("线程%s进入临界区，global_num = %d, X:%d\n", pthr_name, global_num, CSingle::instance().getX());
         sleep();
         printf("线程%s离开临界区...\n", pthr_name);

         /* 写解锁 */
         pthread_rwlock_unlock(&rwlock);

         sleep();
     }

     return NULL;
 }

 int main(void)
 {
     pthread_t tid_read_1, tid_read_2, tid_write_1, tid_write_2;

     /* 创建4个线程，2个读，2个写 */
     if (pthread_create(&tid_read_1, NULL, thread_read_lock, (void *)"read_1") != )
         err_exit("create tid_read_1");

     if (pthread_create(&tid_read_2, NULL, thread_read_lock, (void *)("read_2")) != )
         err_exit("create tid_read_2");

     if (pthread_create(&tid_write_1, NULL, thread_write_lock, (void *)("write_1")) != )
         err_exit("create tid_write_1");

     if (pthread_create(&tid_write_2, NULL, thread_write_lock, (void *)("write_2")) != )
         err_exit("create tid_write_2");

     /* 随便等待一个线程，防止main结束 */
     if (pthread_join(tid_read_1, NULL) != )
         err_exit("pthread_join()");
     if (pthread_join(tid_read_2, NULL) != )
         err_exit("pthread_join()");
     if (pthread_join(tid_write_1, NULL) != )
         err_exit("pthread_join()");
     if (pthread_join(tid_write_2, NULL) != )
         err_exit("pthread_join()");

     return ;
 }

linux读写锁