读写锁

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！
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！

声明

//初始化：
int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, const pthread_rwlockattr_t *restrict attr);

//读写加锁
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);

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

int pthread_rwlock_timedrdlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abs_timeout);
int pthread_rwlock_timedwrlock(pthread_rwlock_t *restrict rwlock, const struct timespec *restrict abs_timeout);

//销毁锁
int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);

例子:

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

/* 初始化读写锁 */
pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
//pthread_rwlock_t rwlock;
/* 全局资源 */
int global_num = 10;

void err_exit(const char *err_msg)
{
 printf("error:%s\n", err_msg);
 exit(1);
}

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

 while (1)
 {
     /* 读加锁 */
     pthread_rwlock_rdlock(&rwlock);

     printf("线程%s进入临界区，global_num = %d\n", pthr_name, global_num);
     sleep(1);
     printf("线程%s离开临界区...\n", pthr_name);

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

     sleep(1);
 }

 return NULL;
}

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

 while (1)
 {
     /* 写加锁 */
     pthread_rwlock_wrlock(&rwlock);

     /* 写操作 */
     global_num++;
     printf("线程%s进入临界区，global_num = %d\n", pthr_name, global_num);
     sleep(1);
     printf("线程%s离开临界区...\n", pthr_name);

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

     sleep(2);
 }

 return NULL;
}

int main(void)
{
 pthread_t tid_read_1, tid_read_2, tid_write_1, tid_write_2;
	pthread_rwlock_init(&rwlock, NULL);
 /* 创建4个线程，2个读，2个写 */
 if (pthread_create(&tid_read_1, NULL, thread_read_lock, "read_1") != 0)
     err_exit("create tid_read_1");

 if (pthread_create(&tid_read_2, NULL, thread_read_lock, "read_2") != 0)
     err_exit("create tid_read_2");

 if (pthread_create(&tid_write_1, NULL, thread_write_lock, "write_1") != 0)
     err_exit("create tid_write_1");

 if (pthread_create(&tid_write_2, NULL, thread_write_lock, "write_2") != 0)
     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) != 0)
     err_exit("pthread_join()");

 return 0;
}