使用读写锁解决读者-写者问题

读写锁

读写锁适合于对数据结构的读次数比写次数多得多的情况.因为,读模式锁定时可以共享,以写 模式锁住时意味着独占,所以读写锁又叫共享-独占锁.

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. 同互斥量以上,在释放读写锁占用的内存之前,需要先通过 pthread_rwlock_destroy对读写锁进行清理工作, 释放由init分配的资源.

读和写:

#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.这3个函数分别实现获取读锁,获取写锁和释放锁的操作.获 取锁的两个函数是阻塞操作,同样,非阻塞的函数为:

#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.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.

代码:

/*********    说明:

*********    读写锁适合于对数据结构的读次数比写次数多得多的情况.因为,

********     读模式锁定时可以共享,以写 模式锁住时意味着独占,所以读写锁又叫共享-独占锁.

********     要让读者与写者之间、以及写者与写者之问要互斥地访同数据集；

*********/



#include <pthread.h>

#include <signal.h>

#include <stdio.h>

//#include "apue.h"



#define R 5 // reader NO.

#define W 5 // reader and writer NO.



pthread_rwlock_t lock; //it's mean writer can writing



int readCount = 0;



void* reader(void *arg)

{

    int n = W;

    int id = *(int*)arg;

    while (n--)

    {

        sleep( rand() % 3 );

        pthread_rwlock_rdlock(&lock);

        printf("reader %d is reading\n", id);

        sleep( rand() % 3 );



        pthread_rwlock_unlock(&lock);

        printf("reader %d is leaving\n", id);

    }

    printf("----reader %d has done----\n", id);

}



void* writer(void *arg)

{

    int n = W;

    while (n--)

    {

        sleep( rand() % 3 );

        pthread_rwlock_wrlock(&lock);

        printf("\twriter is writing\n");

        sleep( rand() % 3 );

        pthread_rwlock_unlock(&lock);

        printf("\twriter is leaving\n");

    }

    printf("----writer has done----\n");

}



int main(int argc, const char *argv[])

{

    int err;

    pthread_t tid[R], writerTid;

    int i;



    err = pthread_create(&writerTid, NULL, writer, (void *)NULL);

    if (err != 0)

    {

        printf("can't create process for writer\n");

    }



    pthread_rwlock_init(&lock, NULL);

    for (i = 0; i < R; i++)

    {

        err = pthread_create(&tid[i], NULL, reader, &i);

        if (err != 0)

        {

            printf("can't create process for reader");

        }

    }

    while(1);

    pthread_rwlock_destroy(&lock);

    return 0;

}