Linux读者和写者（写者优先）

太过爱你忘了你带给我的痛 2022-02-19 18:57 259阅读 0赞

## 写者优先 ##

写者优先与读者优先的不同是：如果读者来，有写者等待，但有其他读者正在读，新读者等。

使用两个互斥锁mutex\_write，mutex\_read和两个信号量sem\_read，sem\_write来确保访问资源的互斥和同步。

#include <stdio.h>
    #include <stdlib.h>
    #include <pthread.h>
    #include<semaphore.h>
    
    #define N_WRITER 5 //写者数目
    #define N_READER 20 //读者数目
    #define W_SLEEP 1 //控制写频率
    #define R_SLEEP  0.5 //控制读频率
    
    
    pthread_t wid[N_WRITER],rid[N_READER];
    int data = 0;
    int readerCnt = 0, writerCnt = 0;
    pthread_mutex_t sem_read;
    pthread_mutex_t sem_write;
    pthread_mutex_t mutex_write;
    pthread_mutex_t mutex_read;
    
    void write()
    {
        int rd = rand();
        printf("write %d\n",rd);
        data = rd;
    }
    void read()
    {
        printf("read %d\n",data);
    }
    void * writer(void * in)
    {
    //    while(1)
    //    {
            sem_wait(&sem_write);
            {//临界区,希望修改 writerCnt,独占 writerCnt
                writerCnt++;
                if(writerCnt == 1){
                    //阻止后续的读者加入待读队列
                    pthread_mutex_lock(&mutex_read);
                }
            }
            sem_post(&sem_write);
            
            
            pthread_mutex_lock(&mutex_write);
            {//临界区，限制只有一个写者修改数据
                printf("写线程id%d进入数据集\n",pthread_self());
                write();
                printf("写线程id%d退出数据集\n",pthread_self());        
            }
            pthread_mutex_unlock(&mutex_write);
            
            sem_wait(&sem_write);
            {//临界区,希望修改 writerCnt,独占 writerCnt
                writerCnt--;
                if(writerCnt == 0){
                    //阻止后续的读者加入待读队列
                    pthread_mutex_unlock(&mutex_read);
                }
            }
            sem_post(&sem_write);
            sleep(W_SLEEP);
    //    }
        pthread_exit((void *) 0);
    }
    
    void * reader (void * in)
    {
    //    while(1)
    //    {
            //假如写者锁定了mutex_read,那么成千上万的读者被锁在这里
                pthread_mutex_lock(&mutex_read);//只被一个读者占有
                {//临界区
                    sem_wait(&sem_read);//代码段 1
                    {//临界区
                        readerCnt++;
                        if(readerCnt == 1){
                            pthread_mutex_lock(&mutex_write);
                        }
                    }
                    sem_post(&sem_read);
                }
                pthread_mutex_unlock(&mutex_read);//释放时,写者将优先获得mutex_read
            printf("读线程id%d进入数据集\n",pthread_self());
            read();
            printf("读线程id%d退出数据集\n",pthread_self());
            sem_wait(&sem_read);//代码段2
            {//临界区
                readerCnt--;
                if(readerCnt == 0){
                    pthread_mutex_unlock(&mutex_write);//在最后一个并发读者读完这里开始禁止写者执行写操作
                }
            }
            sem_post(&sem_read);
            
            sleep(R_SLEEP);
    //    }
        pthread_exit((void *) 0);
    }
    
    int main()
    {
        printf("多线程,写者优先\n");
        pthread_mutex_init(&mutex_write,NULL);
        pthread_mutex_init(&mutex_read,NULL);
        sem_init(&sem_write,0,1);
        sem_init(&sem_read,0,1);
        int i = 0;
        for(i = 0; i < N_READER; i++)
        {
            pthread_create(&rid[i],NULL,reader,NULL);
        }
        for(i = 0; i < N_WRITER; i++)
        {
            pthread_create(&wid[i],NULL,writer,NULL);
        }
        sleep(1);    
        return 0;
    }