Linux读者和写者（写者优先）

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 
#include 
#include 
#include

#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;
}