线程同步-读者写者问题（多线程）

任务描述：
0 推荐在openEuer上实现
1 描述操作系统中“读者-写者”问题，理解问题的本质，提交你理解或查找到的文本资料
2 利用多线程完成reader 和writer
3 在main中测试若干个reader 和writer的测试，提交截图说明代码的正确性

如果一个进程正在某个文件上写入内容，而另一个进程也同时开始在同一文件上写入内容，则系统将进入不一致状态。在特定的时间仅应允许一个进程更改文件中存在的数据的值。
另一个问题是，如果一个进程正在读取文件，而另一个进程同时在同一文件上写入，则可能会导致读取不干净，因为在该文件上写入的进程会更改文件的值，但是读取该文件的过程将读取文件中存在的旧值。因此，应避免这种情况。

1.问题描述

有两组并发进程：读者和写者，共享一个文件F，要求:

- 允许多个读者同时执行读操作
- 任一写者在完成写操作之前不允许其它读者或写者工作
- 写者执行写操作前，应让已有的写者和读者全部退出

2.问题分析

- 共享数据（文件、记录）由Reader和Writer是两组并发进程共享
- Reader和Writer两组并发进程
- 同组的Reader进程可同时对文件进行读，不用互斥
- 写-写互斥
- 读-写互斥

对于线程，有以下流程：定义--创建初始化-线程运行函数--线程退出

对于信号量，有以下流程：定义--创建初始化--信号量wait和signal--信号量销毁


代码实现
实现多读者（多线程）操作

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <semaphore.h>
#include <unistd.h>
 
sem_t rmutex,wmutex;
static void *readerThread(void *arg);
static void *reader3Thread(void *arg);
static void *reader2Thread(void *arg);
static void *writerThread(void *arg);
int readcount = 0;
int n = 0;
int nowLen = 1;
char contentArticle[10][100];
int main(){
 
    pthread_t readerTidp,writerTidp,reader3Tidp,reader2Tidp;
    void *retval;
    
    if(sem_init(&rmutex,0,1)==-1||sem_init(&wmutex,0,1)==-1){
        printf("sem_init error\n");
        return -1;
    }//init semaphore
 
    if(pthread_create(&readerTidp,NULL,readerThread,NULL) !=0||pthread_create(&writerTidp,NULL,writerThread,NULL) !=0||pthread_create(&reader3Tidp,NULL,reader3Thread,NULL) !=0||pthread_create(&reader2Tidp,NULL,reader2Thread,NULL) !=0){
        printf("pthread_create error\n");
        return -2;
    }//init pthread
 
    pthread_join(readerTidp,&retval);
    pthread_join(reader3Tidp,&retval);
    pthread_join(reader2Tidp,&retval);
    pthread_join(writerTidp,&retval);
 
    sem_destroy(&rmutex);
    sem_destroy(&wmutex);
    return 0;
}
 
static void *readerThread(void *arg){
    for(int i = 0;i < 10;i++)
    {
        sem_wait(&rmutex);
        if(readcount == 0)sem_wait(&wmutex);
        readcount = readcount+1;
        sem_post(&rmutex);
    
        //read operatiom
        printf("\n\nI'm reader first Reader thread :...the global variable  n equals to %d\n",n);
        for(int j = 0;j < nowLen-1;j++)
        {
            for(int k = 0;k < 26;k++)
                printf("%c",contentArticle[j][k]);
            printf("\n");
        }
            printf("now the count 0f reader is %d\n",readcount);
        printf("now the length 0f content is %d\n",nowLen-1);
        sleep(5);
 
        sem_wait(&rmutex);
        readcount = readcount-1;
        if(readcount == 0)sem_post(&wmutex);
        sem_post(&rmutex);
        sleep(1);
    }
}
 
static void *reader3Thread(void *arg){
        for(int i = 0;i < 10;i++)
        {
                sem_wait(&rmutex);
                if(readcount == 0)sem_wait(&wmutex);
                readcount = readcount+1;
                sem_post(&rmutex);
 
                //read operatiom
                printf("\n\nI'm reader third  Reader thread :...the global variable  n equals to %d\n",n);
        for(int j = 0;j < nowLen-1;j++)
                {
                      for(int k = 0;k < 26;k++)
                                printf("%c",contentArticle[j][k]);
                        printf("\n");
                }
                printf("now the count 0f reader is %d\n",readcount);
                printf("now the length 0f content is %d\n",nowLen-1);
 
        sleep(5);
                sem_wait(&rmutex);
                readcount = readcount-1;
                if(readcount == 0)sem_post(&wmutex);
                sem_post(&rmutex);
                sleep(8);
        }
}
 
 
static void *reader2Thread(void *arg){
        for(int i = 0;i < 10;i++)
        {
                sem_wait(&rmutex);
                if(readcount == 0)sem_wait(&wmutex);
                readcount = readcount+1;
                sem_post(&rmutex);
 
                //read operatiom
                printf("\n\nI'm reader second Reader thread :...the global variable  n equals to %d\n",n);
        for(int j = 0;j < nowLen-1;j++)
              {
                        for(int k = 0;k < 26;k++)
                                printf("%c",contentArticle[j][k]);
                        printf("\n");
                }
 
                printf("now the count 0f reader is %d\n",readcount);
                printf("now the length 0f content is %d\n",nowLen-1);
 
 
                sem_wait(&rmutex);
                readcount = readcount-1;
                if(readcount == 0)sem_post(&wmutex);
                sem_post(&rmutex);
                sleep(4);
        }
}
 
 
 
static void *writerThread(void *arg){
        for(int i = 0;i < 10;i++)
        {
                sem_wait(&wmutex);
              
               //writer operation 
               n = n+1;
        for(int k = 0;k < 26;k++)
        contentArticle[nowLen-1][k] = 'z'-k;
        nowLen++;
        printf("\n\nWriter thread :writing opration the global variable  n equals to  %d \n",n);
                sleep(2);
               sem_post(&wmutex);
                sleep(3);
        }
}