读者-写者（多线程）
描述问题：
需满足的条件：
- 1.写进程与写进程之间必须互斥的写入数据（因为如果两个写进程同时对共享数据中的区域A中的数据进行写操作的话，会导致数据错误覆盖的问题）
- 2.写进程与读进程之间必须互斥的访问共享数据（因为写进程与读进程如果同时访问共享数据，可能会导致数据不一致的问题。比如：读进程A想要访问共享数据中的B数据，但是写进程C在读进程A访问B数据之前将B数据进行了更新，这就会导致读进程A读不到它想要读到的数据，从而出现数据不一致问题）
- 3.读进程与读进程之间可以同时访问数据，不需要实现互斥的访问共享数据（因为读进程读数据，并不会像之前的生产者消费者问题中的消费者那样改变数据或者是将数据清空，所以多个读进程可以同时访问共享数据）
思路： - 解决写进程与写进程之间必须互斥的写入数据和写进程与读进程之间必须互斥的访问共享数据两个问题。
#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);
}
}
运行：
