读者-写者（多线程）

读者-写者（多线程）

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

读、写问题是另一个非常出名的同步问题，常常用来模拟数据库的数据查询和数据修改两种情况问题。也即，
一个数据库允许有多个访问者同时对其进行数据访问，但是对于数据的修改只能有一个。那么如何来控制这种访问多个，而修改只能一个的数据操作方式？这也涉及到线程间通信和同步问题。

数据库中存在的情况为：

1. 读者、读者可以同时存在；
2. 读者、写者不能同时存在，如果读者在前，则写者等待；否则所有读者等待；
3. 写者、写者不能同时存在，先到先做，后到等待。

测试代码：

read.c：

/*
* 	写者优先
*/
 
# include <stdio.h>
# include <stdlib.h>
# include <time.h>
# include <sys/types.h>
# include <pthread.h>
# include <semaphore.h>
# include <string.h>
# include <unistd.h>
 
//semaphores
sem_t RWMutex, mutex1, mutex2, mutex3, wrt;
int writeCount, readCount;
 
 
struct data {
	int id;
	int opTime;
	int lastTime;
};
 
//读者
void* Reader(void* param) {
	int id = ((struct data*)param)->id;
	int lastTime = ((struct data*)param)->lastTime;
	int opTime = ((struct data*)param)->opTime;
 
	sleep(opTime);
	printf("Thread %d: waiting to read\n", id);	
 
	sem_wait(&mutex3);
	sem_wait(&RWMutex);
	sem_wait(&mutex2);
	readCount++;
	if(readCount == 1)
		sem_wait(&wrt);
	sem_post(&mutex2);
	sem_post(&RWMutex);
	sem_post(&mutex3);
 
	printf("Thread %d: start reading\n", id);
	/* reading is performed */
	sleep(lastTime);
	printf("Thread %d: end reading\n", id);
 
	sem_wait(&mutex2);
	readCount--;
	if(readCount == 0)
		sem_post(&wrt);
	sem_post(&mutex2);
 
	pthread_exit(0);
}
 
//写者
void* Writer(void* param) {
	int id = ((struct data*)param)->id;
	int lastTime = ((struct data*)param)->lastTime;
	int opTime = ((struct data*)param)->opTime;
 
	sleep(opTime);
	printf("Thread %d: waiting to write\n", id);
 
	sem_wait(&mutex1);
	writeCount++;
	if(writeCount == 1){
		sem_wait(&RWMutex);
	}
	sem_post(&mutex1);
 
	sem_wait(&wrt);
	printf("Thread %d: start writing\n", id);
	/* writing is performed */
	sleep(lastTime);
	printf("Thread %d: end writing\n", id);
	sem_post(&wrt);
 
	sem_wait(&mutex1);
	writeCount--;
	if(writeCount == 0) {
		sem_post(&RWMutex);
	}
	sem_post(&mutex1);
 
	pthread_exit(0);
}
 
int main() {
	//pthread
	pthread_t tid; // the thread identifier
 
	pthread_attr_t attr; //set of thread attributes
 
	/* get the default attributes */
	pthread_attr_init(&attr);
 
	//initial the semaphores
    sem_init(&mutex1, 0, 1);
    sem_init(&mutex2, 0, 1);
    sem_init(&mutex3, 0, 1);
    sem_init(&wrt, 0, 1);
    sem_init(&RWMutex, 0, 1);
 
    readCount = writeCount = 0;
 
	int id = 0;
	while(scanf("%d", &id) != EOF) {
 
		char role;		//producer or consumer
		int opTime;		//operating time
		int lastTime;	//run time
 
		scanf("%c%d%d", &role, &opTime, &lastTime);
		struct data* d = (struct data*)malloc(sizeof(struct data));
 
		d->id = id;
		d->opTime = opTime;
		d->lastTime = lastTime;
 
		if(role == 'R') {
			printf("Create the %d thread: Reader\n", id);
			pthread_create(&tid, &attr, Reader, d);
 
		}
		else if(role == 'W') {
			printf("Create the %d thread: Writer\n", id);
			pthread_create(&tid, &attr, Writer, d);
		}
	}
 
	sem_destroy(&mutex1);
	sem_destroy(&mutex2);
	sem_destroy(&mutex3);
	sem_destroy(&RWMutex);
	sem_destroy(&wrt);
 
	return 0;
}