20201317 读者-写者（多线程）问题的思考与研究

操作系统中"读者-写者"问题

1. OS“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

1、允许多个读者可以同时对文件执行读操作。

2、只允许一个写者往文件中写信息。

3、任一写者在完成写操作之前不允许其他读者或写者工作。

4、写者执行写操作前，应让已有的读者和写者全部退出。

semaphore rw = 1;   //实现对文件的互斥访问
int count = 0;
semaphore mutex = 1;//实现对count变量的互斥访问
int i = 0;
writer(){
    while(1){
        P(rw); //写之前“加锁”
        写文件
        V(rw); //写之后“解锁”
    }
}
reader (){
    while(1){
        P(mutex);     //各读进程互斥访问count
        if(count==0)  //第一个读进程负责“加锁”
        {
            P(rw);
        }
        count++;      //访问文件的进程数+1
        V(mutex);
        读文件
        P(mutex);     //各读进程互斥访问count
        count--;      //访问文件的进程数-1
        if(count==0)  //最后一个读进程负责“解锁”
        {
            V(rw);
        }
        V(mutex);
    }
}

利用多线程完成reader和writer

/*
* 	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;
}

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