读者-写者（多线程）

问题

一、问题描述
要求：

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。
二、问题分析

读者写者问题最核心的问题是如何处理多个读者可以同时对文件的读操作。

写者优先

截图

代码

# 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;
}

读者优先

截图

代码

# 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 wrt, mutex;
int 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(&mutex);
    readCount++;
    if(readCount == 1) 
        sem_wait(&wrt);
    sem_post(&mutex);

    printf("Thread %d: start reading\n", id);
    /* reading is performed */
    sleep(lastTime);
    printf("Thread %d: end reading\n", id);

    sem_wait(&mutex);
    readCount--;
    if(readCount == 0)
        sem_post(&wrt);
    sem_post(&mutex);
    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(&wrt);

    printf("Thread %d: start writing\n", id);
    /* writing is performed */
    sleep(lastTime);
    printf("Thread %d: end writing\n", id);

    sem_post(&wrt);
    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(&mutex, 0, 1);
    sem_init(&wrt, 0, 1);
    readCount = 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);
        }
    }

    //信号量销毁
    sem_destroy(&mutex);
    sem_destroy(&wrt);

    return 0;
}