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:
- 读者、读者可以同时存在;
- 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;
- 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;
}