William Stallings 《操作系统内核和设计原理》书中Linux下C语言实现读者写者问题(写者优先)代码

代码能跑，不过实在是观察不出来什么读者优先，或者写者优先。不知道这种优先级的冲突的场景从何而来，也就不知道书上讲得那些情况到底是什么。

特别是在下面代码的writer中，如果引入 sem_wait(&z)的话，writer进程会一直阻塞，从而引起读者也阻塞。去掉后，呈现出读者优先的状况。

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

void * reader(void *) ;
void *writer (void *) ;

sem_t x,y,z,wsem,rsem ;
int readcount=0 ;
int writecount=0 ;

main()
{
    int a=1,b=1;
    system("clear");
    sem_init(&wsem,0,1) ;
    sem_init(&x,0,1) ;
    sem_init(&rsem,0,1) ;
    sem_init(&y,0,1) ;
    sem_init(&z,0,1) ;

    pthread_t r1,r2,r3;
    pthread_t w1,w2,w3,w4,w5 ;

    pthread_create(&r1,NULL,reader,(void *)a);
    a++;
    pthread_create(&r2,NULL,reader,(void *)a);
    a++;
    
    pthread_create(&w1,NULL,writer,(void *)b);
    b++;
    pthread_create(&w2,NULL,writer,(void *)b);
    b++;
    
    pthread_create(&r3,NULL,reader,(void *)a);
    a++;
    pthread_create(&w3,NULL,writer,(void *)b);
    b++;
    
    pthread_create(&w4,NULL,writer,(void *)b);
    b++;
    pthread_create(&w5,NULL,writer,(void *)b); //add a reader
    b++;    
    

    printf("main begin join \n");
    
    pthread_join(r1,NULL);
    pthread_join(r2,NULL);
    pthread_join(w1,NULL);
    pthread_join(w2,NULL);
    pthread_join(r3,NULL);
    pthread_join(w3,NULL) ;
    pthread_join(w4,NULL);
    pthread_join(w5,NULL);    
    
    printf("main end join\n");
    
    sleep(30);
    
    printf("main terminated\n");
}

void * reader(void * arg)
{
    
    int c=(int)arg ;
    printf("\nreader %d is created",c);
    //sleep(1);
    
    //sem_wait(&z) ;
    {
        sem_wait(&rsem) ;
        {
            sem_wait(&x) ;
            readcount++;
            if( readcount == 1){
                sem_wait(&wsem) ;
            } 
            sem_post(&x) ;
        }
        sem_post(&rsem) ;
    }
    //sem_wait(&z) ;
    
    //sleep(1);

    /*Critcal Section */

    printf("\nreader %d is reading\n ",c);
    sleep(10) ;

    /* critical section completd */
    sem_wait(&x) ;
    readcount-- ;
    if(readcount==0)
        sem_post(&wsem) ;
    printf("\nreader %d finished reading,readCount=%d\n",c,readcount);    
    sem_post(&x) ;
}

void * writer(void * arg)
{
    int c=(int)arg ;
    printf("\n--------------------writer %d is created",c);
    sleep(5);

    sem_wait(&y) ;
    writecount++;
    if( writecount == 1){
        sem_wait(&rsem) ;
    } 
    sem_post(&y) ;
                
    sem_wait(&wsem) ;
    printf("\n--------------------writer %d is writing\n",c) ;
    sleep(1);
    sem_post(&wsem) ;
    
    sem_wait(&y) ;
    writecount--;
    printf("\n--------------------writer %d finished writing,writecount=%d\n",c,writecount);
    if( writecount == 0){
        sem_post(&rsem) ;
    } 
    sem_post(&y) ;    
    
}

编译方法 ：  gcc -o pvrww pv-posix-reader-writer.c -lpthread

这里采用posix的信号量机制。

下面修改代码，能达到写者优先的目的：只要有产生了一个写者线程，那么正在运行的读者进程会在执行结束后，立刻被终止调度，让写者

先

运行。

#include 先
#include 
#include 
#include 
#include 
#include 
#include 
#include 

sem_t x,y,z,wsem,rsem ;
int readcount=0 ;
int writecount=0 ;

void * reader(void * arg)
{
    
    int c= *(int *)arg;
    printf("\nreader %d is created",c);
    
    sem_wait(&z) ;
        sem_wait(&rsem) ;
            sem_wait(&x) ;
            readcount++;
            printf("\n-------------readcount=%d----- ",readcount);
            if( readcount == 1){
                
                sem_wait(&wsem) ;
            }
            sem_post(&x) ;
        sem_post(&rsem) ;
    sem_wait(&z) ;

    /*Critcal Section */

    printf("\nreader %d is reading\n ",c);
    
    sleep(10);    

    /* critical section completd */
    sem_wait(&x) ;
        readcount-- ;
        if(readcount==0)
            sem_post(&wsem) ;
        printf("\nreader %d finished reading,readCount=%d\n",c,readcount);    
    sem_post(&x) ;
    pthread_exit(0);
}

void * writer(void * arg)
{
    int c= *(int *)arg;
    printf("\n--------------------writer %d is created",c);
    //sleep(5);

    sem_wait(&y) ;
        writecount++;
        printf("\n--------------------writer %d is created,and writecount ==%d",writecount);
        
        if( writecount == 1){
            if( readcount <= 1 ){
                sem_post(&z) ;
                sem_post(&rsem) ;
            }else{
                sem_wait(&rsem) ;
            }
        } 
    sem_post(&y) ;
    
    sem_wait(&wsem) ;
        printf("\n--------------------writer %d is writing\n",c) ;
        sleep(2);
    sem_post(&wsem) ;
    
    sem_wait(&y) ;
        writecount--;
        printf("\n--------------------writer %d finished writing,writecount=%d\n",c,writecount);
        if( writecount == 0){
            sem_post(&rsem) ;
        } 
    sem_post(&y) ;   
    
    pthread_exit(0); 
}

main()
{
    int a=1,b=1;

    sem_init(&wsem,0,1) ;
    sem_init(&x,0,1) ;
    
    sem_init(&rsem,0,1) ;
    sem_init(&y,0,1) ;
    sem_init(&z,0,1) ;
    
    pthread_t PReader[3];
    pthread_t PWriter[5];
    
    int k =0;
    for(k = 1; k <= 3; k++)        //创建顾客线程
    {
        pthread_create(&(PReader[k-1]),NULL,(void *)reader,&k);
        srand(time(0));
        sleep(rand() % 2 + 1);    //1到3的随机数
    }

    for(k = 1; k <= 5; k++)        //创建顾客线程
    {
        pthread_create(&(PWriter[k-1]),NULL,(void *)writer,&k);
        srand(time(0));
        sleep(rand() % 2 + 1);    //1到3的随机数
    }
     
    for(k = 0; k < 3; k++){
        pthread_join(PReader[k],NULL);      
    }
    
    sleep(30);
      
    for(k = 1; k <= 5; k++){
        pthread_kill(PWriter[k-1],0); 
    }
    
    //sleep(30);
    printf("\nmain terminated\n");
}