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 <stdio.h>#include <stdlib.h>#include <unistd.h>#include <pthread.h>#include <errno.h>#include <sys/ipc.h>#include <semaphore.h>#include <fcntl.h>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 <stdio.h>先#include <stdlib.h>#include <unistd.h>#include <pthread.h>#include <errno.h>#include <sys/ipc.h>#include <semaphore.h>#include <fcntl.h>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");}