学步园

      理发店里有一位理发师、一把理发椅和n把供等候理发的顾客坐的椅子。如果没有顾客，理发师在理发椅上睡觉。一个顾客来到时，叫醒理发师，如果理发师正在理发时有顾客到来，则如果有空椅子可以坐，就坐下来等待，否则离开。

      利用三个信号量和一个控制变量来协调理发师、理发椅和顾客之间的活动。

  1.信号量customers 用来记录等候理发的顾客数，并用坐阻塞理发师进程，初值为0

  2.信号量barbers 记录正在等候顾客的理发师数

  3.信号量waiting 用来记录等候理发的顾客数

  4.信号量mutex 用于互斥，初值为1

/*基于信号量采用多线程技术实现进程同步*/<br /> #include <pthread.h><br /> #include <stdio.h><br /> #include <unistd.h><br /> #include <stdlib.h><br /> #include <semaphore.h><br /> #include <sys/time.h><br /> #include <math.h><br /> #define CHAIRS 5 //椅子数<br /> sem_t customers; //等待服务的顾客信号量<br /> sem_t barbers; //等待顾客的理发师信号量<br /> pthread_mutex_t mutex; //互斥变量<br /> int waiting = 0; //正在等待的顾客数<br /> void *barber(void *arg);<br /> void *customer(void *num);<br /> void cut_hair(void);<br /> double timediff(struct timeval i,struct timeval j);<br /> void seed_random(void);<br /> double flat(void);<br /> double normal(void);<br /> double bursty(void);<br /> int main()<br /> {<br /> int i;<br /> seed_random();<br /> pthread_t barber_t,customer_t;<br /> int error;<br /> error=pthread_create(&barber_t,NULL,barber,NULL);//创建理发师线程<br /> if(error!=0) {<br /> printf("pthread_create is not created.../n");<br /> return -1;<br /> }<br /> while(1) {<br /> usleep(30000);//等待时间如果小于理发师理发时间则会出现等待者过多,否则不会出现等待者过多的现象<br /> error=pthread_create(&customer_t,NULL,customer,NULL);//创建顾客线程<br /> if(error!=0) {<br /> printf("pthread_create is not created.../n");<br /> return -1;<br /> }<br /> }<br /> }<br /> double timediff(struct timeval now,struct timeval earlier)<br /> {<br /> if(now.tv_sec == earlier.tv_sec)<br /> return (now.tv_usec - earlier.tv_usec)/1000000.0;<br /> else<br /> return (1000000*(now.tv_sec - earlier.tv_sec) + now.tv_usec - earlier.tv_usec)/1000000.0;<br /> }<br /> void *barber(void *arg)<br /> {<br /> while(1)<br /> {<br /> sem_wait(&customers);//顾客信号量-1<br /> pthread_mutex_lock(&mutex);<br /> waiting = waiting -1;<br /> sem_post(&barbers);//<br /> pthread_mutex_unlock(&mutex);<br /> cut_hair();//理发<br /> }<br /> }<br /> void cut_hair(void)<br /> {<br /> printf(" Barber:I am cutting the customer's hair.../n");<br /> usleep(100000);//理发时间<br /> printf(" Barber:done./n");<br /> }<br /> void *customer(void *num)<br /> {<br /> pthread_mutex_lock(&mutex);<br /> if(waiting<CHAIRS)<br /> {<br /> waiting = waiting + 1;<br /> sem_post(&customers);<br /> pthread_mutex_unlock(&mutex);<br /> sem_wait(&barbers);<br /> }<br /> else<br /> {<br /> printf(" Waiter is too much.../n");<br /> pthread_mutex_unlock(&mutex);<br /> }<br /> //释放占用的资源<br /> }<br /> void seed_random(void)<br /> {<br /> struct timeval randtime;<br /> unsigned short xsub1[3];<br /> gettimeofday(&randtime,(struct timezone *)0);<br /> xsub1[0] = (ushort)randtime.tv_usec;<br /> xsub1[1] = (ushort)(randtime.tv_usec >> 16);<br /> xsub1[2] = (ushort)(getpid());<br /> seed48(xsub1);<br /> }<br /> double flat()<br /> {<br /> return drand48()/5;<br /> }