信号量实现理发师问题

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