barber

理髮店裏有一位理髮師、一把理髮椅和n把供等候理髮的顧客坐的椅子。如果沒有顧客，理髮師在理髮椅上睡覺。一個顧客來到時，叫醒理髮師，如果理髮師正在理髮時有顧客到來，則如果有空椅子可以坐，就坐下來等待，否則離開。

      利用三個信號量和一個控制變量來協調理髮師、理髮椅和顧客之間的活動。

  1.信號量customers 用來記錄等候理髮的顧客數，並用坐阻塞理髮師進程，初值爲0

  2.信號量barbers 記錄正在等候顧客的理髮師數

  3.信號量waiting 用來記錄等候理髮的顧客數

  4.信號量mutex 用於互斥，初值爲1

/*基於信號量採用多線程技術實現進程同步*/
#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 //椅子數
sem_t customers; //等待服務的顧客信號量
sem_t barbers;  //等待顧客的理髮師信號量
pthread_mutex_t mutex; //互斥變量
int waiting = 0; //正在等待的顧客數
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);//創建理髮師線程
   if(error!=0) {
      printf("pthread_create is not created.../n");
      return -1;
   }
   while(1) {
      usleep(30000);//等待時間如果小於理髮師理髮時間則會出現等待者過多,否則不會出現等待者過多的現象
      error=pthread_create(&customer_t,NULL,customer,NULL);//創建顧客線程
      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);//顧客信號量-1
      pthread_mutex_lock(&mutex);
      waiting = waiting -1;
      sem_post(&barbers);//
      pthread_mutex_unlock(&mutex);
      cut_hair();//理髮
   }
}
void cut_hair(void)
{
   printf("  Barber:I am cutting the customer's hair.../n");
   usleep(100000);//理髮時間
   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);
   }
   //釋放佔用的資源
}
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;
}