Operating System: Synchronization exclusive - barber sleep problems

Problem Description

One barber barber shop, a barber chair and N to wait for customers to sit in a chair. 
If there are no customers in the barber chair barber sleep; 
when there is a customer come when he must wake barber; 
if a customer getting a haircut to the barber, if empty chair, sit down and wait, or else leave. 
With P, V and sync operations to solve the aforementioned problems in a mutually exclusive relationship.

 

 

analysis

Customers will be treated as N producers, is a barber consumers. 
Barber chair and resources are critical, it is a mutually exclusive relationship between the customer; 
barber and customer relations are synchronized.

 

Semaphore set

1、 Semaphore barberReady = 0    互斥量，只能取0或1  
2、 Semaphore accessSeat  = 1    互斥量，如果为1，表明椅子数可以增加或减少，
                                               相当于给椅子加锁，避免两个顾客同时坐一把椅子
3、 Semaphore num_wait     = 0   坐在椅子上等待的顾客数
4、 int  seat_free = n           空着的椅子数目

 

/*顾客进程*/
void customer()
{    
    while(true)
    {
        P(accessSeat);      //  获得对椅子操作的权限
        if(seat_free > 0) {
            seat_free--;    //  获得椅子
            V(num_wait);    //  试图唤醒理发师，
            V(accessSeat);  //  不用再锁着椅子
            P(baberReady);  //  等待理发师ready

            理发;

        }
        else {
             V(accessSeat);  //释放加在椅子上的锁
             离开;
        }
    }
}

/*理发师进程*/
void barber(){
     while(true){
        P(num_wait);      //尝试获得一位顾客，如果没有，去睡觉
        P(accessSeat);    //尝试获得椅子锁，更改空闲椅子数目
        seat_free++;      //空椅子加1
        V(baberReady);    //理发师准备好了
        V(accessSeat);    //无需继续锁着椅子

        理发;

    }
}