Barbershop – First Attempt

(* A barbershop requires synchronization similar to the synchronization necessary in

a computer. In this example there are 50 customers, 3 barbers and one cashier. Only

20 are allowed in the barbershop at one time. There is a sofa for convenience that

seats only 4 customers. The problem with this implementation is when a haircut is

finished, the wrong customer could be signaled to leave which would lead to two

customers sitting in the barber chair – perhaps one sitting on the other’s lap. *)

program barbershop_1;

var max_capacity, sofa, barber_chair, cust_ready, finished, leave_b_chair, payment, receipt,

receipt, coord : semaphore;

procedure customer; procedure barber; procedure cashier;

begin repeat repeat

wait(max_capacity);

enter shop; wait(cust_ready); wait(payment);

wait(sofa); wait(coord); wait(coord);

sit on sofa; cut hair; accept pay;

wait(barber_chair); signal(coord); signal(coord);

get up from sofa; signal(finished); signal(receipt);

signal(sofa); wait(leave_b_chair);

sit in barber chair; signal(barber_chair); until forever;

signal(cust_ready);

wait(finished); until forever;

leave barber chair;

signal(leave_b_chair);

pay;

signal(payment);

wait(receipt);

exit shop;

signal(max_capacity);

end;

begin (* main program executable code *)

max_capacity := 20; sofa := 4; barber_chair := 3; cust_ready := 0; finished := 0;

leave_b_chair := 0; payment := 0 receipt := 0; coord := 3; (* semaphore initializations *)

cobegin

customer; customer; customer … (* 50 times – or 50 customers *)

barber; barber; barber (* 3 barbers *)

cashier; cashier (* 2 cashiers *)

coend;

end.