foil for Exercise 11-6 semaphore solution to sleeping barber problem

waiting : integer :=0; %customers waiting to be cut

guard : semaphore :=1; % to delimit a critical region to protect waiting

customers : semaphore:= 0; %counting semaphore of customers

barber : semaphore :=0; %is barber waiting for a customer (1) or not (0)

WAIT(customers); %sleeps if there are none

WAIT (guard);

waiting := waiting - 1; %otherwise, changes waiting under exclusion

SIGNAL(barber); % and indicates his readiness to cut hair

SIGNAL (guard);

cut hair

WAIT (guard); %test and set waiting under exclusion

if waiting < chairs % if there is a free chair to sit and wait

then { waiting := waiting+1;

SIGNAL(customers) % indicate one more customer

SIGNAL(guard) % release the exclusion on waiting

WAIT(barber); % use the barber resource

have haircut; }

else SIGNAL(guard); % if there are no free chairs just release

% the exclusion on waiting and go away.