Editing Sleeping barber problem (section)

==Solutions==
There are several possible solutions, but all solutions require a [[mutex]], which ensures that only one of the participants can change state at once. The barber must acquire the room status mutex before checking for customers and release it when they begin either to sleep or cut hair; a customer must acquire it before entering the shop and release it once they are sitting in a waiting room or barber chair, and also when they leave the shop because no seats were available. This would take care of both of the problems mentioned above. A number of [[semaphore (programming)|semaphores]] is also required to indicate the state of the system. For example, one might store the number of people in the waiting room.

=== Implementation ===
The following [[pseudocode]] guarantees synchronization between barber and customer and is [[Deadlock (computer science)|deadlock]] free, but may lead to [[Resource starvation|starvation]] of a customer. The problem of starvation can be solved with a [[FIFO (computing and electronics)|first-in first-out (FIFO)]] [[Queue (abstract data type)|queue]]. The [[Semaphore (programming)|semaphore]] would provide two functions: <code>wait()</code> and <code>signal()</code>, which in terms of [[C (programming language)|C code]] would correspond to <code>P()</code> and <code>V()</code>, respectively.{{citation needed|date=January 2022}}

<syntaxhighlight lang="python">
# The first two are mutexes (only 0 or 1 possible)
Semaphore barberReady = 0
Semaphore accessWRSeats = 1     # if 1, the number of seats in the waiting room can be incremented or decremented
Semaphore custReady = 0         # the number of customers currently in the waiting room, ready to be served
int numberOfFreeWRSeats = N     # total number of seats in the waiting room

def Barber():
  while true:                   # Run in an infinite loop.
    wait(custReady)             # Try to acquire a customer - if none is available, go to sleep.
    wait(accessWRSeats)         # Awake - try to get access to modify # of available seats, otherwise sleep.
    numberOfFreeWRSeats += 1    # One waiting room chair becomes free.
    signal(barberReady)         # I am ready to cut.
    signal(accessWRSeats)       # Don't need the lock on the chairs anymore.
    # (Cut hair here.)

def Customer():
  while true:                   # Run in an infinite loop to simulate multiple customers.
    wait(accessWRSeats)         # Try to get access to the waiting room chairs.
    if numberOfFreeWRSeats > 0: # If there are any free seats:
      numberOfFreeWRSeats -= 1  #   sit down in a chair
      signal(custReady)         #   notify the barber, who's waiting until there is a customer
      signal(accessWRSeats)     #   don't need to lock the chairs anymore
      wait(barberReady)         #   wait until the barber is ready
      # (Have hair cut here.)
    else:                       # otherwise, there are no free seats; tough luck --
      signal(accessWRSeats)     #   but don't forget to release the lock on the seats!
      # (Leave without a haircut.)
</syntaxhighlight>