Editing Sleeping barber problem (section)

=== 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>