Programming Project 3:

Barbers and Gamblers





Due Date: ______________________________

Duration:  One week



Overview and Goal


In this project you will gain additional familiarity writing programs involving concurrency control.  You will implement a couple of classical IPC problems to become more comfortable with Semaphores, Mutexes, Condition Variables, and the Monitor approach.



Download New Files


Start by creating a directory for the files you’ll need for this project:




Then download all the files from:




Even though some of the files have the same names, be sure to get new copies for each project.  In general some files may be modified.


Please keep your old files from previous projects separate and don’t modify them once you submit them.  This is a good rule for all programming projects in all classes.  If there is ever any question about whether code was completed on time, we can always go back and look at the Unix “file modification date” information.


For this project, you should get the following files:

















The file Synch.c contains our solution for project 2.  You may use either our version of this file, or the version of Synch.c you created.


We have also included the file




which contains my solution code for Producer-Consumer and Dining Philosophers.  If you had difficulty with it, you can take a look at the solution code.


You will modify and submit the following files:







Task 1:  Implement the Sleeping Barber Problem


An older edition of the Tanenbaum textbook describes the “Sleeping Barber” problem and gives a solution in “C”.  (This material can also be found in a separate file called SleepingBarberProblem.pdf in the p3 directory.)


Translate this into a working KPL program.  You’ll also need to create some code to print out what happens when you run the program.


What will you do for the “cut_hair” and the “get_haircut” methods?  You’ll need at least one print statement in each routine, but be careful: they both should run at more-or-less the same time.


One barber is okay, but how many customers will you model?  Will you just throw a bunch of customers at the barbershop all at once?  This might not test your code very well.


How long will the haircut last?  You can implement waiting with a busy loop, such as


            for i = 1 to 100

              .. want to yield here?...



The goals are:


(1)  Practice creating a KPL class from scratch

(2)  Practice testing your code

(3)  Gain experience with semaphores, mutex locks, and thread synchronization




Task 2:  The Gaming Parlor Problem


Here is the problem scenario: groups of customers come in to a “gaming parlor” to play games.  They go to the front desk to obtain one or more dice, which are used by the group while they are playing their game, and then returned to the front desk.  The front desk is in charge of lending out the dice and collecting them after each game is finished.


The gaming parlor owns only 8 dice, which are available at the front desk before the first group comes in.


The customers can play the following games.  Listed after each game in parentheses is the number of dice required to play that game.


            Backgammon (4)

            Risk (5)

            Monopoly (2)

            Pictionary (1)


You should model the front desk as a monitor with the following entry methods:


            Request (numberOfDice: int)

            Return (numberOfDice: int)


Model each group of customers as a thread.  When a group is ready to play, it must obtain the necessary number of dice.  If the required number of dice is not available, then the group (i.e., the thread) must wait.  You might use a condition variable that “more dice have become available.”


You should model the following eight different groups.  Each group plays one game, as shown below, but each group plays its game 5 times.  Each group must return their dice after each game and then re-acquire the dice before playing again.


            A – Backgammon

            B – Backgammon

            C – Risk

            D – Risk

            E – Monopoly

            F – Monopoly

            G – Pictionary

            H – Pictionary


To simulate playing the game, simply call the Yield method.


This problem is a generalization of the problem of resource allocation where (1) there are a number of resources (dice) but each is identical; (2) every requesting process needs a one or more units of the resource, (3) each requesting thread knows how many units it will need before requesting any units and that info is included in the request, (4) all units are returned before any further requests are made.


In a monitor, each method is either an “entry” method or a local method.  A monitor will always have one Mutex lock, which is associated with entering the monitor.  Every monitor will also have zero or more condition variables, as required by the particular application.


An entry method must always begin by locking the monitor’s mutex.  Before it returns, it must always unlock the mutex.  Within the entry method, the code may Signal some of the condition variables and may Wait on condition variables.  (In some applications, it may make sense to Broadcast to a condition variable.)  However, once inside of an entry method, the code should never touch the monitor’s mutex lock, or try to look inside the condition variables.


You should model the front desk as a monitor and you should use condition variables instead of semaphores.


If deadlock occurs, the program will freeze up and some requests for dice will go unsatisfied forever.  This is a disaster!  Regardless of the order in which the groups make their requests, your solution should be structured such that deadlock can never occur.


Your solution must not be subject to any race conditions.  In other words, regardless of the order in which the groups make their requests and return their dice, each die must never be allocated to more than one group at a time.  It should never be the case that groups are allowed to proceed when there are too few dice.  Likewise, if a group has returned its dice, other groups which are waiting must be allowed to proceed once enough dice have become available.


Starvation can occur if it is possible that one thread can be delayed infinitely by other threads’ requests.  If the game parlor problem is extended to assume that each group will continue to play game after game forever, then starvation might be possible, if you are not careful in your solution.  If some group X comes in requesting a lot of dice, it will be made to wait until enough dice are available.  If it is possible that other groups can come in, request a small number of dice, and have their requests granted, then group X might get delayed forever, since there are never enough dice at the front desk at once to satisfy group X’s needs.  In other words, it might be possible for starvation of X to occur.  In your solution starvation should not be possible.  (Of course, with each group limited to playing only 5 games, all the outstanding dice will always get returned eventually and starvation can never occur, but your solution should also avoid starvation in the presence of a never-ending sequence of Request and Return operations.)


To verify that your code is working, please insert print statements to produce output like this...


Initializing Thread Scheduler...

A requests 4

------------------------------Number of dice now avail = 8

A proceeds with 4

------------------------------Number of dice now avail = 4

B requests 4

------------------------------Number of dice now avail = 4

B proceeds with 4

------------------------------Number of dice now avail = 0

D requests 5

------------------------------Number of dice now avail = 0

E requests 2

------------------------------Number of dice now avail = 0

A releases and adds back 4

------------------------------Number of dice now avail = 4

B releases and adds back 4

------------------------------Number of dice now avail = 8

C requests 5

------------------------------Number of dice now avail = 8

H requests 1

------------------------------Number of dice now avail = 8

B requests 4

------------------------------Number of dice now avail = 8

D proceeds with 5

------------------------------Number of dice now avail = 3



This output makes it fairly easy to see what the program is doing and verify that it is correct.


Below is a method you should use to produce your output.  Feel free to copy this method straight into your program.


  behavior GameParlor


      ... Other methods...


      method Print (str: String, count: int)


        -- This method prints the current thread's name and the arguments.

        -- It also prints the current number of dice available.


          print (

          print (" ")

          print (str)

          print (" ")

          printInt (count)

          nl ()

          print ("------------------------------Number of dice now avail = ")

          printInt (numberDiceAvail)

          nl ()





This method would be called in several places...


At the beginning of the Request method:

     self.Print ("requests", numNeeded)


At the end of the Request method:

     self.Print ("proceeds with", numNeeded)


In the Return method:

     self.Print ("releases and adds back", numReturned)




What to Hand In


Please submit hardcopy of the following files:





Also hand in hardcopy showing your output for both tasks.


For the Sleeping Barber problem, I am not providing any testing material; you’ll have to devise some tests on your own.  Please hand in something showing how you have tested your code.


In LARGE BLOCK LETTERS, write your full name on the first page.  PLEASE STAPLE ALL PAGES!




Basis for Grading


In this course, the code you submit will be graded on both correctness and style.  Correctness means that the code must work right and not contain bugs.  Style means that the code must be neat, well commented, well organized, and easy to read.


In style, your code should match the code we are distributing to you.  The indentation should be the same and the degree of commenting should be the same.