GEORGE MASON UNIVERSITY
Computer Science Department
CS 475 – Concurrent and Distributed Software Systems
Spring 2003
Assignment 2 – Multithreaded Programming
DUE DATE – March 10
Process Synchronization The goal of this exercise to give you some experience writing concurrent pro-
grams.
You have been hired by MetroTrans to synchronize traffic over a narrow light-duty bridge on a public
highway. Traffic may only cross the bridge in one direction at a time, and if there are ever more than
3 vehicles on the bridge at one time, it will collapse under their weight. In this system, each vehicle
is represented by one thread, which executes the procedure OneVehicle when it arrives at the bridge:
OneVehicle(int vehicle_id,int direc, int time_to_cross)
{
ArriveBridge(direc);
CrossBridge(vehicle_id,direc,time_to_cross);
ExitBridge(vehicle_id,direc);
}
In the code above, vehicle id is an integer which uniquely identifies each vehicle (The vehicles arriving
at the bridge should be assigned vehicle ids 1,2,3... and so on ). direc is either 0 or 1; it gives the
direction in which the vehicle will cross the bridge. time to cross is the time it takes a vehicle to cross
the bridge – assume that every vehicle takes 4 seconds to cross the bridge.
Implement a “fair” traffic control policy that imposes a limit (say 5)on the number of vehicles that can
cross the bridge in one direction, while vehicles travelling in the opposite direction are waiting to get
on the bridge. NOTE: if there are no vehicles waiting to get on the bridge in the opposite direction,
your policy should not switch directions. In other words, your policy should be smart enough to
provide fairness if there is traffic in both directions but should never force a vehicle to wait to get on
the bridge if there is no traffic in the opposite direction.
Write the procedures ArriveBridge,CrossBridge and ExitBridge, using mutex locks and condition
variables for synchronization1. The ArriveBridge procedure must not return until it is safe for the
car to cross the bridge in the given direction (it must guarantee that there will be no head-on collisions
or bridge collapses). The CrossBridge procedure should just delay for time to cross seconds and
print out a debug message. ExitBridge is called to indicate that the caller has finished crossing the
bridge; ExitBridge should take steps to let additional cars cross the bridge. In addition, ExitBridge
should update a shared variable departure index, which keeps track of the order in which the cars
leave the bridge, i.e., the first car to leave the bridge has departure index 1, the second car has
departure index 2, and so on. The ExitBridge procedure should also print out the departure index
for that vehicle.
1If you use Java for the assignment, use the equivalent thread synchronization faciltities
1
