1. Announcement
server: server.o common.o $(CC) -o server server.o common.o $(LIBS) -lsock -lpthread and change it to: server: server.o common.o threadpool.o $(CC) -o server server.o common.o threadpool.o $(LIBS) -lsock - pthread

2. Today’s topics Project 2 How to use condition variables in Pthreads.
Classical Synchronization Problems
Sample Questions

3. Using Condition Variable
THREAD A ..
lock(associated_mutex)
If (waiting_condition )
pthread_cond_wait(con_var, associated_mutex)
Unlock (associated_mutex )
THREAD B ..
lock (associated_mutex )
If (signal_condition)
signal(con_var);
Unlock(associated_mutex );

4. How to solve a synchorization problem by using con_var
How many condition variables do we need?
When to make a thread sleep?
When to signal a sleeping thread?
How many associated mutexes do we need?
The waiting_condition & signal condition.
Insert code for wait() & signal().
Insert code for lock(mutex) & unlock(mutex).
Verify & check for deadlock, race condition ..

5. How to solve a synchorization problem by using semaphore??
How many semaphore do we need?
When to make a thread sleep?
When to signal a sleeping thread?
How many mutexes(binary semaphore) do we need?
The waiting_condition & signal condition.
Insert code for up() & down().
Insert code for down(mutex_sem) & up(mutex_sem)
Verify & check for deadlock, race condition ..

6. procedure producer() { while (true) { item = produceItem() if (itemCount == BUFFER_SIZE) {sleep() } putItemIntoBuffer(item) itemCount = itemCount + 1 if (itemCount == 1) { wakeup(consumer) } } procedure consumer() { if (itemCount == 0) { sleep() } item = removeItemFromBuffer() itemCount = itemCount – 1 if (itemCount == BUFFER_SIZE - 1) { wakeup(producer) } consumeItem(item) } }
semaphore fillCount = 0 semaphore emptyCount = BUFFER_SIZE procedure producer() { while (true) { item = produceItem() down(emptyCount) putItemIntoBuffer(item) up(fillCount) } } procedure consumer() { down(fillCount) item = removeItemFromBuffer() up(emptyCount) consumeItem(item)

7. Using cond_var for SleepingBarber
con_var SleepingBarber, waitingCustomers; mutex accessSeats; int NumOfFreeSeats=MAX; procedure Barber() { while (true) { lock(accessSeats); if (NumOfFreeSeats == MAX) _wait(SleepingBarber, accessSeats); signal (waitingCustomers); NumOfFreeSeats++; unlock(accessSeats); }
procedure Custommer() { lock(accessSeats); if (NumOfFreeSeats == 0) { unlock(accessBuff); //Leave withou haircut } else { if (NumOfFreeSeats == MAX) _signal(SleepingBarber); NumOfFreeSeats --; wait (waitingCustomers, accessSeats); unlock(accessSeats);

8. Using semaphore for SleepingBarber
+ Semaphore Customers = 0
+ Semaphore Barber = 0
+ Semaphore accessSeats (mutex) = 1
+ int NumberOfFreeSeats = N //total number of seats
BARBER
while(true) {
down(Customers) //
down(accessSeats) NumberOfFreeSeats++
up(Barber);
up(accessSeats) //we don't need the lock on the chairs anymore
//here the barber is cutting hair }
procedure Custommer() { down(accessSeats) //tries to get access to the chairs if ( NumberOfFreeSeats > 0 ) { NumberOfFreeSeats-- up(Customers) up(accessSeats) down(Barber) //now it's this customers turn, but wait if the barber is busy //here the customer is having his hair cut } else { //there are no free seats up(accessSeats) //customer leaves without a haircut }