CS202: HW 4: More concurrency practice

These problems should be done on your own. You get credit if you turn in something, and we are not checking the correctness. So these exercises exist purely to reinforce the material, not to evaluate you.

Getting help from AI (besides being ruled out by the course policies) will actually hurt you, since the point of these questions is for you to gain the practice and experience of working through the problems.

You will need that general skill (of absorbing something by practicing it) in life, and in this semester you will need the specific problem-solving skills that are emphasized in these homeworks.

Smokers

void chooseIngredients(int *paper, int *tobacco, int *match);

Auction

Auction auction;  // global

class Auction {

    public:
        Auction();  
        ~Auction();
        void RegisterBid(uint32_t amount);
        uint32_t SelectWinner();

    private:
        uint32_t bids[NUM_BIDS]; // bids[i] == 0 means that slot i is free

        // ADD MORE HERE





};


Auction::Auction()
{
    memset(bids, 0, sizeof(bids));

    // FILL THIS IN






}

void
Auction:RegisterBid(uint32_t amount)
{
    // We use 0 to mean "slot free"
    assert(amount > 0); 

    // FILL THIS IN
    





}



uint32_t
Auction:SelectWinner()
{

    // FILL THIS IN







}

Sleeping barber

This is a potentially useful (if convoluted) example to exercise your understanding of how to use mutexes and conditional variables. It is a well-known concurrency problem. The writeup and solution are due to Mike Dahlin (who used to be on the faculty at The University of Texas at Austin). He asked this question on a midterm once.

Work through the problem on your own; we will post the solution next week.

A shop has a barber, a barber chair, and a waiting room with NCHAIRS chairs. If there are no customers present, the barber sits in the chair and falls asleep. When a customer arrives, the customer wakes the sleeping barber. If an additional customer arrives while the barber is cutting hair, the customer sits in a waiting room chair if one is available. If no chairs are available, the customer leaves the shop. When the barber finishes cutting a customer’s hair, the barber tells the customer to leave; then, if there are any customers in the waiting room, the barber announces that the next customer can sit down. Customers in the waiting room get their hair cut in FIFO order.

The barber shop can be modeled as 2 shared objects:

• A BarberChair, with the methods napInChair(), wakeBarber(), sitInChair(), cutHair(), and tellCustomerDone(). The BarberChair must have a state variable with the following states: EMPTY, BARBER_IN_CHAIR, LONG_HAIR_CUSTOMER_IN_CHAIR, SHORT_HAIR_CUSTOMER_IN_CHAIR.

  • Note that neither a customer or barber should sit down until the previous customer is out of the chair (state == EMPTY).

  • Note that cutHair() must not return until the customer is sitting in the chair (LONG_HAIR_CUSTOMER_IN_CHAIR).

  • Note that a customer should not get out of the chair (that is, return from sitInChair()) until the customer’s hair is cut (SHORT_HAIR_CUSTOMER_IN_CHAIR).

  • The barber should get in the chair (BARBER_IN_CHAIR) only if no customers are waiting.

  • You may need additional state variables.

• A WaitingRoom, with the methods enter() and callNextCustomer().

  • enter() returns WR_FULL if the waiting room is full or (immediately or eventually) returns MY_TURN when it is the caller’s turn to get their hair cut

  • callNextCustomer() returns WR_BUSY or WR_EMPTY depending on if there is a customer in the waiting room or not. Customers are served in FIFO order.

Thus, each customer thread executes the code:

Customer(WaitingRoom *wr, BarberChair *bc)
{
    status = wr->enter();
    if (status == WR_FULL) {
        return;
    }
    bc->wakeBarber();
    bc->sitInChair();   // Wait for chair to be EMPTY
                        // Make state LONG_HAIR_CUSTOMER_IN_CHAIR
                        // Wait until SHORT_HAIR_CUSTOMER_IN_CHAIR
                        // then make chair EMPTY and return
    return;
}

The barber thread executes the code:

Barber(WaitingRoom *wr, BarberChair *bc)
{
    while (1) { // A barber’s work is never done
        status = wr->callNextCustomer();
        if (status == WR_EMPTY) {
            bc->napInChair(); // Set state to BARBER_IN_CHAIR; return with state EMPTY
        }
        bc->cutHair(); // Block until LONG_HAIR_CUSTOMER_IN_CHAIR;
                       // Return with SHORT_HAIR_CUSTOMER_IN_CHAIR
        bc->tellCustomerDone(); // Return when EMPTY
    }
}

Handing in the homework

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