4330/6310 THIRD ASSIGNMENT Spring 2015

The problem (I) A post office with a single queue and several nameless clerks: Clerk 0 Customers waiting Clerk 1 …

The problem (II) Can represent the n clerks by a single semaphore  post_office Its initial value will be the total number of clerks helping customers Its current value will indicate the number of idle clerks

Your program (I) Main program will  Read a line with a customer's name, arrival_delay, and service_time  Sleep for arrival_delay seconds  Fork a child Wait until all children have terminated Print statistics

Your program (II) Your child processes will  Print a message  Do a P() on the post_office semaphore  Print a message  Sleep for service_time seconds  Print a message  Do a V() on the post_office semaphore  Terminate by doing _exit(0);

Collecting statistics Before terminating, your program should display:  The total number of customers that got serviced Can be maintained by the main program  The number of customers that did not have to wait Will require some extra coding  The number of customers that had to wait Trivial

Knowing if a customer had to wait A customer process will wait if all clerks were busy when it arrived at the post office  Easiest way is to check the value of the semaphore post office sem_getvalue(mysem,&value);

Customer processes revisited (I) Your child processes will  Print a message  Get the current value of the post_office  If all clerks are busy increment the number of customers who had to wait  Do a P() on the post_office semaphore  Print a message  Sleep for service_time seconds  Print a message  Do a V() on the post office semaphore  Terminate by doing _exit(0);

Using a shared variable The number of customers that have to wait is  A global variable shared by all your processes  Must be stored in a shared memory segment  Must be accessed in mutual exclusion Add a second semaphore ("mutex") to your program

Customer processes revisited (II) Your child processes will  Print a message  Get the current value of the post_office semaphore  If all clerks are busy then Do a P() on a mutex Increment the number of customers who had to wait Do a V() on a mutex  Do a P() on the post_office semaphore  Print a message  Sleep for service_time seconds  Print a message  Do a V() on the post office semaphore  Terminate by doing _exit(0);