Question

A file is to be shared among different processes, each of which has a unique number. The file can be accessed simultaneously by several processes, subject to the following constraint: the sum of all unique numbers associated with all the processes currently accessing the file must be less than n. Write a monitor to coordinate access to the file. 3. Exercise 6.29 (10 points) Your solution should use condition variables. Assume a broadcast() operation can be invoked on a condition variable c to resume all processes suspended on c. Hint: Your monitor should contain two functions: one function is called before a process accesses a file and the other function is called after a process accesses a file.

Answer 1

Answer:

monitor fileSharer

{

       enum {THINKING, WAITING, READING} state[N];

       condition self[N];

       int total;

       void open(int i) {

               state[i] = WAITING;

               if (i + total >= N)

               { self[i].wait(); }  // Leaves monitor

               state[i] = READING;

               total += i;

       }

       void close(int i) {

               state[i] = THINKING;

               total -= i;

               // Can signal one waiting proc whose ID won't break bank.

               for (int x = N - total - 1; x >= 0; x--) {

                       if (state[x] == WAITING) {

                               self[x].signal(); break;

                       }

               }

       }

       initialization.code() {

               for (int i = 0; i < N; i++) {

                       state[i] = THINKING;

               }

               total = 0;

       }

}