# COMP1521 18s1 Exam Q9

A.  i) The `mx' semaphore is there to ensure that only one reader at a
       time executes the code in between the `wait(mx)' and `signal(mx)'
       calls.  So only the first reader correctly waits for the writer
       to finish (if needed) and only after the last reader has finished
       reading can the writer start writing.

   ii) The `w' semaphore is to ensure that only one process at a time is
       writing (readers can't read here) and if the resource is being
       read by at least 1 reader, the writers cannot write.


B.  i) sem_init(mx, 0, 1);

   ii) sem_post(w);


C.  The writer could be pre-empted after the `signal(w)' call, so `w'
    has value 1.  If there are multiple readers, the first reader will
    decrement the `w' semaphore and start reading.  It is pre-empted and
    another reader starts reading.  That reader is pre-empted and
    another reader starts and so on.  If the reader processes have a
    higher priority than the writer, the scheduler will likely choose to
    run the reader processes thus starving the writer of the shared
    resource.