Question

I’m trying to implement the pseudo solution below in java as part of an assignment. The pseudo is for a Readers Preference program, specifically the reader process itself. There is an accompanying writer process, but to keep things spartan I didn’t bother pasting it.

Process Reader[i=1 to M] {
  while (true) {
    /* Implementing <await (nw == 0) nr = nr+1;> */
    P(e);
    if (nw > 0) {dr = dr+1; V(e); P(r);}
    nr = nr + 1;
    if (dr > 0) {dr = dr-1; V(r);}
    else V(e);
    read the database;
    /* Implementing <nr = nr-1;> */
    P(e);
    nr = nr - 1;
    if (nr == 0 and dw > 0) {dw = dw-1; V(w);}
    else V(e);
  }
}

Originally I assumed that the line:

/* Implementing <await (nw == 0) nr = nr+1;> */

Was some kind of commenting on what was occurring, but having just re-read it I think it’s supposed to be an if statement to control the P(e) locking semaphore. Below is what I’ve implemented in my code based on the above assumption.

if (nw == 0) {
  nr = nr++;
  try {
    e.acquire();//P(e)
  } catch (InterruptedException e) {
  }
}//end if

The output from the rest of the code has been a bit of a mess, but it seems to work. I used delays with a thread.sleep in order to make the output to console more readable and again it seemed ok, though I was wary about using delays as that seemed quite likely to interfere with the results, since I was giving the writer processes a longer delay. In theory though, the locking system should be sound and the delays should just make the output to console more readable.

So, am I correct to use the if statement above?

Can you give me any tips on outputting data to the console to prove it works as it should (readers preference/writers preference)

Answer 1

The line

/* Implementing <await (nw == 0) nr = nr+1;> */

is an atomic process, the code below carries that out atomically.

If anyone was interested about the other question, just add print statements under the acquire/release.