Solving Homework 7, part 2 See Diagram of interactions: https://www.dropbox.com/s/8bk1kiidhkxxhph/WritersPriority.png

WritersPriority.png

writerspriority.txt

Brian Anderson
Com S 352, Spring 2013
Assignment 7, Question 2
=========================================================================
2. (30pts) Modify the monitor pseudo-code for the reader-writer problem 
on Page 8 of 2013-3-4's lecture to solve the "writer's priority" 
version of the problem defined as follows:

  (1) At any time, the shared file can be accessed by 
      one writer exclusively, or one or multiple readers concurrently.

  (2) Whenever there are both reader and writer processes 
      arriving or waiting to access the shared file, a writer 
      has higher priority than reader to access the shared file. 
      
==========================================================================

Monitor reader-writer
begin
  activeReaderCount: integer          //renamed to activeReaderCount
  busyWriting:       boolean          //renamed to busyWriting
  OKtoread:          condition        
  OKtowrite:         condition


  /* -- READING -----------------------------------------------------------------------  */
  prodcedure startread;
  begin
    if busyWriting then OKtoread.wait;
    activeReaderCount := activeReaderCount + 1;     
    OKtoread.signal;    // all readers can join in now....
  end startread
  
  proecedure endread;
  begin
    activeReaderCount := activeReaderCount - 1;
    if activeReaderCount = 0 then OKtowrite.signal;
  end endread
  
  
  /* -- WRITING --------------------------  */
  procedure startwrite;
  begin
    if busyWriting or activeReaderCount <> 0 then OKtowrite.wait;
    busyWriting := true;
  end startwrite
  
  procedure endwrite;
  begin
    busyWriting := false;
    if OKtowrite.queue then OKtowrite.signal;     // If another write queued, signal it
    else if OKtoread.queue then OKtoread.signal;   // else signal read
  end endwrite
  
  /* -- INITIALIZE -------------------------- */
  begin
    activeReaderCount := 0;
    busyWriting := false;
  end
end reader-writer