CMSC 421 Operating Systems Lecture Notes
(c) 1994 Howard E. Motteler


Interprocess Communication Example: Readers and Writers
--------------------------------------------------------

A simple, semi-fair readers and writers solution using a state
variable with any form of mutual exclusion.

States:

  Free:  no active readers or writers
  AR:    one or more readers are active
  AW:    one writer is active

S is single semaphore, used for mutual exclusion, and readcnt 
is the number of active readers

Initially S = 1, readcnt = 0, state = Free, and k the desired
time between tests of the state variable

WRITER:  while (alive) {
            
              wait(S);
              while (state ==  AR || state == AW) {
                signal(S);  sleep(k);  wait(S);
                }
              state = AW;
              signal(S);
            
              write_stuff();
            
              wait(S);
              state = Free;
              signal(S);
              }

READER:  while (alive) {

              wait(S);
              while (state == AW) {
                signal(S);  sleep(k);  wait(S);
                }
              state = AR;
              readcnt++;
              signal(S);
            
              read_stuff();
            
              wait(S);
              readcnt--;
              if (readcnt == 0) state = Free;
              signal(S);
              }


Similarly to the TSL instruction, this solution does not
guarantee bounded waiting for a fixed bound.  If the process
scheduler is fair, however, no reader or writer is locked out
permanently.