The Readers and Writers Problem.

(Concurrent readers OR exclusive Writer).

All readers execute reader-entry at start and reader-exit on end of reading zone. Similar for writers. Mutex semaphores are initialized to 1 and others (rsem, wsem) are initialized to 0. the variables rc, wc, rwc and wwc are counters (reader count, reader waiting count etc) and a inited to 0.

PROGRAM 1:   Has writer starvation.

Reader Entry                                      Writer Entry

P(mutex);                                                 P(wsem)

  rc++;

  if rc=1 then P(wsem);

V(mutex);

Reader Exit                                         Writer Exit

P(mutex);                                                    V(wsem)

  rc--;

  if rc=0 then V(wsem);

V(mutex);

PROGRAM 2: Has reader starvation

Reader Entry

P(rsem);

V(rsem)

P(rmutex);

   rc++

   if rc=1 then P(wsem);

V(rmutex);

Reader Exit

P(rmutex);

   rc--;

   if rc=0 then V(wsem);

V(rmutex);

Writer Entry

P(wmutex);

 wc++;

 if wc=1 then P(rsem);

V(wmutex);

P(wsem);

Writer Exit

V(wsem);

P(wmutex);

   wc--;

   if wc=0 then V(rsem);

V(wmutex);

PROGRAM 3: Correct Solution - version A

Reader Entry

P(mutex);

   if (wwc>0) or (wc>0) then begin

            rwc++;

            V(mutex);

            P(rsem);

            P(mutex);

            rwc--;  end;

  rc++;

V(mutex);

Reader Exit

P(mutex);

  rc--;

  if (rc=0) and (wwc>0) then V(wsem);

V(mutex);

                                          RW3

Writer Entry

P(mutex);

if (rc>0)or(wc>0)or(rwc>0)or(wwc >0)

         then begin

            wwc++;

            V(mutex);

            P(wsem);

            P(mutex);

            wwc--; end;

 wc++;

 V(mutex);

Writer Exit

  P(mutex);

   wc-- ;  

   if (rwc>0) then

      for i:=1 to rwc do V(rsem)

   else if (wwc>0) then V(wsem);

  V(mutex)                             

PROGRAM 4: Correct Solution - version B

Reader Entry

P(mutex);

   if (wwc>0) or (wc>0) then begin

            rwc++;

            V(mutex);

            P(rsem);

            rwc--;

      end;

  rc++;

  if rwc>0 then V(rsem)

  else V(mutex);

Reader Exit

P(mutex);

  rc--;

  if (rc=0) and (wwc>0) then V(wsem);

  else V(mutex);                         RW4

Writer Entry

P(mutex);

if (rc>0)or(wc>0)

         then begin

            wwc++;

            V(mutex);

            P(wsem);

            wwc--;

         end;

    wc++;

 V(mutex);

Writer Exit

  P(mutex);

   wc-- ;  

   if (rwc>0) then V(rsem)

   else

      if (wwc>0) then V(wsem);

      else V(mutex)