Semaphores
   Critical section : S
   Read gate        : R
   Write gate       : W

Shared variables
   ar                                     /* number of active readers */
   rr                                    /* number of running readers */
   wexists       /* flag indicating whether the writer process exists */

Initial values
   S = 1                                /* only one process at a time */
   R = 5                                   /* five concurrent readers */
   W = 0              /* update process should be explicitly woken up */
   ar = rr = wexists = 0

Code for reader (search process)
   P(S);                                    /* enter critical section */
   ++ar;                            /* a new reader has become active */
   if ((!wexists) && (rr < 5))            /* proceed to the read area */
      ++rr;                           /* this process will be running */
   V(S);                                    /* leave critical section */

   P(R);                                                 /* wait on R */

   /*  read request granted, so this process will continue until end  */

   read database;

   P(S);                                    /* enter critical section */
   --ar; --rr;           /* this running reader is going to terminate */
   if (!wexists)
      if (ar > rr)                       /* there is a waiting reader */
         ++rr; /* a waiting reader will switch from active to running */
         V(R); /* do this anyway (even if there is no waiting reader) */
   else         /* do nothing, unless this is the last running reader */
      if (rr == 0), then V(W);                  /* wake up the writer */
   V(S);                                    /* leave critical section */

Code for writer (update process)
   P(S);                                    /* enter critical section */
   wexists = 1;
   Set R = 0;     /* no new active reader will be granted read access */
   if (rr > 0)
      V(S);                                 /* leave critical section */
      P(W);                  /* wait until all running readers finish */
   else
      V(S);                                 /* leave critical section */

   /* write request granted, so this process will continue until end  */

   update database;

   P(S);                                    /* enter critical section */
   Let t = min(5,ar);       /* the count of active readers to wake up */
   Do V(R) exactly t times;              /* wake up t waiting readers */
   rr = t;                                /* count of running readers */
   Set R = 5 - t;   /* remaining count of readers that can be allowed */
   wexists = 0;
   V(S);                                    /* leave critical section */