1	This monitor is a model of a database with multiple readers and
2	writers. The high−level goal here is (a) to give a writer exclusive
3	access (a single active writer means there should be no other writers
4	and no readers) while (b) allowing multiple readers. Like the previous
5	example, this one is expressed in pseudocode.
6	
7	    // assume that these variables are initialized in a constructor
8	    state variables:
9	    AR = 0; // # active readers
10	    AW = 0; // # active writers
11	    WR = 0; // # waiting readers
12	    WW = 0; // # waiting writers
13	
14	    Condition okToRead = NIL;
15	    Condition okToWrite = NIL;
16	    Mutex mutex = FREE;
17	
18	    Database::read() {
19	        startRead(); // first, check self into the system
20	        Access Data
21	        doneRead(); // check self out of system
22	    }
23	
24	    Database::startRead() {
25	        acquire(&mutex);
26	        while((AW + WW) > 0){
27	            WR++;
28	            wait(&okToRead, &mutex);
29	            WR−−;
30	        }
31	        AR++;
32	        release(&mutex);
33	    }
34	
35	    Database::doneRead() {
36	        acquire(&mutex);
37	        AR−−;
38	        if (AR == 0 && WW > 0) { // if no other readers still
39	            signal(&okToWrite, &mutex); // active, wake up writer
40	        }
41	        release(&mutex);
42	    }
43	
44	    Database::write(){ // symmetrical
45	        startWrite(); // check in
46	        Access Data
47	        doneWrite(); // check out
48	    }
49	
50	    Database::startWrite() {
51	        acquire(&mutex);
52	        while ((AW + AR) > 0) { // check if safe to write.
53	            // if any readers or writers, wait
54	            WW++;
55	            wait(&okToWrite, &mutex);
56	            WW−−;
57	        }
58	        AW++;
59	        release(&mutex);
60	    }
61	
62	    Database::doneWrite() {
63	        acquire(&mutex);
64	        AW−−;
65	        if (WW > 0) {
66	            signal(&okToWrite, &mutex); // give priority to writers
67	        } else if (WR > 0) {
68	            broadcast(&okToRead, &mutex);
69	        }
70	        release(&mutex);
71	    }
72	
73	    NOTE: what is the starvation problem here?
74