## Introduction

This algorithm uses a monitor to solve the first readers-writers problem.

## Algorithm

```1	readerwriter: monitor
3			writing: boolean;
6	begin
8		if writing then
10	end;
12	begin
14		if readcount = 0 then
15			oktowrite.signal;
16		end;
17	procedure entry beginwrite;
18	begin
19		if readcount > 0 or writing then
20			oktowrite.wait;
21		writing := true;
22	end;
23	procedure entry endwrite;
24	var	i: integer;
25	begin
26		writing := false;
27		if readcount > 0 then
28			for i := 1 to readcount
30		else
31		oktowrite.signal;
32	end;
33	begin
34		readcount := 0; writing := false;
35	end.
```

lines 1-4: Here, readcount contains the number of processes reading the file, and writing is true when a writer is writing to the file. Oktoread and oktowrite correspond to the logical conditions of being able to access the file for reading and writing, respectively.

lines 7-9 In this routine, the reader announces that it is ready to read (by adding 1 to readcount). If a writer is accessing the file, it blocks on the condition variable oktoread; when done, the writer will signal on that condition variable, and the reader can proceed.

lines 13-15 In this routine, the reader announces that it is done (by subtracting 1 from readcount). If no more readers are reading, it indicates a writer may go ahead by signalling on the condition variable oktowrite.

lines 19-21 In this routine, the writer first sees if any readers or writers are accessing the file; if so, it waits until they are done. Then it indicates that it is writing to the file by setting the boolean writing to true.

lines 26-31 Here, the writer first announces it is done by setting writing to false. Since readers have priority, it then checks to see if any readers are waiting; if so, it signals all of them (as many readers can access the file simultaneously). If not, it signals any writers waiting.

line 34 This initializes the variables.

Send email to cs150@csif.cs.ucdavis.edu.

Department of Computer Science
University of California at Davis
Davis, CA 95616-8562