First Readers Writers Problem

Introduction

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

Algorithm

	1	readerwriter: monitor

	2	var	readcount: integer;

	3			writing: boolean;

	4			oktoread, oktowrite: condition;

	5	procedure entry beginread;

	6	begin

	7		readcount := readcount + 1;

	8		if writing then

	9			oktoread.wait;

	10	end;

	11	procedure entry endread;

	12	begin

	13		readcount := readcount - 1;

	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

	29				oktoread.signal;

	30		else

	31		oktowrite.signal;

	32	end;

	33	begin

	34		readcount := 0; writing := false;

	35	end.

Comments

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 pri-
ority, 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.