Producer Consumer Problem

Introduction

This algorithm uses ADA to solve the producer/consumer (or bounded-buffer) problem.

Algorithm

This process (task, to ADA) manages the buffer.

1	task boundedbuffer is
2		entry deposit(data: in item);
3		entry extract(data: out item);
4	end;
5	task body boundedbuffer is
6		buffer: array[0..n-1] of item;
7		count: integer range 0..n := 0;
8		in, out: integer range 0..n-1 := 0;
9		begin
10		loop
11			select
12				when count < n =>
13					accept deposit(data: in item) do
14						buffer[in] := data;
15					end; 
16					in := (in + 1) mod n;
17					count := count + 1;
18				or when count > 0 =>
19					accept extract(data: out item) do
20						data := buffer[out];
21					end;
22					out := (out + 1) mod n;
23					count := count - 1;
24			end select;
25		end loop;
26	end.

27		boundedbuffer.deposit(nextp);

28		boundedbuffer.extract(nextc);

Comments

lines 6-8 As usual, buffer is the buffer, and count the number of items currently in the buffer; in and out are the indices indicating where deposits go or where extractions come from.

lines 13-17 If there is room in the buffer (when count < n) this process will accept a request to deposit an item in it (accept deposit ...); it then updates its variables.

lines 18-23 If there is an item in the buffer (when count > 0) this process will accept a request to extract an item from the buffer (accept extract ...); the item is returned via the parameter list. This procedure then updates its variables.

line 24 If both of the above two when conditions are true, and both a producer and consumer has invoked a procedure named by an accept statement (called "an open accept statement"), the system will select one to be executed in some fair manner (such as first-come-first-serve). If only one of the conditions is true, and the procedure named in an accept statement in the body of the when statement is open, that one will be executed. If both of the when conditions are false, an error condition occurs (this usually terminates the process.)