CIS 307: Protected Bounded Buffers

Protected Bounded Buffers are treated in Tanenbaum Figure 2.11

We assume that we are given an Abstract Data Type, or class, BBuffer that is probably a queue. It has operations InitBBuffer, BEnqueue, and BDequeue. It can hold N objects of type item.
We want to define another ADT called PBBuffer which will be usable by concurrent processes. [Remember that the standard implementation for Bounded Buffers is unsafe when used in a concurrent program.]
Protected Bounded Buffers can be used in all Producer-Consumer situations.

	constant N = ....;
	type item  = ....;
	type PBBuffer = record
			 full, empty, mutex : semaphore;
			 b : BBuffer;
			end;
	procedure InitPBBuffer(R : in out PBBuffer) is
	begin
	   InitSem(R.full,0); InitSem(R.empty,N); 
	   InitSem(R.mutex,1); InitBBuffer(R.b);
	end;
	procedure PBEnqueue (R : in out PBBuffer; I : item) is
	begin
   1.	   P(R.empty);
   2.	   P(R.mutex);
	   BEnqueue(R.b,I);
	   V(R.mutex);
	   V(R.full);
	end;
	procedure PBDequeue (R : in out PBBuffer; I : out item) is
	begin
   3.	   P(R.full);
	   P(R.mutex);
	   BDequeue(R.b,I);
	   V(R.mutex);
	   V(R.empty);
	end;

Notes:

• In 1. the calling process makes sure that the buffer is not full.
• In 3. the calling process makes sure that the buffer is not empty.
• If statements 1. and 2. are reversed we may have a deadlock.
• Notice what we have done: We have assumed a class, BBuffer, and defined a new class, PBBuffer. We built a new abstraction in terms of existing abstractions.