CS 536 Class Notes for April 19, 1999 by S. Nicole Little

Topics for Today 
	- Heap Allocation 
	- Heal deallocation

A heap is a big block of space   _ _____________________
				| |			|
				| |	  heap 		|
				|_|_____________________|
				 ^
				 |
				size of the heap 

	 _ _____ _ _____ _ _____ _____
       	|1|	|2|	|3|	|     |	
	|0|	|0|	|0|	|     |	
	|0|_____|0|_____|0|_____|_____|   
         \	 |	 /	\   
	  \      |      /	 \
        headers for 	        free space	
	   allocated space    	
	
	In order to deallocate the 200 bytes 
		- keep a free space list (FSL - linked list)
		- simply place the oject (200) into the free space list
		- ordered by adding (unordered list)

	Ways to allocate space when a free space list is being implemented 
	1) First fit 
		- looks for the first place where he allocation fits
	 	- leaves lots of little gaps that may become too small to 
			be practical
	2) Best Fit
		- looks for the best fit - as close to fit as possible
		- takes time, must look thru the whole list
	3) Circular Fit
		- eliminates front end clutter, spreads out allocation

	Note - adjacent free space can be merged

	
USER CONTROLLED DEALLOCATION

	Two pointers
		- free(ptr) assigned to the printer after dispose 
		- dispose(ptr) assigned to the object to be disposed
	Problem
		- logging space, what has is still needed and what is not

		- example 
			p = new int;
			q = p;
			delete p;  // q space is also deallocated
			*q = 123   // What happens here?
					No way to Predict

	Therefore we move to system control of deallocation

SYSTEM CONTROLLED DEALLOCATION

	- very popular
	- reliable
		
	How does it work?
		-system tracks objects and decides when they are garbage
			+ tracks creation and deletion of pointers
			+ free memory - objects that cannot be reached

	Options
		1) Refernece Counting
			a) _ _ ________    keep track of how many pointers are
			  | | |        |     pointing to an object
			  | | |  heap  |
			  |_|_|________|
			   / \
		       count  header(size)
			b) Code that amnipulates pointers will also adjust 
				counter (i.e. null assignment subtract count)
			c) If you go out of scope it is essentially deleted
			d) drawbacks
				- more expensive operations
				- makes you pay incrementally
		
		2) Mark and Sweep
			a) two passes
			b) pay almost nothing until heap is almost running out
			c) idea
				- look at heap state(when) almost out of memory
				- mark the active object of the heap
					+ global pointers
					+ locals	
					+ heap objects
				- sweep 
					+ check if marked
					+ if not marked, add to free space
			d) how do we decide what to mark?
				- look for big numbers that look like heap 
					pointers 
				- may mark things that are not active but 
					all active things will be marked