Lecture 8
--------------------

Announce
----------------
- hw5 due
- hw6 posted, due monday
- hw1,2,3 grades, sorry
- project1 extra credit clarification

Today
-------------
- More Pointers
- UNIX Process Memory
- Dynamic Memory
- C Structures

Review
------------
- pointer defn
- pointer uses
	- access variables outside curr scope
	- 'return' more than one value
	- 'pass' more than one value
	- dynamic memory allocation

More Pointers
-----------------

- array and pointer relationship
	- name of array pointer to addr of memory
	- p = a;
	- a = p <-- NO!

- ddim arrays and pointer relationship 
	- say wanted to store all of the months

	- char months[12][10];

	- char *months[12];

	- important to DRAW these data structures

	- why pointers?  save memory and time, can reshuffle!

- pointers and types
	- all pointers store address, which is some range

	- why are they typed?

	- pointers must be typed for dereferencing purposes.

- POWERFUL pointer uses: pointers can be used to make a program dynamic and generic
	- can use to grab arbitrary chunks of memory
	- can use to make variable function calls

	- function pointers

	- ex: built in qsort
		- need to compare two elements during computation
			- elements could be numbers
			- elements could be strings
			- could want to sort small to big, big to small

		- write different qsort for all known types?

		- we can define our OWN types, then what?

		- write a compare function, pass it to qsort as a parameter
		
ex:
	qsort(void *base, int num, int size, int (*compar)(void *, void *)) {

	int a;

	a = compar(base[0], base[1]);
}
	
	- dynamic memory pointers


Dynamic Memory
------------------------------

- motivation: we've been statically allocating memory!

- picture of memory used for a process
** 

stack
heap
data segment
text segment

- works fine when we have global/constant variables and functions that operate on them

- also works fine when we use variables within the scope of functions

- what happens when we need more memory?
	- all our variables are defined in the data seg or in the text/stack area

- we have to borrow memory from the heap!

- malloc function
	- returns a void *
		- untyped pointer, your resp to cast

	- takes the number of bytes to allocate
	- returns a NULL on error (no more memory)

ex: enough space for 8 integers;

	int *myarray;

	myarray = malloc( <what goes here>? );

	sizeof(int) * 8;

- why is this cool? you can put variables into malloc!!

Memory Management
----------------------

- stack picture
	- call a function
	- new stack frame
	- function calls malloc
	- function exits
	- eek!
	- or just redefine the ptr, eek!

- In C, programmer is responsible for doing memory management
- must care for your pointers and free unused memroy yourself
- free(ptr);


C Strings
-----------------------------

- quickly, c strings are char arrays
- usually we use char * to manage strings (variable length)
- programmers have coded many useful functions to deal with strings
- use '\0' to terminate by convention

- strdup(), strlen(), strcat(), strcpy(), strstr(), strcmp(), etc


C Structures
-----------------------------
- arrays are OK but all the same type!
- another way to encapsulate multiple variables

- struct

ex:
	struct playing_card {
		int value;
		char color;
		char *suit;
	};

- defined outside main/other functions

- once defined, can now be declared just like other variables

	struct playing_card mycard;

	struct playing_card deck[52];

	
- accessing struct members
	
	mycard.value = 10;
	mycard.color = 'R';
	mycard.suit = strdup("hearts");

- accessing struct members via a struct pointer

	struct playing_card *cptr;

	cptr = &mycard;

	cptr->value = 5;
	cptr->color = 'B';
	cptr->suit = strdup("clubs");

- operations on structs are illegal as they are undefined!
	- +, -, /, *

- assignments are legal, but watchout!

** draw picture

	- ask them what happens

	struct playing_card newcard;

	newcard = mycard;

- better to define a function to copy contents of one struct to another

	card_copy(struct playing_card *dest, struct playing_card *source) {

		dest->value = source->value;
		dest->color = source->color;
???
		dest->suit = strdup(source->suit);

	}