will include the material from this week

will not include the material from next week

wget http://courses.cs.washington.edu/courses/cse374/16wi/hws/clint.py

chmod +x clint.py

./clint.py SOURCE_FILE

-g option for gcc compiles with debugging information

gdb EXECUTABLE to start debugger

run ARGS to run the program

backtrace to see the chain of function calls that lead to this point

frame N to switch to function call N (0 is the current call)

list to see the surrounding code

print VAR_NAME to see the value of a variable

quit to end the debugger (it may ask about terminating the running program)

cheatsheet: http://courses.cs.washington.edu/courses/cse374/16wi/gdb_refcard.pdf

manual: http://sourceware.org/gdb/current/onlinedocs/gdb/

deeper look in a future lecture

motivation: consider representing a 2D point or a linked list

we need some way to group multiple pieces of data together

a C struct is exactly this: pieces of data bundled together

point example

struct point {
int x;
int y;
};

struct point p;
p.x = 2;
p.y = 2;

struct point makepoint(int x, int y) {
struct point temp;
temp.x = x;
temp.y = y;
return temp;
}
struct point pt = makepoint(2, 2);

temp is copied when it is returned

exercise: write the definition for a string struct that contains a string and its length

struct string {
char *str;
int len;
};

we can avoid putting struct everywhere by using typedef

typedef just makes an alias for an existing type

typedef struct point Point;
Point p;

can also incorporate it into the definition:
typedef struct point {
int x;
int y;
} Point;

exercise: write a function add_point that takes two points and returns their sum

Point add_point(Point p1, Point p2) {
p1.x += p2.x;
p1.y += p2.y;
return p1;
}

p1 copied on the way in and on the way out

A struct can contain other structs

typedef struct rect {
Point p1;
Point p2;
} Rect;

You can have arrays of structs

Point points[10];

and, of course, pointers to structs

Point *pp = &p;

need to deference to access memebers

void add_point(Point *p1, Point *p2) {
(*p1).x += (*p2).x;
(*p1).y += (*p2).y;
}

need ( ) because . has higher precedence than *

this is very common, so C gives us nicer syntax

void add_point(Point *p1, Point *p2) {
p1->x += p2->x;
p1->y += p2->y;
}

exercise: define a struct for a bank account that includes an account number, first name, last name, and balance. Then write a function update_balance that takes a bank account and an integer indicating the change in balance

typedef struct account {
int account_number;
char *first_name;
char *last_name;
float balance;
} Account ;

void update_balance(Account *acc, int delta) {
acc->balance += delta; // may cause balance to go negative
}