Software Engineering 3156

6-Nov-01

#18: Language I, CVEs, and Databases

Phil Gross

2

Administrivia

Upcoming events Today, 4pm, Interschool (7th floor CEPSR)

– Graduate School panel

Tomorrow, 6pm, 415 CEPSR– Me doing C++ again, hopefully better this time

Monday, 11-12:15, Interschool– Phil Wadler: should be a good talk

3

Also Monday

6pm, location TBA– Janak gets his wireless on– All you ever wanted to know about wireless comm– Phones, Ethernet, Bluetooth– Unleashing his gadget-freak side

4

Mini-Case Studies

iPod installer– Apple screws up big time– Unix is powerful, but Unix is lame– rm -rf "$2Applications/iTunes.app" 2> /dev/null

IRIX 5.1– How to make a bad product much worse– At great expense– http://yarchive.net/risks/sgi_irix.html

5

Collaborative Virtual Environments

Many people are on-line these days E-mail works Chat works, but doesn’t scale too well People want to see other people Visual systems are highly optimized for 3D

– Get vastly more data– Extra dimensions

6

Solutions?

Video conferencing– Aka Brady Bunch Model– Doesn’t scale much past two or three

Virtual environments– Avatars

Usually trivial

– Environment Usually static

7

Problem Domains

Web, web, web– http://www.cs.brown.edu/memex/ACMCSHT/51/51.html

Military simulation– All military scales, platoon to division– Mechanized and not

Distributed Software Development– CHIME– Complex back-end sources

8

Databases in Nutshell

One of the oldest computer applications Historically, armies of clerks filed and retrieved Early computers (i.e. IBM) changed this

– Founded on inventions to speed 1890 census– Further developments after intro of Social Security

in 1935 (26 million records)– As always with computers, primarily military uses at

first

9

First Database Systems

1950s: Idea of a database, as hardware-independent entity

1960: COBOL established 1968: IBM’s IMS for Apollo project 1971: Codasyl (Conference on Data Systems

Languages) model

10

IMS and Codasyl

IMS is hierarchical– Data has parents, children, siblings– Move through a tree-like structure

Codasyl model is a network model– Think arbitrary pointers

Programmer as navigator

11

Relational Model

Ted Codd "A Relational Model of Data for Large Shared

Data Banks," 1970 Data is independent of storage Queries are non-procedural and specified

against the entire data setNot an immediately successful idea

12

Ah, Politics

IBM had massive investments in IMS– “Strategic” product

Codd was at IBM San Jose– Supposed to be building disk drives, not coming up

with new software paradigms 10 years pass

– 1973-9: Ingres at Berkeley Open source success story!

– 1974-9: IBM develops System R and SQL

13

Modern Databases

Everybody came from Ingres– Oracle, Sybase, Informix

SQL first commercially released by Oracle– Despite having been developed by IBM

1980: First relational database from IBM Ingres lives on: first postgres, then postgresql

– Blows out mysql IMHO

14

So What’s It All About

Data is stored as rows in tables Tables have columns

– Table employee has columns name, address, id-number, and salary

– One row is “’phil’, ‘118th st’, 3456, 120000”– Another row is “’janak’, ‘Long Island’, 5678, 15000”

Set of tables organized into Database

15

SQL is Easy: Four Commands

Select, Insert, Update, and Delete– Where clauses can get tricky, though

Select * from employee where salary > 20000 Insert into employee values(‘Suhit’, ‘Roosevelt

Island’, 1010, 60000) Update employee set salary = 120000 where

name = ‘Janak’ Delete from employee where id=1010

16

Theoretically Clean

All operations are on tables All return values are row sets Query is actually sent as ascii over the wire

– Best strategy to answer is computed by database

Structure of database also stored in “system” tables

Table Tables, table Columns

17

Joins

Relate two tables based on common data column

Table with author-ID, author-full-name Table with paper-title, date, author-ID Print out paper-title, author-full-name Normalized: author changes name, only gets

changed once Beware Social Security Numbers!

18

Why Is This Important

Software Engineering deals with large-scale systems

Virtually all large-scale systems have a database component

Often the centerpiece of a system– And an organization

Also, DBAs have the highest salaries in the IT field

19

Also, Remember IMS

Remind you of anything? Anything else? Everything comes back in style I thought IMS was dead, btw; It’s not

– IMS serves 200 million end users, managing over 15 billion Gigabytes of production data and processing over 50 billion transactions every day.

– One customer reports 3000 days uptime

20

Language I: C

Hello, world#include <stdio.h>

main() {

printf(“hello, world\n”);

} #include thingy not a C language statement printf not C language either

21

To Compile:

cc hello.c Produces program…

– Called a.out– !!??

cc –o hello hello.c will produce program called hello

22

Simple/Stupid Compilation

Compiler can’t find dependencies Run-time can’t find dynamic libraries

– Well, it can now, but not originally Import becomes a literal include Compiles to native code All files resulting from compilation are

physically attached to one another to create executable (static linking)

23

Like Java

Or rather, Java (and C++) are wholesale ripoffs of C

While, if-else, for, switch-case (original sin) Operators, % for mod, ++ and --, prefix and

postfix, no exponentiation operator

24

Datatypes

Integers, and a little floating point– Signed and unsigned char, int, short, long

No booleans: 0=false, nonzero=true Different sizes

– Not precisely defined! Characters just aliases for bytes Machine addresses are a native type

– “pointers”

25

Pointers

Can get address of any variable– The ‘&’ operator

Can value stored at any address– The ‘*’ operator

Declarations:int *ip; /* ip will “point” to an int */

void *vp; /* vp will “point” to something… */

26

More Pointers

int x = 7;

ip = &x;

printf(“%d\n”, *ip); Prints 7

*ip = 9 /* assignment */

printf(“%d\n”, x); Prints 9

27

Pointer Arithmetic

If ip == 4, after ip++, ip == 8– assuming ints are four bytes

This is the meaning of the “type” of a pointer Doesn’t really have arrays, just pointers

int a[7];

a ≡ &a[0]

++a ≡ &a[1]

28

C Likes Pointers

No strings, just char arrays terminated with 0 Char pointers flying all over the place in a C

program– usually pointing to strings

Note three meanings of 0– Falseness– End-of-String char– Null pointer (guaranteed invalid)

29

The StringCopy Puzzler

void strcpy(char *dst, char *src) {

while (*dst++ = *src++)

;

} Assignment returns value Dereference binds tighter than post-increment Strings are 0 terminated arrays of chars 0 ≡ false

30

C is Minimal

i.e. nearly nonexistent runtime services No array/pointer bounds checking

– Except for processor halt if you try to dereference 0

Carefree casting– Reinterpret bits as you please: You’re the boss!

No exception mechanism Certainly no garbage collection

31

Memory management

Everything is primitive by default If you want a block of bytes, just ask

void *malloc(int size)int *myString = (int *)malloc(1024 * sizeof(int));

myString is all mine, until I free it– Exactly once, please

And if I double-free or don’t free?– I’m F*(&%$!ed

32

Pointers Not Null-initialized

char *cp; Copy a bunch of stuff to cp Perfectly legal Totally undefined Disaster Need to allocate space to cp

33

“Right” way

char *cp = (char *) malloc (SOMESIZE);

Now I can “safely” copy stuff to cp Why all the quotes? What if SOMESIZE isn’t big enough…

34

EZ Aggregation

struct point { int x; int y; };

struct point pt; pt.x = 3; pt.y = 4;

35

Struct Meets Pointers

struct point *ppt = &pt;

(*ppt).x == 3 Or….

ppt->x == 3 Wacky “arrow” syntax And then there are function pointers, but we

won’t go there…