Software Engineering 3156

19-Nov-01

#21: Language II

Phil Gross

2

Administrivia

Calendar tweak Wednesday class Research Fair

– Nov 30th, 10am-4pm– http://www.cs.columbia.edu/acm/research

Intro to Perl & CGI– Nov 20th, 6-8pm

3

Implementation Notes

Chat– Formal/modal– Log messages– Should have distinguished…

ChangeOther for bots– MapDelta?– RequestActorChange?

4

And More…

Death sequence– Attacked with death=true– KickGame– DeletedObject– If actor, LDAP deletion

“Raw” sockets: anyone going to be using these?

Integration is … interesting

5

Advanced Programming

You can’t take it Because Prof. Schulzrinne is taking hints from

our syllabus So, going to try to cover as much of his stuff as

possible http://www.cs.columbia.edu/~hgs/teaching/ap/

6

Review of C

Manual compilation and linking– Multiple .o files manually linked into executable

Statements and operators the same as Java Fewer datatypes

– Everything is really some size of int or float– Including, in particular, booleans and chars– Very weak typing

7

EZ Aggregation

struct point {

int x;

int y;

};

struct point pt;

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

8

Pointers

Integers that represent a machine address Typed, sorta

– Pointer to int, pointer to char

Declare, dereference with *– int *p;

Define with &– int q; p = &q; /* p contains q’s address */

9

More Pointers

int x = 7;

ip = &x;

printf(“%d\n”, *ip); Prints 7

*ip = 9 /* assignment */

printf(“%d\n”, x); Prints 9

10

Pointer Arithmetic

If ip == 4C84, after ip++, ip == 4C88– 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]

11

C Likes Pointers

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

program Pointers = modifiable args Note three meanings of 0

– Falseness– End-of-String char– Null pointer (guaranteed invalid)

12

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

13

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

14

Pointers Not Null-initialized

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

15

“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…

16

Systems Programming

Unix, Windows (and JVM) all written in C/C++ Couldn’t write a JVM for a bare system in Java Need access to underlying system resources Located at particular memory locations Operating system provides (very) basic

services System calls

17

System Calls

Man page section 2, not 3 That is the sum total of what an OS does

– It usually also comes with a bunch of software

Systems programming involves dealing directly with the system calls, or just above

18

Typical System Calls

File access Time and date Process management Signals Networking Security

19

File I/O

open, creat, close read, write

– Pure byte-oriented

20

Unix and Devices

Everything’s a file Devices are “special files” Character or block oriented Character devs include some weird things

– Mice– Audio

Block devices are storage Mknod

21

Filesystems

Sorta part of the OS Usually abstracted and modularized Primitive version has one filesystem per

“volume” Unix model has “mount points” Hard links vs. soft links

22

Special Filesystems

/proc– Virtual filesystem– Access to all kinds of information– Originally processes

NFS– Network filesystem– RPC based

Journalling Filesystems

23

Processes

The fork model Copy on write Vfork

24

Virtual Memory and Paging

Memory allocated by “pages” When full, some pages are copied to disk Replacement targets picked by some strategy

– What strategy?

Can leverage this yourself with “memory mapped files”

25

IPC

Message/queue Shared memory/semaphore

26

Signals

“software interrupts” Asynchronous model Not as nice as threads…

27

Sockets

Way ugly Highly confusing I have examples…

28

C++

C with classes And function overloading And operator overloading And an amazing standard library