COMS W3156: Software Engineering, Fall 2001

1. COMS W3156:Software Engineering, Fall 2001 Lecture #22: C, C++, OS, etc… Janak J Parekh janak@cs.columbia.edu

2. Administrativia • Prototype is in: enough for integration • Our work never finishes: now working on reference model • If problems with the prototype, come see us early • Final exam: any questions? • Open book doesn’t mean easier… • Reminder: Research Fair this Friday • http://acm.cs.columbia.edu/research • Intro to Java Server pages and Servlets tomorrow • See CS front doors…

3. Next class • Finish everything • Monday will be MMM and final exam review • Can you believe the semester has less than two weeks left?

4. Today’s class • A little bit more on C • C++ • Begin operating systems • Begin design patterns, if time

5. Struct • Basically, a class without methods struct Point { int x; int y; }; /* note the semicolon */ int main(void) { struct Point myPoint; myPoint.x = 5; myPoint.y = 5; }

6. Miscellany on structs • (*myP).x looks ugly • C has a bizarre shortcut: myP -> x • Yes, a dash followed by a greater than • Always remember that a -> b is equivalent to (*a).b • Precedence of operators • It’s cumbersome to have to say struct Point every time • Use typedefs: define “aliases” • typedef struct Point PointT; • or even typedef struct Point *PointP;

7. More miscellany on structs • Since not everything happens in main, malloc’ing structs is very common • The sizeof operator works on pretty much any datatype, be it structs or primitive datatypes • Guaranteed tip: you will come across PointP myP = (PointP)malloc(sizeof(PointP)); • and be glad that you were attending this class…

8. Function prototypes • As I said, C’s compiler is not all that smart • If you have: int main(void) { int i = foo(); } int foo(void) { return 5; } • It will not work

9. Function prototypes (II) • Need to clue in the compiler by putting the following prototype before main: int foo(void); • Looks like a Java interface construct • Often, you collect the prototypes into a header (.h) file, then #include it into your various source (.c) files

10. On various .c files… • A common way to build multi-source-file code in C is to • build a set of .h files that export functionality (i.e. “public” methods) • #include these .h files • Compile and link the .c files together • As long as you have a prototype, the compiler will not care about the definition • Like compiling against an Interface • The Linker will, though…

11. Useful libc commands (I) • printf: Output text: like System.out.println • printf(“%d\n”, intval); • printf(“%d%f\n”, intval, floatval); • man printf • scanf: Input text: no BufferedReader “crap” • scanf(“%d”, &intval) • note the ampersand

12. Useful libc commands (II) • gets(string) • gets a line of text and puts it in string • Should technically use fgets(stdin, string) • Why not &string? • puts() also exists: more like println, actually • Can’t use + as a concatenator • atoi(string) • Roughly equivalent to Integer.parseInt, although not nearly as smart • No exceptions

13. C++ • Not strictly a superset of C, but C++ compilers backwards-compatible with C • Certain subtle language differences • C with classes, but a whole lot more • Function, operator overloading • Powerful “template” functionality • Exceptions

14. Quick primer to C++ • g++ is the GNU C++ compiler • Actually a C++ plugin to gcc • Can feed C code to it, will work fine • Can integrate C and C++ code pretty easily • However, ideally, you want to avoid using many C constructs

15. Hello, world in C++ #include <iostream> using namespace std; int main(void) { cout << “Hello world\n”; } • Note use of namespaces, streams • << operator is overloaded to allow combination/concatenation of strings with streams • Can do cout << “foo” << “bar” << endl;

16. Class declarations in C++ class Rectangle { private: double width; double height; public: Rectangle(int width, int height); ~Rectangle(); void setWidth(int width); int getArea(); // and so on… }

17. Implementing methods • Inline in the class declaration • Or, use the class as a prototype and implement elswhere void Rectangle::setWidth(int w) { width = w; }

18. Constructors and destructors • You have both, not just one • Motivation of destructor: cleanup the object, free any memory, etc. • Don’t use C++’s default destructor if you new anything • You should delete anything you new • Don’t use malloc/free unless absolutely necessary

19. Inheritance in C++ • class C: public class A { /* stuff */ } • Motivation: make A’s members public in C • if “: private class A”, A’s members become private in C • Java only has the public mechanism • Multiple inheritance! • class C: public class A, private class B {…} • Can be extremely useful, but beware of “the Death Diamond”, among other things

20. Polymorphism in C++ • If B extends A, and you new a B but “call it” an A: • In Java, if a method in A is overridden, will actually call the overloaded one automatically • In C++, will not by default: must declare a method virtual; also, cannot pass objects call-by-value into method • Should also make destructor virtual

21. Memory allocation in C++ • new, delete • When you allocate memory, you must also • write a destructor • define a assignment operator • define a copy operator • Why? • In C++, default copy/assignment is bitwise • Your pointers will just get “copied”, instead of new memory being allocated

22. C++ miscellany • Operator overloading: nice, but must be careful • References in function prototypes (finally!) • Templates: “type”ify data structures • STL: Standard Template Library • You now have “string”s! • Lots of other data structures • Somewhere between C and Java API’s • Very, very fast if used correctly • Wish we had more time to cover this…

23. Systems programming • Most software written in C/C++ • UNIX • Windows • Java VM’s • Why? • Need access to underlying system resources • But multiple programs need those resources • An operating system’s primary responsibility is to manage resources

24. Managing resources • An OS often abstracts away the resources, and builds an API to access them • “Virtualization” • Operating system’s API: system calls • Why don’t you always see them? • The C, C++, Java libraries some of the system call work for you

25. System calls • See man pages in section 2 • FYI: man 2 open • That’s the prime responsibility of an OS to its applications • Of course, it’s more complicated than that • Most functionality we consider standard to an OS is actually bundled, however • Really cool: truss or strace

26. Typical system calls • File access (managing disks) • Process management (managing processor) • Memory allocation (managing memory) • Date/time (managing clock) • At a higher level: • Networking • Security

27. Example system calls: file I/O • open, create, close • These work with file descriptors • Actually generalized in UNIX to be far beyond files: sockets represented by file descriptors • read, write • Purely byte-oriented • Do a man on these…

28. UNIX • Core concept: everything is a file • Devices are “special files”, stored in /dev • UNIX devices are character or block oriented • tty vs. disk, for example • mice, audio? • mknod command

29. Speaking of files… • Filesystems are often considered a core functionality of the OS • However, it’s usually separately modularized: convenience abstraction layer • Two common models: • Rooted (UNIX and cousins): mount points • Non-rooted (DOS, Windows) • Links…

30. Special filesystems • “Virtual” filesystems • /proc • In Linux and Solaris • Has a number of useful sets of information, no longer just “processes” • /tmp • On certain machines, actually ramdisk • Is this really virtual? • NFS filesystems

31. Processes • The “fork” model: start off a new process as a duplicate of an existing one • Copy-on-write methodology: only create as much of the new process as necessary • Multitasking of processes • Cooperative: “hey, let me know when you’re done, so I can give time to another process” • Preemptive: “yo, your slot is up” • Interprocess communication: messages/queues, shared memory, pipes, domain sockets

32. Memory management • Allocation: assign memory blocks to processes, as well as dynamic memory allocation from a “heap” • Segmentation: prevent programs from overwriting each other’s memory • Virtual memory: abstract away memory into “pages” which may be swapped in and out of disk • What to swap requires “replacement strategy” • Memory-mapped files

33. Networking • At first, write directly to network hardware (yuck!) • TCP/IP, socket API built on top of most OS’s (BSD socket API) • You have no idea how nice Java sockets are in comparison to C/BSD sockets…

34. Shell • Not really part of the OS, but expected to be bundled with it • Front-end to operating system • Allow for convenient file management and process control • bash, cmd just make a bunch of system calls… • Explorer: a GUI front-end • eshell: now this is freaky

35. What does this mean for you? • C/C++: small program to be written for HW4 • OS’s: covered lightly on FE, if at all • You’ll learn about each of these in the actual operating systems class