1. 4061 Session 29 (5/1)

2. Today Review, Looking Ahead DRM Teaching/TAing evaluations

4. Admin TCLUG –Networking opportunity –First meeting in a year...good time to start –Tomorrow, this room, 6:30pm - 8:00pm –Learn about linux on a flash drive + food/drink after the meeting Quiz 5 Homework 5 Office Hours Next Week

5. Trip Report Persuasive Computing Conference iParrot and Pet Plant –Odd form factors for computing Increasing interest in computing among women –Avatars as role models: Gender, attractiveness, age, ….and species VR Lab The dog clicker

6. Review: Shell Scripting Why? One goal of this class is to become comfortable in *nix. –Hopefully, we are all at becoming comfortable at the command line, now Quick and dirty tool to make life easier Used for chaining other applications together –If you need to code an algorithm, use Python

8. “It is easier to port a shell than a shell script.” -- Larry Wall

9. Directions: Shell Scripting I have seen very little movement in shell scripting. However, CLI continues to diminish in importance. –Does this also mean that shell scripts will decline in importance?

10. Review: System Calls System calls are the programmer’s interface to operating system features –files –processes –etc. OS Protection and Process Isolation –System calls: Kernel mode vs. user mode –Base and limit registers What happens when you make a system call?

11. read(fd, &buffer, nbytes)

12. Directions: System Calls We (programmers) are being taken further from the system calls API by more abstract (usable?) languages –Bytecode-based languages and virtual machines Specialized system calls continue to be added to support emerging technologies and needs –E.g. epoll_create

13. Review: Unix Security The Unix security model is simple but powerful –Many argue that it lacks flexibility Files have owners and groups Permissions: r, w, x –Owner, owner group, everyone else –x treated a bit differently for files and directories files: can you execute? dirs: can you read the files in the dir?

15. Directions: Unix Security New methods for authentication –fingers, eyes, smart cards New (old) methods for declaring permissions may pop up in specialized distros –To make the permissions model more flexible –E.g., access control lists associate a list of rules with each file (users a and b may read and write, user c may read) Personal belief: usability of security gains importance

16. Review: Processes in Unix A process is an abstraction of a running program The OS is responsible for managing processes: largely in the PCB –Who owns the process? –When should it next get the CPU? –What is the state of the process? –Which resources does the process have open? From a user’s perspective, a process is a logically contiguous piece of memory –program code, data, stack, heap

17. Review: Processes in Unix Fork, exec, wait Processes form hierarchies Daemon processes Processes can be in one of several states:

19. http://www.cs.unm.edu/~dlchao/flake/doom/

20. Directions: Processes Scheduling has been receiving attention –Scheduling efficiency under load has been addressed - the current scheduler is O(1) –But fairness is still an issue (e.g. for interactive processes)

21. Review: I/O Files serve as abstractions for bytes on disk, terminals, network connections... Low-level I/O: open, close, read, write Opening/Closing a file –OS tracks open files and offsets by process –File descriptor table, System file table, and i- nodes Reading/writing a file –Buffering: in the kernel or in user-space

22. Review: Unix File Systems Disks are organized into partitions, over which sits a file system –A typical file system in Unix contains space set aside to track i-nodes, and space for storage I-nodes contain both information about a file (use “stat” to see more) and pointers to blocks on disk Directories are special files which contain file names A given i-node may be referenced by more than one path name –Soft and hard links

29. Review: IPC There are a bunch of ways to share information between running processes –pipes and fifos one-way buffers used by the shell –signals asynchronous communication used by apache admin scripts –sockets reliable two-way communication used by ftp server

31. Review: Threads Lightweight-processes –More efficient to start –Easier to share data Threads share the heap, code, and static data with the process, but get their own stack and registers posix: create, join It’s important to write reentrant code, or else use synchronization...

32. Review: Synchronization Multiple threads that write to shared data structures are trouble We need atomic operations to ensure synchronization, otherwise races happen –e.g. TSL can be used mutex: a room that can only be entered by one thread at a time semaphore: a room that can be entered by a set number of threads at a time CV: a condition that must be true before your thread is allowed to proceed

33. Thank You! I learned a lot this semester I hope you’ve gotten value out of the course Build things, stay curious, read, talk with other geeks Stop by to say hi