Cp217

CP217 - Final Exam Review

By: Robin Sharma

Unix Architecture:

- System resources:

1. CPUs

2. memory, disk

3. devices: i/o

4. process mgmt

5. IPC

6. timers

7. Networking

- OS is a resource manager. It takes the form of a set of software routines that allow users and application programs to access system resources in a safe, efficient and abstract way

- kernel is in direct control of the underlying hardware. The kernel provides low-level device memory and processor mgmt functions, eg. dealing with interrupts from hardware devices, sharing the processor among multiple programs, allocating memory for programs etc.

System Calls:

- basic hardware-independent kernel services are exposed to higher-level programs through library of system calls.

Application Programs: ex. word processors, spreadsheets etc. make use of system calls

System Utililty programs:

- simple but useful application programs that come with the OS, eg. programs which find text inside a group of files), make us of system calls.

Shells and GUIs

- applications and system utilities are launched using a shell

System programs and programming:

- system programs are those that direclty inferface with the kernel services and system calls

Examples:

- ls, cd, pwd, who

- shell, text editor, compiler, system daemons, core utils etc.

- used by other programs or by users directly

- system programming is to write these types of system programs

Who:

purpose: list users currently logged on

output: logname, terminal, time, from where

How does it work?

open utmp

while(~eof) {

read record

display info

}

close file

Where is this information?

*utmp - holds entries for currently active user login sessions

wtmp - where all log entries are simply appended to the end of the file, acts as historcial utmp

btmp - records failed login attempts

struct utmp {

char ut_user[8]; /* User login name */

char ut_id[4]; /* /etc/inittab id(usually line #) */

char ut_line[12]; /* device name (console, lnxx) */

short ut_pid; /* short for compat. - process id */

short ut_type; /* type of entry */

struct exit_status ut_exit; /* The exit status of a process */

/* marked as DEAD_PROCESS. */

time_t ut_time; /* time entry was made */

};

Question: How do I read stucts from a file?

ans: use open(), read(), and close()

fd = open(name, mode) <-- creates a connection to a file

name = char *

mode = O_RDOLNY, O_WRONLY, O_RDWR

returns -1 on error OR a file descriptor int on success

n = read(fd, array, numchars)

transfer numchars from file to the array

returns number of chars actually transferred or -1 on error