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Teaching Operating
Systems With
Programming and
Freeware
Lecture 1: Introduction, IPC and Lab1
A workshop by
Dr. Junaid Ahmed Zubairi
Visiting Associate Professor
CIT, Agriculture University, Rawalpindi
Workshop Overview
Operating Systems Course Outline
Topics Suited for Programming
Assignments
Process Model and IPC(Lab1)
Concurrency Issues (Lab2)
Processor Scheduling (Lab3)
Disk Scheduling and RAID
Programming Project
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Workshop References
Operating Systems Internals and
Design Principles by William Stallings,
4th Edition Prentice Hall 2001
Modern Operating Systems by Andrew
Tanenbaum
Linux Programmer’s Guide by S. Goldt,
S. Meer, S. Burkett and M. Welsh March
1995
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Course Outline
A typical undergraduate operating
systems course would include:
Process and thread models
 Concurrency and deadlocks
 Uni and multiprocessor scheduling
 Realtime systems
 Memory management
 Disk scheduling

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
The Need for
Programming
Operating systems are software programs
Various algorithms and mechanisms are
implemented in operating systems to manage
the computer
The students will get a better understanding
of the main concepts if they are given
programming assignments
Some institutions require the students to
develop a full working operating system
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Topics Suited for
Programming
Assignments
Following topics are considered suitable
for programming assignments
Process and thread models
Concurrency issues, semaphores
Deadlocks and resolution
Processor scheduling
Disk scheduling
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Requirements of an
Operating System
Interleave the execution of several
processes to maximize processor
utilization while providing reasonable
response time
Allocate resources to processes
Support interprocess communication
and user creation of processes
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Process
Also called a task
Execution of an individual program
Can be traced
The diagram shows a currently active
process. What will be the next process
to execute?
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Two-State Process
Model
Process may be in one of two states
Running
 Not-running

Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Not-Running Process in
a Queue
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Process Creation
Submission of a batch job
User logs on
Created to provide a service such as
printing
Process creates another process
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Process Termination
Batch job issues Halt instruction
User logs off
Quit an application
Error and fault conditions
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Processes
Not-running

ready to execute
Blocked

waiting for I/O
Dispatcher cannot just select the
process that has been in the queue the
longest because it may be blocked
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
A Five-State Model
Running
Ready
Blocked
New
Exit
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Using Two Queues
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Suspended Processes
Processor is faster than I/O so all processes
could be waiting for I/O
Swap these processes to disk to free up more
memory
Blocked state becomes suspend state when
swapped to disk
Two new states


Blocked, suspend
Ready, suspend
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
One Suspend State
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Two Suspend States
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Reasons for Process
Suspension
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Group Worksheet
Please complete the group worksheet 1
and hand it over in 15 minutes. A
maximum of 3 members are allowed per
group
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Process Creation
Assign a unique process identifier
Allocate space for the process
Initialize process control block
Set up appropriate linkages

Ex: add new process to linked list used for
scheduling queue
Create of expand other data structures

Ex: maintain an accounting file
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Execution of the
Operating System
Non-process Kernel


execute kernel outside of any process
operating system code is executed as a separate
entity that operates in privileged mode
Execution Within User Processes


operating system software within context of a user
process
process executes in privileged mode when
executing operating system code
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
UNIX SVR4 Process
Management
Most of the operating system executes
within the environment of a user
process
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
UNIX Process States
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Programming
Assignment 1 Lab 1
In order to understand the processes in
a better way, it is recommended that the
participants become familiar with
UNIX/Linux user interface and then
write a program that uses fork
command to start several processes.
We will use a red hat linux server.
Please login to workshop group
accounts and change your passwords.
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
UNIX/Linux User
Interface
UNIX/Linux user interface is simple and
easy to use
Mostly the user logs in to the default
“bash” shell
Use “ls –al” to list all directories and files
Use “ls –F” to see files and directories
separately
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
UNIX/Linux User
Interface
Use “cat filename” to see contents of a
file
Use “more filename” to see a file longer
than a page
Use “w”, “who”, and “finger” to see who
else is logged on
Try “chfn”
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
UNIX Utility Programs
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Processes in UNIX
Process creation in UNIX.
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
POSIX Shell
A highly simplified shell
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
The Editor
“pico” is the best text mode editor
available under Linux
Pico allows you to start entering text
after you reach the text insertion point
using arrows
Pico has some commands that can be
given with control-key combinations,
including the command to save and exit
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Practice Problem 1 Lab1
Using pico, type the source code given
into your Linux accounts and save and
exit
Using “gcc”, compile and run the
program
(Example: gcc myforks.c –o myforks)
Comment on the output of the program
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Concurrent Process
Creation Example
#include <stdlib.h>
#include <stdio.h>
int sum;
main() {
int i;
sum=0;
fork();
for (i=1; i<=5; i++) {
printf(“the value of i is %d\n”,i);
fflush(stdout);
sum+=i;
}
printf(“the sum is %d\n”, sum);
exit(0);
}
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Explanation
When the program reaches the line with
statement fork(), the system duplicates
the process and allows both the original
and duplicate processes to execute
forward
The original process is called “parent”
and the duplicate process is called
“child”
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Parent-Child
Identification
It is easy to identify the parent and the
child. The value returned by fork() is
examined. If it is zero, it is the child else
it is the parent
Consider the same program with the
identification of parent and child
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Parent-Child
Identification
#include <stdlib.h>
#include <stdio.h>
int sum;
main() {
int i;
sum=0;
if (fork()) printf("This is parent\n");
else printf("This is child\n");
for (i=1; i<=5; i++) {
printf(“the value of i is %d\n”,i);
fflush(stdout);
sum+=i;
}
printf(“the sum is %d\n”, sum);
exit(0);
}
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Programming
Assignment 2 Lab 1
Rewrite the program so that the parent
and the child do different activities and
display different messages on the
screen. For example, parent could run a
loop to display all odd integers from 1 to
100 and the child could display all even
integers from 1 to 100.
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Interprocess
Communication Using
Pipes
Processes communicate among
themselves using messages, sockets
and pipes
In this workshop, we will learn how to
use UNIX pipes for interprocess
communications
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Pipes
Two processes connected by a pipe
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
UNIX Pipes
The most common example is the use
of pipes in shell commands. For
example: “ls –al | grep cc | wc –l”
In this line, three commands are
connected together using pipes.
Let us analyze this line and all
commands one by one
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Pipes
“ls –al” command lists all the contents of the
current directory
“grep cc” searches for and outputs the lines in
the result of the previous command that
contain the search pattern cc
“wc –l” counts the number of lines that have
the search pattern cc
Thus pipes connect output of one command
to the input of the next command
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Unnamed Pipes
The pipe that we used in shell command is the
unnamed pipe
We can also create unnamed pipes in our C
programs with:
#include <unistd.h>
int pipe(int fd[2]);
Returns 2 file descriptors in the fd array.
fd[0] is for read
fd[1] is for write
Returns 0 on successful creation of pipe, 1 otherwise.
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Reading and Writing
When reading from a pipe, the unused
end of the pipe (write end) is closed.
if write end of the is still open and there
is no data, read() will sleep until input
become available.
Please compile and execute the source
code given using gcc (execute with
./filename)
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Example of Unnamed
Pipe
#include <stdio.h>
#define READ 0
/* The index of the “read” end of the pipe */
#define WRITE 1
/* The index of the “write” end of the pipe */
char * phrase = “Stuff this in your pipe and smoke it”;
main ()
{
int fd[2], bytesRead;
char message [100]; /* Parent process’s message buffer */
pipe ( fd ); /*Create an unnamed pipe*/
if ( fork () == 0 ) /* Child is the Writer */
{
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Example Code
Continued
close (fd[READ]); /* Close unused end*/
write (fd[WRITE], phrase, strlen ( phrase) +1);
close (fd[WRITE]); /* Close used end*/
}
else /* Parent is the Reader */
{
close (fd[WRITE]); /* Close unused end*/
bytesRead = read ( fd[READ], message, 100);
printf ( “Read %d bytes: %s\n”, bytesRead, message);
close ( fd[READ]); /* Close used end */
}
}
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
IPC Using Named Pipes
Named pipes are created as special
files. They are also called FIFO (First-in
First-out)
Named pipes can be created from the
shell with (for example)“mknod myfifo p”
This command results in the creation of
a named pipe called myfifo with a file
type p (try it out)
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Named Pipes
Named pipes can be created within
programs by using a command:
mknod ( “mypipe”, SIFIFO, 0 );
Once a named pipe is created,
processes can open(), read() and write()
them just like any other file.
There is a difference however from
normal files
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Named Pipes
opens for reading will block until a process
opens it for writing.
opens for writing will block until a process
opens it for reading.
Thus the IPC can be achieved in a smooth
way using named pipes
Compile and run next two programs to get a
feel for how the named pipes operate. It is
assumed that a named pipe “mypipe” already
exists
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Named Pipe Example:
Writer
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
char * phrase = “Stuff this in your pipe and smoke it”;
int main ()
{ int fd1; fd1 = open ( “mypipe”, O_WRONLY ); write
(fd1, phrase, strlen ( phrase)+1 ); close (fd1);
}
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002
Named Pipe Reader
Reader
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main ()
{
int fd1;
char buf [100];
fd1 = open ( “mypipe”, O_RDONLY ); read ( fd1, buf, 100 );
printf ( “%s\n”, buf ); close (fd1);
}
Operating Systems Workshop CIT, Arid Agriculture University Aug 2002