Operating-System Structures

« A View of Operating System Services

· Communications –Processes may exchange information, on the same computer or between computers over a network. Communications may be via shared memory or through message passing

· Error detection – OS needs to be constantly aware of possible errors

§ May occur in the CPU and memory hardware, in I/O devices, in user program

§ For each type of error, OS take the appropriate action to ensure correct and consistent computing

§ Debugging facilities enhance the user’s and programmer’s abilities to efficiently use the system

· Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them.

Many types of resources such as CPU cycles, main memory and file storage have special allocation code,

others such as I/O devices may have general request and release code

· Accounting - To keep track of which users use how much and what kinds of computer resources

· Protection and security - owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other

§ Protection involves ensuring that all access to system resources is controlled

§ Security from outsiders requires user authentication, defending external I/O devices from invalid access attempts.

§ For a system to be protected and secure, precautions must be instituted throughout it.

A chain is only as strong as its weakest link.

« User Operating System Interface

· Command Line Interface (CLI) or command interpreter allows direct command entry

§ Sometimes implemented in kernel, sometimes by systems program

§ Sometimes multiple flavors implemented – shells

§ Primarily fetches a command from user and executes it

· Sometimes commands built-in, sometimes just names of programs

If the latter, adding new commands doesn’t require shell modification.

· User-friendly desktop Graphical User Interface (GUI) metaphor interface

§ Usually mouse, keyboard, and monitor

§ Icons represent files, programs, actions, etc

§ Various mouse buttons over objects in the interface cause various actions

(Provide information, options, execute function, open directory (folder)

§ Invented at Xerox PARC

· Many systems now include both CLI & GUI interfaces

§ Microsoft Windows is GUI with CLI “command” shell

§ Apple Mac OS has GUI interface with UNIX kernel underneath and shells available

§ Solaris is CLI with optional GUI interfaces (Java Desktop, KDE)

« System Calls

· Programming interface to the services provided by the OS

· Typically written in a high-level language (C or C++)

· Mostly accessed by programs via a high-level Application Program Interface (API)

· Three most common APIs are

§ Win32 API for Windows,

§ POSIX API (all versions of UNIX, Linux, and Mac OS)

§ Java API for the Java virtual machine (JVM)

Example of System Calls:

Examples:

ê Example 1: copy

Description: Open an existing input file and copy it to non-existing output file.

v C-version

http://www.cs.odu.edu/~cs471w/code/misc/copy.c

int
main(int argc, char **argv)
{
int n, in, out;
char buf[1024];
char InputFile[1024];
char OutputFile[1024];
struct stat st;

printf("enter Input File Name: ");
scanf("%s", InputFile);
if (stat(InputFile, &st) != 0) {
perror("InputFile does not exit");
exit(1);
}
printf("enter Outut File Name: ");
scanf("%s", OutputFile);
if (stat(OutputFile, &st) == 0) {
perror("OutputFile exists");
exit(1);
}

if ((in = open(InputFile, O_RDONLY)) < 0) {
perror(InputFile);
exit(1);
}

if ((out = open(OutputFile, O_CREAT | O_WRONLY, 0755)) < 0) {
perror(OutputFile);
exit(1);
}

while ((n = read(in, buf, sizeof(buf))) > 0)
write(out, buf, n);

close(out);
close(in);
printf("copied %s to %s\n", InputFile, OutputFile);
exit(0);
}

To execute:

% copy

NOTE: 0755 means file permission (0 for -) should be:

rwx r-x r-x

However, the shell varaible umask (1 for -) may change the final permission.

For example in /home/cs471w/.cshrc the value of umask 0133:

rw- r-- r--

The file permission should not exceed the umask value and therefore the file created will have permission: rw- r-- r--

v Java-version

http://www.cs.odu.edu/~cs471w/code/misc/jcopy.java

public class jcopy {
public static void main(String[] args) throws IOException {

BufferedReader stdin = new BufferedReader( new InputStreamReader(System.in));

System.out.println("enter Input File Name: ");
String InputFile = stdin.readLine();
File infile = new File(InputFile);
InputStream fin = new FileInputStream(infile);

System.out.println("enter Outut File Name: ");
String OutputFile = stdin.readLine();
File outfile = new File(OutputFile);
OutputStream fout = new FileOutputStream(outfile);

int len;
byte[] buf = new byte[1024];
while ((len = fin.read(buf)) > 0) {
fout.write(buf, 0, len);
}
fin.close();
fout.close();
System.out.println("copied " + InputFile + " to " + OutputFile);
}
}

To execute:

% java jcopy

v Python-version

http://www.cs.odu.edu/~cs471w/code/misc/copy.py

#! /usr/bin/python
import sys
print ("enter Input File Name: ")
fname = sys.stdin.readline()
InputFile=fname.strip('\n')
print(InputFile)
print ("enter Output File Name: ")
fname = sys.stdin.readline()
OutputFile=fname.strip('\n')
print(OutputFile)
try:
fin = open(InputFile,'r')
fout = open(OutputFile,'w')
for line in fin:
fout.write(line)
fin.close()
fout.close()
except:
print ('Something went wrong.')

To execute:

% copy.py

ê Example 2: save:

Description: save 2^nd argument (a string) into 1^st argument file.

If the file exists it is truncated, otherwise it is created.

It then displays the content of the file to tty (standard output or file descriptor 1).

v C-version:

http://www.cs.odu.edu/~cs471w/code/misc/save.c

int
main(int argc, char *argv[])
{
int sfd;
int n;
char buf[1024];
int FileLength;
char *FileName = argv[1];
char *Message = argv[2];

if ((sfd = open(FileName, O_RDWR | O_TRUNC| O_CREAT, 0600)) < 0) {
perror(FileName);
exit(1);
}

write(sfd, Message, strlen(Message));
lseek (sfd,0, SEEK_SET); // beginning of file
printf("\nFile '%s' contains:\n", FileName);
fflush(stdout);
n=read(sfd, buf, sizeof(buf));
write(1,buf, n);
FileLength = lseek(sfd, 0, SEEK_END); // end of file
printf("\n\nFile '%s' length: '%d'\n", FileName, FileLength);
}

To execute:

% save savefile “how are you?”

v Java-version

http://www.cs.odu.edu/~cs471w/code/misc/jsave.java

public class jsave {
public static void main(String[] args) throws IOException {

File FileName = new File(args[0]);
String Message = args[1];

OutputStream fout = new FileOutputStream(FileName);
fout.write(Message.getBytes());

System.out.println("File " + FileName + " contains:");

BufferedReader fin = new BufferedReader( new FileReader(FileName));
String str;
while((str = fin.readLine())!= null)
System.out.println(str);

long length = FileName.length();
System.out.println( FileName + " lenght is: " + length);

}
}

To execute:

% java jsave savefile “how are you?”

v Python-version

http://www.cs.odu.edu/~cs471w/code/misc/save.py

#! /usr/bin/env python

import sys
fname = sys.argv[1]
msg = sys.argv[2]
try:
fout = open(fname,'w')
fout.write(msg)
fout.close()
fin = open(fname,'r')
for line in fin:
print(line)
fin.seek(0,2) # move the cursor to the end of the file
size = fin.tell()
print(size)
except:
print ('Something went wrong.')

To execute:

% save.py savefile “how are you?”

API – System Call – OS Relationship

Standard C Library Example:

C program invoking printf() library call, which calls write() system call

System Call Parameter Passing

Three general methods used to pass parameters to the OS

1. Simplest: pass the parameters in registers

2. Parameters stored in a block in memory, and address of block passed as a parameter in a register.

This approach taken by Linux and Solaris.

3. Parameters pushed onto the stack by the program and popped off the stack by the operating system

Block and stack methods do not limit the number or length of parameters being passed

Types of System Calls:

ü Process control

ü File management

ü Device management

ü Information maintenance

ü Communications

ü Protection

Examples of Windows and Unix System Calls

« System Programs

· System programs provide a convenient environment for program development and execution.

They can be divided into:

§ File management

§ Status information

§ Programming language support, loading and execution.

§ Communications

§ Application programs

· Most users’ view of operating system is defined by system programs, not the actual system calls

· Provide a convenient environment for program development and execution

Some of them are simply user interfaces to system calls; others are considerably more complex

· File management:

Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories

· Status information:

§ System info - date, time, amount of available memory, disk space, number of users

§ Others provide detailed performance, logging, and debugging information

§ Typically, these programs format and print the output to the terminal or other output devices

« Operating System Design and Implementation

· User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast

· System goals – operating system should be easy to design, implement, maintain, flexible, reliable, error-free, and efficient

· Important principle to separate:

Policy: What will be done?
Mechanism: How to do it?

The separation of policy from mechanism is a very important principle,

it allows maximum flexibility if policy decisions are to be changed later

Traditional UNIX System Structure:

The UNIX OS consists of two separable parts

· Systems programs

· The kernel

4 Consists of everything below the system-call interface and above the physical hardware

4 Provides file system, CPU scheduling, memory management, and other operating-system functions;

« Virtual Machines

· A virtual machine provides an interface identical to the underlying bare hardware

· The operating system host creates the illusion that a process has its own processor and (virtual memory)

· Each guest provided with a (virtual) copy of underlying computer

VMware Architecture:

The Java Virtual Machine:

« Operating-System Debugging

· Debugging is finding and fixing errors, or bugs

· OSs generate log files containing error information

· Failure of an application can generate core dump file capturing memory of the process

· Operating system failure can generate crash dump file containing kernel memory

· Kernighan’s Law:

Debugging is twice as hard as writing the code in the first place.

Therefore, if you write the code cleverly you are not smart enough to debug it.

« Operating System Generation

· Operating systems are designed to run on any class of machines; the system must be configured for each specific computer site

· SYSGEN program obtains information for specific configuration of the hardware system

· Booting – starting a computer by loading the kernel

· Bootstrap program –stored in ROM, locate the kernel, load it into memory, and start its execution