Os1

OS course – lecture 1 Nezer J. Zaidenberg

Announcements  Lecturer : Prof. Amir Averbuch  Tuturing assistant :Mr. Nezer Zaidenberg  Ex. Grader : TBD  The course form has changed since previous

years. It is recommended that you will attend lectures even if you taken this course previously

Grades  40% exercises  70% exam  The exam will include multiple choice

questions. Most of which will be based on the homework!

References  The main books for this course is  Understand the Linux kernel 3rd edition (For

kernel space)

 Advanced programming for the UNIX

environment 3rd edition by Richard W. Stevens (For user space)

 Additional books (that will be noted on

specific lectures)

 UNIX filesystems  Beej Manual for network programming

Course goals  The goal of this course is to gain practical

understanding of the OS

 The course is very practical with lots of

homework and exam based on the homework

 The course will focus on UNIX OS and

particular Linux

 The exercises will be mainly coding work or

reviewing kernel and low-level user code.

What this course is not about  Coding (to the extent of coding standards,

best coding practices or efficiency)

 MPP (beyond simple algorithms)  Network algorithms  Encryption  Etc.  This code is on OS and OS related

programming only

What is OS  Once upon a time there was no Operating systems  Developers used to develop hardware and software,

together with everything needed to make the application run

 As computers become more standard and tasks

become common several tasks stood out as needed by all (or practically all) software.

 Those common tasks became “OS” and were

supplied by 3rd party vendors

 Commercial software used to require specific OS.

This became one of the first usages of “code reusability”

Common tasks  Memory management  Hardware (driver) support  Task scheduling on multi process environments  File management  User management (on multi-user systems)

Software providers developed a single software for all those components (and more) – the OS This software became the base for all further software development

What the OS includes 

Core OS function  Process management  User management  File management  Memory management  Hardware management  Network management



Additional functions are added and removed according to political and marketing scenarios  Initial UNIX implementation had built in C compiler (sold separately with almost all modern OS)  Most UNIX OS comes with full range of servers not really part of “OS” such as ftp server, telnet server, daytime server, printing server and even HTTP server, J2EE platform, etc.  Many OS also come with GUI (windows, OSX aqua, X Windows) and other user tools (calculator, Text processor etc.)  Microsoft wished to add Internet Explorer, Windows media player etc. as part of the OS  The European union prevented the inclusion of ms media player in all windows sold in europe… (anti trust reasons)



The course will deal with “CORE OS” functions only. Those services are usually managed by the OS “Kernel”

What is UNIX 

UNIX is a general term for lots of things



UNIX may refer to standards such as POSIX that defines OS interface that many OS follow



UNIX is also general name for a family of many proprietary OS that were tested for standard completion. (however, even most modern OS does not fully implement all aspects of the standard)



Open source OS (such as Linux) that were not tested by standard committee (but tend to have the save API often referred to as UNIXlike



Most modern UNIX OS are brunched from either system V revision 4 or BSD 4.4 (both ancient versions of UNIX)



Today, for most people, UNIX means an OS with specific set of API and applications regardless of standards. So UNIX and UNIX-like OS’s are considered UNIX. (at least for this course)

UNIX history 

UNIX (acr. for uniplexed information) was initially pun on multics (OS that is virtually extinct) and was the name of OS that was developed at bell labs The OS was initially distributed in source form so that anybody can modify and customize it.



UNIX was developed by Bell labs for their PDP11 computers.



The source was later brunched into two main trunks SYS-V and BSD code (with both brunches often borrowing code from each other)



Most modern Unices are usually based on either of those brunches.



Commercial UNIX today exist only in high end server and desktop market. (for example 2/3 of PC’s sold for over 1000$ are Macintosh – a UNIX brand)



Open source UNIX tends to rule several market segments (Linux and apache rule the http serving and Linux practically runs on all Wireless routers.) and is heavy favorite on several others (running database servers)

UNIX OS in the wild SVR4 Unices 

Solaris – Unix by SUN Microsystems



HP-UX – Unix by HP



Tru64 – old unix by Digital and later Compaq, still maintained by HP



Linux – The most popular open source OS (sponsered by RedHat, IBM, Silicon graphics, Novell and more)

BSD Unices 

OS X and Darwin – Apple OS (OS X is the proprietary version and darwin is the open source version)



AIX – Unix by IBM



FreeBSD, NetBSD, OpenBSD – Open source BSD distributions

NOT UNIX per-se 

VxWorks – embedded OS, follows Posix standard. (but usually UNIX doesn’t refer to embedded system though it can)

GNU source and the FSF  GNU – acr. For “GNU’s not UNIX”  GNU – a public license granting permission to use,

modify and redistribute code provided it remains GNU  FSF – acr. For “free software foundation”  FSF – distribute the source for most of the free UNIX applications (The unix look and feel). Promote the GNU license  Linux kernel – the “main functionality” of the OS developed by volunteers (not the FSF, but the group is connected) under the the GNU license

The many types of free  MIT license – allow you to do what ever you want     

with the code BSD license – allow modification and redistribution (modification may not be free). You must keep credits to original author GNU GPL – modifications must be free GNU LGPL – modification must be free, but linking with GNU source is not modification (license used by GNU libraries) Other free licenses - check on FSF website Commercial Open source – you may view the source but not use it commercially or you may view the source, but modification belong to specific company or modifications may not be redistributed. Are not considered “free” (such as Apple public license, Sun Public license, Netscape public license, MySQL)

Free as in beer – free as in speech  Beer – when you get a free beer – you can drink it.

But not order the entire bar for a round of free beer!  Speech – when you hear an idea you can repeat it and incorporate it in your own speeches.  In neither case – free does not refer to distribution and support fees which may apply.  The open source community looks for “free as in

speech” software. And only “free as in speech” is considered free. (GPL, LGPL, BSD, MIT are all “free” while commercial free licenses are not considered “free”.)

What is Linux distribution  Linux distribution is a gathering of software

from several sources, compiling them and branding them by single company

 All Linux distribution include the Linux kernel

and most GNU sources (thereby Linux distribution….)

 Linux distributions by Red hat SuSe, debian,

Ubunto, slack ware have been popularized some gained commercial support.

 Some Linux distribution include only free

content some include open source (but not free) as well and some include commercial content

 On this course we will focus on fedora core 9

OS code and User code  OS code – usually invoked in “the kernel” is

the code that “makes the computer work”. Including drivers, memory management etc.

 User code – most code we write  In this code we will use the terms Userland

and kernelspace to distinguish between them.

 We will write both kinds of code in the

exercises

UNIX OS internals  UNIX OS is built on top of two basic

principals  Process  File

 All OS services are implemented as either

process or file. (for example the system logger is a “process” while a socket is a file.)

 This is one of the first implementations of

the interface design pattern

Some unix file handling code  Open(2)  Close(2)  Read(2)  Write(2)

To learn about those file type “man 2 open” to get the function C info.

Library code relation with OS code  Library functions, such as printf, are

invoking OS code to complete their work

 For example printf  Printf(3) (short for print formatted) parses

the input and format the text. Then it calls for write.

 Printf is library function (and not OS

function)

The difference between system and library function OS function

Library function

Lowest level interface

Usually calls OS function

Section 2 of man

Section 3 of man

System dependent and may not exist on different systems (or used with different name or (parameters Almost always bug free

Programming language dependent (but not system (dependent

Almost always efficient

Almost always efficient

Usually – kernel space code

Usually user space code

Almost always bugfree

fopen(3) implementation   FILE *fopen(char *name, char *mode) {        int fd;        FILE *fp;  if (*mode != ‘r’ && *mode != ‘w’ && *mode != ‘a’)            return NULL;        for (fp = _iob; fp < _iob + OPEN_MAX; fp++)            if ((fp->flag & (_READ | _WRITE)) == 0)                break;        /* found free slot */        if (fp >= _iob + OPEN_MAX)   /* no free slots */            return NULL;        if (*mode == ‘w’)            fd = creat(name, PERMS);        else if (*mode == ‘a’) {            if ((fd = open(name, O_WRONLY, 0)) == -1)                fd = creat(name, PERMS);            lseek(fd, 0L, 2);        } else            fd = open(name, O_RDONLY, 0);        if (fd == -1)         /* couldn’t access name */            return NULL;        fp->fd = fd;        fp->cnt = 0;        fp->base = NULL;        fp->flag = (*mode == ‘r’) ? _READ : _WRITE;        return fp; }

Conclusion  So OS code is the underlying layer of the

library function  Homework – (not for submission)  Examine printf(3) code – find the OS function

call references.  Follow all functions printf calls!