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Unix&Network programming

"Study Of Multiuser Operating System and their Features

Course Instructor : K.JAVUBAR SATHICK

University : BSA University

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Contents to be covered:

Process Environment

Main function

Process Termination.

Command-line arguments.

Environment List

Memory layout of C program in unix.

Memory Allocation

DEPARTMENT OF COMPUTER APPLICATIONS

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Introduction

In this chapter we examine the environment of a single process.

We also see how the main function is called when program is executed.

How command-line argument are passed to the new progam.

How the typical memory layout looks like.

How to allocate the additional memory.

How to use the environment variable.

Various ways for process to terminate.


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Main function

A c program starts execution with a function called main.

The prototype for the main function is int main(int argc, char *argv[]);

Where argc is the no. of command-line arguments and argv is an array

of pointers to the arguments.

When C program is executed by kernel -by any one of the exec fn.


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Process Termination

There are eight ways for a process to terminate.

Normal termination occurs in five ways.

Abnormal termination occurs in three ways.

Five normal termination ways are as follows,

1. Return from main fn.

2. Calling exit fn.

3. Calling _exit or _Exit fn.

4. Return of the last thread from its start routine.

5. Calling pthread_exit from the last thread.


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Process Termination

Abnormal termination occurs in three ways.

Three Abnormal termination ways are as follows,

1. Calling abort fn.

2. Receipt of signal.

3. Response of the last thread to cancellation request.


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Exit functions

Three functions terminate a program or process normally are as follow

_exit() and _exit() which returns to the kernel immediately,

and exit(), which performs certain clean up processing and then returns to

the kernel.


#include // standard library header file

Void exit(int status); // performs clean up work

Void _Exit(int status);// no clean up work, returns im to ker

#include // unix standard library header file

Void _exit(int status);// no clean up work, returns im to ker

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Command-Line arguments& Environment ListCommand-Line Argument:

The term command-line argument can be informally defined as anyparameter that is typed after the name of a program in a command

interpreter (such as a DOS box or Unix shell).

For example, if you typed the command rmfile.s here "file.s" is a

command-line argument which tells the program rm to remove the file "file.s".

This is the part of normal operation of the unix system shells.

Environment List:

Each program is also passed an environment list, like the argument list.

The environment list is an array of character pointers, with each pointercontaining the address of a null-terminated c string.

The address of the array of pointers is contained in the global variable

environ: extern char ** environ


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Environment List

environment ptr environment list environment string

Environ: HOME=/home/sar\0

PATH=/bin:/usr/bin\0

SHELL=/bin/bash\0

USER=sar\0

LOGNAME=sar\0

ENVIRONMENT CONSISTING OF FIVE C CHARACTER STRINGS


NULL

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Memory layout of c programThe memory layout of C program as the following segments

1. Text Segment.

2. Intialized data segment.

3. Unintialized data segment.

4.Stack.

5.Heap.

1. Text Segment:

This segment as machine instruction that the cpu executes.

Usually, the text segment is sharable so that only a single copy needs to bein memory for frequently executed programs such as c compiler, the shellso on....


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Memory layout of c program2. Intialized data segment:

This segment usually called simply the data segment, containing variablesthat are specifically initialized in the program. For eg int maxcount=35;

3. Uninitialized data segment:

This segment often called as the bss segment block started by symbol.

Data in this segment are initialized by the kernel before the program startsexecuting.

4. Stack:

In this segment, the automatic variables are stored, along with informationthat is saved each time when the function is called.

The space for automatic and temporary variable is stored on the stack.


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Typical Memory Arrangement5. Heap:

In this segment dynamic memory allocation usually takes place.

And the heap is located between uninitialized data segment and stacksegment.

high address command-line agr and env. Var

initialized to zero by execread from program file by

exec

Low address


Stack

heap

Uninitialized data

Initialized data

text

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Memory AllocationThree functions is used for Memory allocation;

1. Malloc: which allocates the specified no. Of bytes of memory. The initialvalue of the memory is indeterminate.

2. Calloc: which allocates space for a specified no.of objects of a specified

size. The space is initialized to all 0 bits.3. Realloc: Which increase or decrease the size of previously allocatedmemory.


#include

Void *malloc(size_t size);

Void *calloc(size_t nobj,size_t size);Void *realloc(void* ptr,size_t newsize);

Void free(void * ptr);

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Process controlContents to be covered;

Process Identifier.

Fork function.

Vfork function.

Wait and wait pid function.

Race condition.

Six exec function.


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Process control-process identifier Every process has unique process id.

Unique pid can be reused, once a process terminates their ids become

candidates for reusability.

This prevents a new process from being mistakenly referred again.

Pid with 0 is scheduler process and it is often referred as swapper.

In addition to the process id,there are other identifier for every process they are

as follows,

A. pid_t getpid(void) // process id of the calling process.

B. pid_t getppid(void) //parent process id of the calling process.

C. uid_t getuid(void) //real user id of the calling process.

D. uid_t geteuid(void) // effective user id of the calling process so on


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Fork function An existing process can create a new process by calling the function known as

fork which is syntactically referred as Pid_t fork(void);

It creates a new process called child process.

It returns 2values : 1st value throws 0 for child process which indicates

that process can have only single parent process. 2nd value throws pid ofthe child process for parent process the reason is process can have more

than one child.

Reason for 0 in child process is child can have only one parent.

Child process can always call getppid to obtain the pid of its parent.

The child process is copy of parent.


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Vfork function & wait and wait pid function Same calling sequence as that of fork function.

The vfork function is intended to create new process when the purpose ofnew process is to exec a new program.

It creates a new process without copying the address space of parent

into child.Wait & wait pid fn:

When a process terminates either normally or abnormally, the kernelnotifies the parent process by sending the SIGCHILD signal to the parent.

Because the child process termination is asynchronous event.so parentmight ignore this signal or it can be handled.

Ifit decides to handle then parent might call wait and waitpid fn.


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wait and wait pid function Role of wait and waitpid fn:

It blocks all the child process which are still running.

Returns immediately with termination status of the child.

Returns immediately with an error.


#include

pid_t wait(int *statloc);

pid_t waitpid(pid_pid, int *statloc,int options);

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Difference between wait and wait pid function The wait fn can block the caller until a child process terminates,

whereas waitpid prevents it from blocking.

The waitpid does not wait for the child that terminates first.

features of waitpid fn:

1. the waitpid fn let us wait for one particular process,whereas wait fnwill return the status of any terminated child.

2. the waitpid fn provides the non-blocking version of wait.

3. the waitpid fn provides support for job control.


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Race condition/ Exec fn This race condition occurs when multiple processes are trying to do

some work with shared data and final outcome depends on order inwhich process run.

Fork fn initiates race condition because parent and child processcompetes.

Six Exec fn:

The exec function is used for execution of the process.

When a process calls one of the exec functions, that process iscompletely replaced by the new program starts executing at its main

function.

The process Id does not change across an exec, because a newprocess is not created; exec merely replaces the current process-itstext,data,heap and stack segments with a brand new program fromdisk.


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Exec function #include

Int execl(const char* pathname, const char *arg0,./*(char *)0 */);

Int execv(const char *pathname, char *const argv[]);

Int execle(const char *pathname,const char *arg0,/* (char *) 0, char*const

envp[]*/);

Int execve(const char *pathname,char *const argv[], char *const envp[]);

Int execlp(const char *filename, const char *arg0,./* (char *) 0 */);

Int execvp(const char *filename, char *const argv[]);

1. The first difference in these fns is that the 1st four fn take a pathname

argumet,whereas the last 2 take a filename argument. When a filenameargument is specified,

2. If filename contains a slash, it is taken as a pathname, otherwise the executable

file is searched for in the dir specified by path environment variable.


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Relationship of six exec function

Only one of these 6 fns,execve is a system call within the kernel. The other 5 are

just library fn that eventually invoke this system call.

build argv build argv build argv

try each use

PATH environ


execlp

execvp execv

execl

execve

(system call

Within the kernel)

execle

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Features Of Unix Operating System Multitasking

Multitasking refers to OS that executes multiple tasks simultaneously.

UNIX refers to a task as a process.

A user can run several commands in background while executing anotherin the foreground.

When a background task is being executing, user can continue doing

another task

e.g., printing a large document can be performed in the background while

editing some other document in the foreground.


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Features Of Unix Operating System Multi-user OS

Multi-user refers to an OS that allows multiple users to use the system

simultaneously.

The theory of multi-user system is to approach 100% computer utilization

while reducing the cost per user.

A single user cannot use the printer, disk, memory or CPU 100% of the

time. But multiple users can increases the use of these devices and

resources by having an OS that manages the resources for them.


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Features Of Unix Operating System Portability

UNIX is highly portability is the ability of the software operating on one machine

to operating as efficiently on another, different machine.

Job Control

Job Control on UNIX refers to the ability to control which job is executed in the

foreground, background or is suspended.

Using Job control can increase the productivity of a user by allowing multiple

tasks to be juggled back and forth between background, foreground andsuspended states.


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Features Of Unix Operating System Hierarchical Structure

UNIX uses a hierarchical structure to store and maintain files. This structure

allows maximum flexibility for storing information to resemble its real life

structure. Multiple users may be grouped by corporate departments

The UNIX Shell

The shell is a very powerful and dynamic UNIX utility. It is the primary interface

to the OS (kernel). It can be interactively programmed or it can be used to write

scripts to solve simple to complex problem.

Pipes and Filters

Pipes and filters contribute to the power of UNIX. These enable several

commands or utilities to be combined to perform complex functions.


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Features Of Unix Operating System

Device Independence

UNIX system considers all devices connected to it as files. It hides the machine

architecture from the users, making it easier to write programs that run on

different hardware implementations.

System Security

Being a multi-user OS, UNIX offers protection to one users information from the

user without permission i.e., Invalid users cannot access data, making it easier

to write programs that run on different hardware implementations.

Communication

The UNIX system has several built in programs, enabling the user to

communicate, transfer files across different UNIX systems and between UNIX

and other OS system.


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Features Of Unix Operating System

Programming Capability

UNIX is a highly programmable operating system.

UNIX facilities the user to develop both system and application programs.

Programming in UNIX is of two types they are Shell programming andProgramming in language (Like C, C++, Java, etc).

The programs written in shells are called as Shell files

The command statements in the programs are called as shell Script.


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Unix System Overview Logging in:

we log in with the name and password where password will be read by the

internal components of os such as kernel,shell etc.

home/mcaii/myfolder

Files &Directories:

File system is a hierarchial arrangements of directories and files.

Directory is a file that contains directory entries.

Some of the attributes of files are 1. type of file,2. size of file,3.owner of thefile,4. permission for file accessing so on

The stat and fstat functions return a structure of information containing all the

attributes of a file.


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Unix System Overview

File Name: Name in a directory which is used refer the file is called as Filename.

Path Name:A sequence of one or more file names separated by slash can be

referred as path name. A path name begin with a slash is often called as

absolute pathname or relative path name.

Working & Home directory: Every process as working directory.Some times

referred as current working directory(CWD). When we login, the working

directory is set to our home directory. It is obtained when we enter a password.

Program & Processes:A program is an executable file residing on the disk in a

directory. An executing instance of program is often referred as process. Every

process is referred by the unique ID called process id and it is a non-negativeinteger.

Process Control: There are 3 primary functions is used for process control they

are fork.exec and waitpid.


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File I/O

Most of the file I/O on unix system can be performed using only five functions:

Open

Read

Write

lseek

and close

File Descriptor: It is denoted by fd

To the kernel, all open files are referred to by File descriptors. A file descriptor is

a non-negative integer.

When we open an existing file or create a new file,the kernel returns a file

descriptor to the process

When we want to read or write a file. we identify the file with fd that was

returned by open or creat fn as an argument to either read or write


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File I/O Open Function:

A file is opened or created by calling the open function

The pathname is name of the file to open or create.

The oflag argument is combination of one or more following operations which is

controlled by the header.

O_RDONLY -open for reading only.

O_WRONLY -open for writing only. ONE OF THESE 3 CONSTANTS

O_RDWR -open for reading and writing. MUST BE SPECIFIED.


#include

int open(const char *pathname,int oflag/*mode_t mode*/);returns file descriptor if OK,-1 on error

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File I/O The following constants are optional

O_APPEND,O_CREAT,O_EXCL,O_TRUNC,O_NOCTTY and O_NONBLOCK

Creat Function:

A new file can also be created by calling the creat fn.


#include

int creat(const char *pathname,mode_t mode);

returns file descriptor opened for writing only if OK,-1 on error

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File I/O Close Function:

An open file is closed by calling the close fn.

Lseek Function:

Every open file has an associated current file offset, normally a non

negative integer that measures the number of bytes from beginning of thefile.

An open files offset can be set explicitly by calling lseek fn.


#include

int close(int filedes);Returns:0 if OK,-1 on error

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File I/O

The interpretation of the offset depends on the value of the whence argument.

If the whence is SEEK_SET,the files offset is set to offset bytes from

beginning of the file.

If the whence is SEEK_CUR,the files offset is set to its current value plusthe offset.

If the whence is SEEK_END,the files offset is set to the size of the file plus

the offset.


#include

off_t lseek(int filedes,off_t offset,int whence);

Returns: new file offset if OK,-1 on error

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File I/ORead function:

Data is read from open file with read function.

Write Function:

Data is written to an open file with write function


#include

ssize_t read(int filedes, void *buf, size_t nbytes);

Returns:no of bytes read,0 if eof,-1 on error

#include

ssize_t write(int filedes, const void *buf, size_t nbytes);Returns:no of bytes written if OK,-1 on error

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