Understanding of Unix Programs

Using system () call

#include <stdio. h>

#include <stdlib. h>

Main (int arg c , char ** arg v)

{

Int I ;

If  (argc ==  2)

{

Print f (“Usage :  ./a. out         <command 1>   <command 2> ………….”);

Return ;

}

For (I =1 ; argv [i] ; I ++ )

System (arg v [i] );

}

Output

./a .out ls date cal

1. out ass1.c ass2. C ass3. C cmd exec  cmd sys p1.c p2 thu Aug 12  00:28:23 1^st 2010

August 2010

Mon   tue wed  thu   fri  sat 

EX:- 1

# include <stdio.h>

#include <unistd.h>

Main ()

{

Print f (“Before : process id  = % d , parent id = % d/n ”, get p id (), get  p pid ());

Print f (“Enter to call fork ”);

Get char ();

Fork ();

Print f (“After  : process id  = % d , parent id = % d/n ”, get p id (), get  p pid ());

}

Output

Before :

process id = 2069 , parent id = 1835

Enter to call fork

After :

process id = 2071, parent id = 2069

After :

process id = 2069, parent id = 1835

Main ()

1835  Bash

2069  process

Ex :-2

# include <stdio.h>

#include <unistd.h>

Main ()

{

Print f (“Before : process id  = % d , parent id = % d/n ”, get p id (), get  p pid ());

Fork ();

Fork ();

Print f (“After  : process id  = % d , parent id = % d/n ”, get p id (), get  p pid ());

}

Bash : 1835

Main ()

Process : 2156

Output:

Before:           process id = 2156 ,  parent id = 1835

After  :           process id = 2158 , parent id = 2157

After  :           process id = 2157 , parent id = 2156

After  :           process id = 2159 , parent id = 2156

After  :           process id = 2156 , parent id = 1839

Last print f is using  common code, so it is called duplicate process

That is sharing the common code by multiple process.

Observation 1. C

Main ()

{

Print f (“1 : %d %d /n”, get p id (), get p pid () );

Fork ();

Print f (“2: %d %d /n”, get p id (), get p pid () );

Fork ();

Print f (“3 : %d %d /n”, get p id (), get p pid () );

Fork ();

Print f (“14: %d %d /n”, get p id (), get p pid () );

Output :-

1. Observation 2.c

# include <stdio . h>

Main ()

{

Int ret;

Ret = fork ();

If (ret = = -1)

{

P error (“fork ”);

Return ;

}

Else if (ret = = 0 )

{

Print f (“Exclusive child code /n ”);

Exit (0);

}

Else

{

Print f (“Exclusive parent code/ n ”);

Exit (0);

}

}

Output :-

Exclusive child code

Exclusive parent code

      ↓

Fork on success returns two values “Zero ” & “Non - zero” collected by child process

     ↓

Parent process

4.If memory is not sufficient then fork causes error  Race condition  in O.S

If the out come of a multiprocessing environment involved with 2 tasks  “T1” & “T 2” , is different from then condition  is  occurred.

• Child process always run in background.
• Parent process  always run in background.

If the parent exited before child then it is called orphan.

If the child exited but parent is running  it is “ZOMBIE” state.

 That is exited value is not collected by parent.

The exit status must be collected only by wait (); wait (), will be discussed later.

W. a. p to execute an application which will be duplicated with four new jobs. That is total no. of jobs hence must be is

(i) x  x +1  x +2  x +3.

Hence two process should  not have common  parent id.

#include <stdio.h>

#include <unistd.h>

Main ()

{

If (fork ())

{// parent process

Print f ( “parent : entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

Sleep (11);

Print f ( “parent : entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

}

Else

{

//child 1 process

If (fork ())

{

Print f ( “child 1 : entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

Sleep (1);

Print f ( “child 1 : entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

}

Else

{

If (fork ())

{// child 2 process

Print f ( “child 2 : entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

Sleep (3);

Print f ( “child 2: entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

}

Else

{//child 3 process

Print f ( “child 3: entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

Sleep (2);

Print f ( “child 3 : entered process id : % d, parent id + % d/n), get p id (), get p pid()”);

}

}

}// end of ‘ else ’  paired with main ‘if’

}// end  of main

Output

Output :-

W. a. p to execute pwd , cal, date by child 1 , child 2 & child 3 with random delays and observe the sequence  of output

#include <stdio.h>

#include <unistd.h>

Main ()

{

If (fork ())

{

If (fork ())

{

Int t ;

Brand (get p id ());

T = rand () % 10 +1;

Sleep (t);

Execlp (“pwd”, ‘pwd ’, NULL);

}

Else

{

Int  t ;

Brand (get p id ());

T = rand () % 10 +1;

Sleep (t);

Execlp (“cal”, ‘cal ’, NULL);

}

}

Else

{

Int t ;

Brand (get p id ());

T = rand () % 10 +1;

Sleep (t);

Execlp (“date”, ‘date ’, NULL);

}

}

Here we can declare ‘t’ as  common local / global variable. It does not be a problem due to concept of “copy – on - write”.

Output :-

July 2011

SU   MO TU  WE  TH  FR  ST

Sat Jul 284:05:34 1^st 201

/home /b59p2/UNIX/fork

Name

# include <stdlib.h>

Rand, rand –r, brand – pseudo- random number generator int rand (void) ;

Int rand-r  (unsigned int * seed p);

Void brand  (unsigned int * seed);

Return Value

The  rand () and rand – r function return a value between 0 and RAND – MAX.

The brand () function return no  value.

• The rand () function returns a pseudo- random number generator integer between 0 and RAND – MAX.
• The Brand () function sets its argument as the seed for a new sequence of pseudo- random number generator integers to be returned  by rand ()
• These sequence are repeatable by calling brand() with same seed value.
• If no seed value is provided , the rand () function is automatically seeded with a value of 1.