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Understanding of Unix Programs

12 September, 2018

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Q. W.a. p to accept any number of commands from the command line and execute them in the same sequence as they are provided.   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 1st 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 2 Bash 2069 2 process     Screenshot_1    

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 Screenshot_2   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 :-   Screenshot_3      

1 2470 2439
2 2471 2470
2 2470 2439
3 2473 2470
4 2474 2473
4 2473 2470
3 2470 2439
4 2475 2470
4 2470 2439
3 2472 2471
3 2471 1
4 2477 2471
4 2471 1
4 2476 2472
4 2472 1

    Screenshot_4    

1 2550 2439
2 2550 2550
3 2552 2550
4 2553 2552
4 2552 2551
3 2551 2550
4 2554 2551
4 2551 2550
2 2550 2439
3 2555 2550
4 2556 2555
4 2505 2550
3 2550 2439
4 2557 2650
4 2550 2439

     

  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); } }   Screenshot_5     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.

2If the parent exited before child then it is called orphan. 2If the child exited but parent is running  it is “ZOMBIE” state. 2 That is exited value is not collected by parent.   Screenshot_6 Screenshot_7     The exit status must be collected only by wait (); wait (), will be discussed later.   2W. 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 2 x +1 2 x +2 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  

Parent : Entered Process id =  3272 Parent id = 2439
Child 1 : Entered Process id =3273 Parent id =  3272
Child 2 : Entered Process id =3274 Parent id = 3273
Child 3 : Entered Process id =3275 Parent id =3274
Child 3 : Exiting Process id =3275 Parent id =3274
Child 2 : Exiting Process id =3274 Parent id =3273
Child 1 : Exiting Process id =3273 Parent id =3274
Parent  : Exiting Process id =3272 Parent id =2439

    Screenshot_8     Screenshot_9       Output :-  

Child 3 : Entered Process id =  2054 Parent id = 2063
Child 2 : Entered Process id =2055 Parent id =  2053
Child 1 : Entered Process id =2056 Parent id = 2053
Parent  : Entered Process id =2053 Parent id =1858
Child 3 : Exiting Process id =2054 Parent id =2053
Child 2 : Exiting Process id =2055 Parent id =2053
Child 1 : Exiting Process id =2056 Parent id =2053
Parent  : Exiting Process id = 2053 Parent id =1853

    2W. 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 1st 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.

 

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