Session 5 : Operating System Programming Concepts : Lab 5 - Exec System Call

Lab: 5 Exec System Call 

Exec Family Examples: 

execl

execl(“/usr/bin/wc”,”wc”,”-w”,(char*)0);

execle

char *env[] = {“HOME=/usr/home”,”LOGNAME=home”,(char *)0};

execle(“/bin/ls”,”ls”,”-l”,(char*)0,env);

execlp

execlp(“sort”,”sort”,”-k”,”2”,”-n”,”ex4_4.txt”,”-o”,”ex4_4_sorted.text”,0);

execv

char *cmd[] = {“ls”,”-l”,(char*)0};

execv(“/bin/ls”,cmd);

execve

char *cmd[] = {“ls”,”-l”,”-a”,(char *)0};

char *env[] = {“HOME=/usr/home”,”LOGNAME= home”,(char*)0};

execve(“/bin/ls”,cmd,env);

execvp

char *cmd[] ={“ls”,”-l”,(char*)0};

execvp(“ls”,cmd);

Learning goals: in this laboratory activity you will learn how to use
waitpid and exec system calls.
You will understand how to wait for a specific child and how to
excute command in your own programs. 

Exercise 1 

Using system calls fork and waitpid write a program that receive a number n
from the command line and do the following: 

1.creates n processes 

2.the first child created sleep n second, the second child sleep n-1
seconds and so on.....(the last child sleep 1 second) 

3.the parent wait for “all” the child(it waits sequentially
starting from the first created to the last – you have to use waitpid system call) 

Exercise 2 

Modify the exercise 1 using wait instead waitpid . How many times the parent
has to call “wait” ? 

Does this program behaves as the previous one? Check it out!  

Exercise 3 

Run, in background mode, the programs you wrote in the exercises 1 and 2
and using ps command try to figure out how many zombie process you have. 

Exercise 4 

Download ex4_4.txt file then write a C program which will execute the
following series of Linux Commands:

1. cp {path}/ex4_4.txt . #path is the path you downloaded ex4_4.txt file
– it has to be different from “.” 

2. sort -k 2 -n ex4_4.txt -o ex4_4_sorted.txt 

3. wc -w ex4_4.txt 

4. cat ex4_4.txt 

 Commands 2 and 3 have to be executed concurrently, find below the
corresponding execution 

graph:

At the end of the excution of all of its children the parent has to
print a message. 

Hint: consider the 4 commands as 4 different processes(which are created by
the parent) 

Exercise 5 

Try to figure out what is the output of the following program: 

#include
<stdio.h> 

int
main (int argc, char ** argv){ 

int
i = 0; 

printf
("%d %d\n", getpid(), ++i); 

execl
(argv[0], argv[0], 0); 

printf
("end\n"); 

return
(1); 

}

Then compile and run it and see what's going on. 

Summary

At the end of this laboratory activity you should have understood how to
use waitpid and exec system calls . You should have understood the difference between wait and
waitpid system calls, and the difference between zombie and orphan process too