System call of UNIX fork and exec

1. System Call
Fork()

2. Fork()
▪ Defination:
▪ In computing, the fork is an operation whereby a process creates a copy of itself.
▪ it is usually called a system call, implemented in the Kennel.
▪ Simply We can say that fork is the Primary method of process Creation
▪ It takes no arguments and returns a process ID

3. Overview
▪ In multitasking operating systems, processes (running programs) need a way to
create new processes, to run other programs.
▪ In Unix systems equipped with virtual memory support (practically all modern
variants), the fork operation creates a separate address space for the child. The
child process has an exact copy of all the memory segments of the parent
process.

4. Purpose of fork()
▪ The purpose of fork() is to create a new process, which becomes
the child process of the caller. After a new child process is
created, both processes will execute the next instruction following
the fork() system call.
▪ Unix will make an exact copy of the parent's address space and give it to
the child. Therefore, the parent and child processes have separate address
spaces.

5. A=1
B=2
Example of Fork ()
Process
A=1
B=2
Parent
Child
A=7
B=5
A=1
B=2
Exit
Change the
Child
Process
value

6. One Process Not effect the
other Process
If Child Process Return
nothing

7. A Process executes the code
Fork()
Fork()
Fork()
The total number of the child process created is
(A) 3 (B) 4 (C) 7 (D) 8
Example of Fork ()

8. Example of Fork () Main Process
Child Process
1.Fork ()
2.Fork ()
3.Fork ()
1 2 3 4 5 6 7 8
Answer is 7

9. System Call
“exec” family

10. exec()
In computing, exec is a functionality of an operating
system that runs an executable file in the context of an
already existing process, replacing the previous
executable.

11. Why “exec” functionality ?
▪ Is fork enough to create a new process with
different code and data segment in the same
process id ?

12.  Forking creates a process but it is not enough to run a
new program, to do that the forked child needs to overwrite
its own image with the code and data of the new program
and mechanism is exec() creation.

13. When “exec()” call ?
▪ No new process create.
▪ Replace the data segment and code segment of current process.
▪ Process ID remain same.
▪ Calling process data and code is gone .
▪ Cannot return to the caller .

14. How exec work ?
int main(){
doWorkOne();
call B.exe via exec
doWorkTwo();
return 0;
}
int main(){
doWorkOne();
call B.exe via exec…..
doWorkTwo(); ××
return 0;
}
A.cpp / A.exe
int main(){
doWorkThree();
return 0
}
B.cpp / B.exe
int main(){
doWorkThree();
return 0;
}

15. C language prototypes
int execl(char const *path, char const *arg0, ...);
int execle(char const *path, char const *arg0, ..., char const *envp[]);
int execlp(char const *file, char const *arg0, ...);
int execv(char const *path, char const *argv[]);
int execve(char const *path, char const *argv[], char const *envp[]);
int execvp(char const *file, char const *argv[]);
 “ l ” is specified as a list of arguments.
 “ v “ is specified as a vector (array of character pointers).
“ e “ environment is specified as an array of character pointers.
“ p “ user's PATH is searched for command, and command can be a shell program
If exec return value<0 then Error occurred in “exec” call .
Else successfully call the “exec”

16. Applications
• If any program need run a new process in the same context
(i.e : open a program from another program).
• Continuously run new process from child process (i.e : Terminal ).

17. Thank You….