Thread: A small question about fork( )

Hello experts,
I am using fork() in my code but I am confused which output comes first child or parent?
I did the following code .My book shows parent first but my linux shows child first.Can anyone tell me why?
Code:

Code:

#include <stdio.h>
int main(){
int pid;
printf("I am original process with pid is %d ,My parent (Terminal) pid is %d\n",getpid(),getppid());
pid=fork();
if(pid>0)//parent starts
    {
    printf("I am parent,my pid is %d and my parent pid is %d\n",getpid(),getppid());
    }
else 
    {
    printf("I am child,my pid is %d and my parent pid is %d\n",getpid(),getppid());
    }
//both parent and child executes the next printf
printf("PID %d terminates \n",getpid());    
}

output:

Code:

mlhazan@dEBx:~/Desktop$ gcc myFork.c 
mlhazan@dEBx:~/Desktop$ ./a.out 
I am original process with pid is 13426 ,My parent (Terminal) pid is 28555
I am child,my pid is 13427 and my parent pid is 13426
PID 13427 terminates 
I am parent,my pid is 13426 and my parent pid is 28555
PID 13426 terminates 
mlhazan@dEBx:~/Desktop$

Either way is fine. Both processes are running simultaneously (or so it appears) and thus there's no guarantee which will complete first. It's essentially a roll of the die when both processes are doing the same thing. For example, on my system, I've gotten both the parent printing first and the child printing first.

You can wait for the child to finish with waitpid().

Originally Posted by matsp

Like the others say When you fork a process, it is entirely arbitrary whether the current (forking) process or the new (forked) process is the one to run when the kernel call to fork is finished. The choice is made by the scheduler [that is the function in the OS kernel that selects which process/thread [[or processes/threads if the system has multiple processors]] to run at the moment], and schedulers by definition are quite complex, so trying to understand what happens in a Linux/Unix or Windows scheduler is non-trivial.

A closely related concept is described here:
http://www.flounder.com/badprogram.h...%20immediately
Although that is about threads in Windows, the reasons for why Windows does/doesn't start the new thread is also in the scheduler.

--
Mats

Thanks to everyone .I got my answer