Why fork is executing the program twice from start???

mayjune · 05-25-2011, 03:56 PM

Hi,
I am learning about OS and I wrote this simple forking program...
Here's the code..

Code:

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
        pid_t pid;

        printf ("\n\n~~~~~~~~~~~~At the start of the programm..... My pid  = %d and my parent id = %d", getpid(), getppid());

        pid = fork();

        printf ("\nforked done my pid = %d and my parent pid = %d\n", getpid(), getppid());

        if (pid < 0)
        {
                fprintf (stderr, "Fork Failed");
                return -1;
        }


        else if (pid == 0)
        {
                printf("\n\nhello i am a child process my pid = %d and my parent pid = %d\n", getpid(), getppid());
        }

        else
        {
                wait(NULL);
                printf ("\n\nWait Complete my pid = %d and my parent pid = %d\n", getpid(), getppid());
        }

        return 0;
}

Now the problem is, first statement - At the start of the programm..... is being printed twice!!!!!

Here's the final output

Code:

mankaj@mankaj:~$ cc new2forking.c -o new2forking
mankaj@mankaj:~$ ./new2forking 


~~~~~~~~~~~~At the start of the programm..... My pid  = 4239 and my parent id = 1762
forked done my pid = 4239 and my parent pid = 1762
~~~~~~~~~~~~At the start of the programm..... My pid  = 4239 and my parent id = 1762
forked done my pid = 4240 and my parent pid = 4239


hello i am a child process my pid = 4240 and my parent pid = 4239


Wait Complete my pid = 4239 and my parent pid = 1762
mankaj@mankaj:~$

How can it print the first statement, that is BEFORE the fork() statement twice????

I am really confused. What am I doing wrong? Or is it suppose to be this way?

I am running Natty 11.04 btw..

Thanks a lot for your help

clvic · 05-25-2011, 04:27 PM

Yes this is really confusing... but try editing the first printf, add a "\n" at the end, like this:
printf ("\n\n~~~~~~~~~~~~At the start of the programm..... My pid = %d and my parent id = %d\n", getpid(), getppid());

You won't get the message repeated twice. That's because a printf without a "\n" accumulates the characters in a buffer. Then, the fork clones the buffer itself along with the whole process.
Finally, as soon as the buffer is emptied (because of a "\n" in your case), then all messages are printed out, the old ones along with the new ones.

PinoyAko · 05-25-2011, 05:14 PM

Quote:

Originally Posted by clvic

Yes this is really confusing... but try editing the first printf, add a "\n" at the end, like this:
printf ("\n\n~~~~~~~~~~~~At the start of the programm..... My pid = %d and my parent id = %d\n", getpid(), getppid());

You won't get the message repeated twice. That's because a printf without a "\n" accumulates the characters in a buffer. Then, the fork clones the buffer itself along with the whole process.
Finally, as soon as the buffer is emptied (because of a "\n" in your case), then all messages are printed out, the old ones along with the new ones.

VPN for Android

That was something new. Thanks for answering his question because I also learned from this

mayjune · 05-25-2011, 05:36 PM

Oh my GOD!!!
I can't believe that!!! How in the world you knew that??? lol I mean that is Soooo Not Obvious!!!
Thanks a ton. I kept looking at the program for so long....had no clue what to do...

Is there anything else that I "should" be aware of, such obsure stuff that you won't expect to happen!!!!??

Thanks a lot....you saved tons of hours for me....

mayjune · 05-25-2011, 05:52 PM

Ok how about this.... i have only removed the \n at the end of the bolded line...

Code:

int main()
{
        pid_t pid;

        printf ("~~~~~~~~~~~~At the start of the programm..... My pid  = %d and my parent id = %d", getpid(), getppid());

        pid = fork();

        printf ("\n\nforked done my pid = %d and my parent pid = %d", getpid(), getppid());

And the output is

Code:

mankaj@mankaj:~$ cc new2forking.c -o new2forking
mankaj@mankaj:~$ ./new2forking 
~~~~~~~~~~~~At the start of the programm..... My pid  = 3077 and my parent id = 2948

~~~~~~~~~~~~At the start of the programm..... My pid  = 3077 and my parent id = 2948

forked done my pid = 3078 and my parent pid = 3077

hello i am a child process my pid = 3078 and my parent pid = 3077
forked done my pid = 3077 and my parent pid = 2948

Wait Complete my pid = 3077 and my parent pid = 2948

Why the order has changed? What \n makes the buffer empty? First \n or last \n of a printf statement? I didn't get this part...

Thanks a lot..

sundialsvcs · 05-25-2011, 08:56 PM

Quote:

Originally Posted by mayjune

Oh my GOD!!!
I can't believe that!!! How in the world you knew that??? lol I mean that is Soooo Not Obvious!!!
Thanks a ton. I kept looking at the program for so long....had no clue what to do...

Is there anything else that I "should" be aware of, such obsure stuff that you won't expect to happen!!!!??

Thanks a lot....you saved tons of hours for me....

The old guy looks at you with a brief, dumbfounded stare, but quickly recovers. He pushes himself back from the desk in his cube and leans forward, pointing to a slightly rough spot on his now-glistening cranium. "Believe it or not, young'un, at one time way back then I still had hair, and that spot right there is where I yanked quite a bit of it out ... (little did I know at the time how much I'd miss it) ... solving a problem that's almost exactly like yours." He cracks a surprisingly youthful smile. "And that is why I spend a part of my day, every day, on linuxquestions.org."

mayjune · 05-26-2011, 05:45 AM

Quote:

Originally Posted by sundialsvcs

The old guy looks at you with a brief, dumbfounded stare, but quickly recovers. He pushes himself back from the desk in his cube and leans forward, pointing to a slightly rough spot on his now-glistening cranium. "Believe it or not, young'un, at one time way back then I still had hair, and that spot right there is where I yanked quite a bit of it out ... (little did I know at the time how much I'd miss it) ... solving a problem that's almost exactly like yours." He cracks a surprisingly youthful smile. "And that is why I spend a part of my day, every day, on linuxquestions.org."

Lol....

clvic · 05-28-2011, 05:47 AM

Quote:

Originally Posted by mayjune

Why the order has changed? What \n makes the buffer empty? First \n or last \n of a printf statement? I didn't get this part...

Thanks a lot..

I think this depends on the specific implementation of the printf, depending on your version of C... I bet the buffer usually flushes at the last "\n" of a printf call.
The order in which you see the prints from the processes also depends on the scheduler, so it can't be considered reliable.

Quote:

Is there anything else that I "should" be aware of, such obsure stuff that you won't expect to happen!!!!??

Well, the answer is difficult, so I can only suggest reading a good book... I found this reference on how a fork should behave on the web, this seems quite explanatory:
http://pubs.opengroup.org/onlinepubs...ions/fork.html

Nominal Animal · 05-28-2011, 09:12 PM

Quote:

Originally Posted by mayjune

Code:

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
        pid_t pid;

        printf ("\n\n~~~~~~~~~~~~At the start of the programm..... My pid  = %d and my parent id = %d", getpid(), getppid());

At this point, before forking, you need to make sure all pending output is output. Add

Code:

        fflush(stdout);
        fflush(stderr);

to explicitly flush both standard output and standard error streams.

Quote:

Originally Posted by mayjune

How can it print the first statement, that is BEFORE the fork() statement twice????

At fork() time, all memory structures are duplicated. This includes any data pending in output buffers. Since the statement was in the output buffer for standard output (as noted by civic, above), both processes will include the first statement in their output when they finally flush the buffers.

Add explicit fflush() for each open output stream before fork() calls to avoid this situation.