Statement before fork() printing twice

Question

I was experimenting with fork() and re-direction to check whether the re-directions done in the parent apply to the child too. I wrote the following simple program

#include<stdio.h>
#include<unistd.h>
#include<stdlib.h>

int main ()
{
    freopen( "error.txt", "w+t", stdout ); // From now on, stdout = error.txt
    printf (" ERROR!  WHY DONT U UNDERSTAND?\n");
    if ( fork() == 0 ) 
    {   
        printf(" I AM CHILD\n");
        exit(0);
    }   
    else-
    {   
        printf (" EITHER I AM A PARENT OR SOMETHING GOT SCREWED\n");
    }   


    return 0;
}

The output ( error.txt ) I got is

ERROR!  WHY DONT U UNDERSTAND?
EITHER I AM A PARENT OR SOMETHING GOT SCREWED
ERROR!  WHY DONT U UNDERSTAND?
I AM CHILD

Surprisingly, ERROR! WHY DONT U UNDERSTAND? is printing twice even though it appears much before the fork() is called and should only be printed once by the parent.

Can anyone shed some light on this?

I'm not sure about this one, but make sure you flush io-buffers, before the fork. maybe the buffers get copied to the child. — lupz, May 22 '12 at 10:34

Alex Bakulin · Accepted Answer · 2012-05-22T10:37:45.417

9

Since after reopen the stream is non-interactive, it's fully buffered and doesn't flush on '\n'. Before fork is called the buffer still contains the message, and after fork this buffered message was duplicated (because both processes got their own copies of stdout) and then flushed by both the parent and the child. See part 7.19.3 of C standard.

You can avoid such behavior by calling fflush just before fork.

edited May 22 '12 at 10:37

answered May 22 '12 at 10:31

Alex Bakulin

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I dint know that `\n` doesn't work as an automatic flusher after redirection. Thanks and +1 – Pavan Manjunath May 22 '12 at 10:35
You could also use `setvbuf` to reconfigure `stdout`. – Maxime Chéramy Mar 23 '15 at 14:37

score 3 · Answer 2 · answered May 22 '12 at 10:29

3

It's because of buffering. Do a fflush right after printf.

Both processes end up with the same copy of stdio's internal stuff and both proceed to flush it at exit. You might also prevent it from happening if you call _exit in the child.

answered May 22 '12 at 10:29

cnicutar

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If I change `exit()` to `_exit()`, the child is not printing anything. ie `I AM CHILD` is missing from the output. Whats the difference between `exit()` and `_exit()` ? – Pavan Manjunath May 22 '12 at 10:40
1

@Stacker `_exit` doesn't flush stdio buffers. – cnicutar May 22 '12 at 10:41

score 1 · Answer 3 · answered May 22 '12 at 10:31

1

flushing the buffer will solve the problem. use fflush just after the print statement.

answered May 22 '12 at 10:31

Hemant Metalia

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score 0 · Answer 4 · answered May 22 '12 at 10:33

0

It seems that the ERROR! WHY DONT U UNDERSTAND is still buffered after forking and gets written by both processes.

If you add

fflush(stdout);

right after your first printf() the internal buffer is flushed and it only appears once in your file.

answered May 22 '12 at 10:33

dwalter

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Statement before fork() printing twice

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