Statement before fork() printing twice [duplicate]

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?

Answer 1

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.

Answer 2

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.