erroneous "hello" from forked "hello world" process!

[Marc Fiuczynski]

For my Operating System Design class we had to write some trivial 
programs using the fork() and wait() commands.  The teaching assistant
came across an interesting problem.  After compiling the following:

 
#include <stdio.h>

main ()
{
	int pid = 1;
	printf("Hello\n");
	pid=fork();
	if(pid==0){
		printf("World\n");
	}
}

We should get the output:

Hello
World

however, when redirecting the output of a.out to a file and then doing a 
cat on the file we would get either:

a.out > filename
cat filename

Hello
Hello 
World

or 

Hello
World 
Hello

does anyone know what is going on?   Is there a problem because there are
two processes output being redirected to the filename or what?  Any help
with this would be appreciated.  
-- 
Marc Fiuczynski
mef at remus.rutgers.edu