Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!helios!csseq!randy
From: randy@csseq.tamu.edu (Randy Hutson)
Newsgroups: comp.lang.c
Subject: Re: erroneous "hello" from forked "hello world" process!
Keywords: fork(), wait(), Hello World
Message-ID: <8661@helios.TAMU.EDU>
Date: 1 Oct 90 03:13:49 GMT
References: <Sep.30.21.09.46.1990.2881@romulus.rutgers.edu>
Sender: usenet@helios.TAMU.EDU
Organization: Computer Science Department, Texas A&M University
Lines: 53

In article <Sep.30.21.09.46.1990.2881@romulus.rutgers.edu> mef@romulus.rutgers.edu (Marc Fiuczynski) writes:

>#include <stdio.h>
>
>main ()
>{
>	int pid = 1;
>	printf("Hello\n");
>	pid=fork();
>	if(pid==0){
>		printf("World\n");
>	}
>}

[Marc explains that when the above program is executed and redirected to
a file, the expected string "Hello\nWorld\n" is not in the file, but instead
either "Hello\nHello\nWorld\n" or "Hello\nWorld\nHello\n".]

After a fork, a child process will inherit the stdio buffers of its
parent.  In your case, printf("Hello\n") was not sufficient to flush
the stdio buffer of the parent process, so "Hello\n" was written to a
buffer but not to the file.  Right after the fork, "Hello\n" was in the 
stdio buffers of both processes.  Then after the child executed 
printf("World\n"), its buffer contained "Hello\nWorld\n" while the
parent's buffer still contained only "Hello\n".  The order in which
the two processes terminate (with their buffers being flushed) is not 
defined, hence you sometimes got "Hello\nHello\nWorld\n" (the parent
exited first) and other times got "Hello\nWorld\nHello\n" (the child 
exited first).

You will probably get the output you desire if you don't redirect
the output of your program.  This is because most (at least) terminal 
drivers are newline buffered, and the '\n' in printf("Hello\n") is
sufficient to flush the buffer.  In any case, a "correct" version
of your program follows with a fflush(stdout) executed right after
the first printf:


#include <stdio.h>

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


Randy Hutson
randy@csseq.tamu.edu