3.5.rc2 tty buffering bug ?

3.5.rc2 tty buffering bug ?

[Dave Jones]

I found something odd that happens with redirection.

this code...



#include <stdio.h>

void main()
{
	printf("begin\n");

	if (fork() != 0)
		printf("child\n");
	else
		printf("parent\n");

	printf("end\n");
}

usually exits before the child gets to print anything, so stdout looks like..

begin
parent
end

but when redirected to a file, sometimes it prints what we expect, but other times
(if the child exits first) it seems to print..

begin
child
end
begin
parent
end


How does 'begin' get printed a second time ?

	Dave

Re: 3.5.rc2 tty buffering bug ?

[Dave Jones]

On Thu, Jun 14, 2012 at 12:13:31PM -0400, Dave Jones wrote:
 
 > #include <stdio.h>
 > 
 > void main()
 > {
 > 	printf("begin\n");
 > 
 > 	if (fork() != 0)
 > 		printf("child\n");
 > 	else
 > 		printf("parent\n");
 > 
 > 	printf("end\n");
 > }
 > 
 > How does 'begin' get printed a second time ?

Someone pointed out that an fflush is needed before the fork.
Somehow I've managed to go all these years without knowing that.

	Dave

 

Re: 3.5.rc2 tty buffering bug ?

[Andreas Schwab]

Dave Jones <davej@redhat.com> writes:

> How does 'begin' get printed a second time ?

It's buffered in stdio, which is then cloned.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

Re: 3.5.rc2 tty buffering bug ?

[Alan Cox]

> How does 'begin' get printed a second time ?

You've been away from user space too long. Read the documentation for
stdio. Stdio is buffering, both forks get the buffer, both write it.

Try the experiment with write()

Alan

Re: 3.5.rc2 tty buffering bug ?

[Cong Wang]

On Fri, Jun 15, 2012 at 12:13 AM, Dave Jones <davej@redhat.com> wrote:
> but when redirected to a file, sometimes it prints what we expect, but other times
> (if the child exits first) it seems to print..
>
> begin
> child
> end
> begin
> parent
> end
>
>
> How does 'begin' get printed a second time ?
>

setbuf(3) said:

"
Normally  all files are block buffered. [...]
If a stream refers to a terminal (as stdout normally does) it is line buffered.
The standard error stream stderr is always unbuffered by default.
"
So when you redirect stdout to a file, "begin\n" is buffered too, both
in child and in parent.