3.5.rc2 tty buffering bug ?

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

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

Dave

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

Andreas.

Andreas Schwab, [email protected]
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

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

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.