Re: [9fans] echo -n

[Joel Salomon <chesky@plan9.jp>]

> No, [a zero-length write] doesn't close the pipe.  It sends a
> zero-length message, causing the reader to get a zero-length message
> returned from read.  The pipe is still completely usable; the problem
> is that the reader interprets this zero return as EOF, as is the
> convention.
>
> It's too bad that both of these conditions are signaled the same way,
> but the pipe is *not* closed.
>
> There are two options available for dealing with this: change your
> writer not to write empty messages (as you have), or change the reader
> to ignore them.  For the latter, just ignore a return value of 0 and
> read again.  If the pipe is really closed, then after returning 0
> three times, read will start returning -1.

Why is read from a closed pipe not considered an error, though?
	close(fd[0]);
	for(int i=0; i<5; i++){
		n = read(fd[1], buf, 8);
		print("%d: read %ld bytes: %r\n", i, n);
	}
produces:
	0: read 0 bytes:
	1: read 0 bytes:
	2: read 0 bytes:
	3: read -1 bytes: i/o on hungup channel
	4: read -1 bytes: i/o on hungup channel
where I expected errstr to be set on the first read after pipe
closure.  Is there really no way to immediately determine that a
pipe has been closed?  Why would read be so implemented?

--Joel