From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <e7ce9dadd2e8a46dea26d8199b72ff02@plan9.jp>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] echo -n
From: Joel Salomon <chesky@plan9.jp>
Date: Mon, 20 Nov 2006 11:37:51 -0500
In-Reply-To: <ee9e417a0611200446u79131412xeb756d8b812305a9@mail.gmail.com>
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> 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