linux-kernel - [PATCH man-pages] pty.7: clarify asynchronous nature of PTY IO.

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Date:	Wed, 6 May 2015 08:59:07 +1000
From:	NeilBrown <neilb@...e.de>
To:	Nic Percival <Nic.Percival@...rofocus.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Cc:	Michael Matz <matz@...e.de>,
	Peter Hurley <peter@...leysoftware.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH man-pages] pty.7: clarify asynchronous nature of PTY IO.


A PTY is not like a pipe - there may be delayed between
data being written at one end and it being available at the other.

This became particularly apparent after
 commit f95499c3030f ("n_tty: Don't wait for buffer work in read() loop")

in Linux 3.12

Signed-off-by: NeilBrown <neilb@...e.de>

---

Peter: does this seem reasonable and accurate to you?

MichaelK:  Would you prefer the commit ID in the man page.  It isn't so much
   a deliberate change as a code improvement which caused problems for certain
   use cases which depended on undefined behaviour.
   Thread at https://lkml.org/lkml/2015/5/1/35

NeilBrown


diff --git a/man7/pty.7 b/man7/pty.7
index 1332d11d9ca2..6c9ae182925c 100644
--- a/man7/pty.7
+++ b/man7/pty.7
@@ -56,6 +56,12 @@ terminal emulators,
 and
 .BR expect (1).
 
+Data flow between master and slave is handle asynchronously, much like
+data flow with a physical TTY.  Data written to the slave will be
+available at the master promptly, but may not be available
+immediately.  Similarly there may be a small processing delay between
+a write to the master, and the effect being visible at the slave.
+
 Historically, two pseudoterminal APIs have evolved: BSD and System V.
 SUSv1 standardized a pseudoterminal API based on the System V API,
 and this API should be employed in all new programs that use

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