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 <ne...@suse.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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