Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.mail.uucp
Subject: Re: Bidirectional Modem Lines under SunOS 4.0.1
Message-ID: <1403@auspex.auspex.com>
Date: 8 Apr 89 22:27:06 GMT
References: <160@osc.COM> <743@key.COM> <2209@laidbak.UUCP> <1395@auspex.auspex.com> <3748@ficc.uu.net>
Reply-To: guy@auspex.auspex.com (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 20


 >> Opens on the "dial-in" device block until carrier comes up. [...] Opens on
 >> the "dial-out" device succeed even if carrier isn't up, [...] In addition,
 >> once the dial-out device is open, any opens on the dial-in device [...] will
 >> *NOT* complete when carrier comes up!...
 >
 >This is likely to cause problems if you're trying to implement callback,
 >isn't it?

Huh?  Its sole purpose is to prevent two programs ("getty" and the
program that's dialing out) from gaining simultaneous access to the same
serial line.  Giving them that simultaneous access is likely to cause
problems; for one thing, they'll both be writing to it, and getting
their output intermingled, and for another thing, they'll both be
*reading* from it, and input will be randomly distributed among them.... 

If by "implement callback" you mean "implement a scheme by which
computer A calls computer B and tells computer B to call computer A
back", it doesn't cause any problems with that at all; the portion of
the description which you elided explains why that works.