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From: davem@sdcrdcf.UUCP (David Melman)
Newsgroups: comp.unix.questions,comp.unix.wizards
Subject: O_NDELAY flag on open system call on 4.2BSD
Message-ID: <3472@sdcrdcf.UUCP>
Date: Tue, 25-Nov-86 18:07:29 EST
Article-I.D.: sdcrdcf.3472
Posted: Tue Nov 25 18:07:29 1986
Date-Received: Wed, 26-Nov-86 20:43:35 EST
Reply-To: davem@sdcrdcf.UUCP (David Melman)
Distribution: world
Organization: System Development Corporation R&D, Santa Monica
Lines: 17
Keywords: system calls
Xref: mnetor comp.unix.questions:196 comp.unix.wizards:186

I want to be able to check for carrier on a tty port without blocking.
I thought I would use the O_NDELAY flag in an open call, but I found
that the open still hangs, e.g 

>open (ttyline, O_WRONLY|O_NDELAY, 0);

Specifically, the tty port is a DEC DMZ.  
We are running 4.2bsd with Chris Torek's DMZ driver.

I'm doing this in order to determine whether a LaserJet printer is connect
or not.  It is shared between a our Vax and a micro.
How can I do this?

David Melman
UNISYS 

UUCP: {hplabs,cbosgd,ihnp4}!sdcrdcf!davem