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From: thorinn@skinfaxe.diku.dk (Lars Henrik Mathiesen)
Newsgroups: comp.unix.wizards
Subject: Re: pty bugs & features
Message-ID: <1990Sep8.234330.19036@diku.dk>
Date: 8 Sep 90 23:43:30 GMT
References: <5992@muffin.cme.nist.gov> <3948@auspex.auspex.com> <6038@muffin.cme.nist.gov>
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Organization: Department Of Computer Science, University Of Copenhagen
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In article <6038@muffin.cme.nist.gov> libes@cme.nist.gov (Don Libes) writes:
>Why do pty's return EIO instead of 0 upon EOF?

In my opinion, there is no such thing as an EOF when reading from a
(master) pty. After all, the pty is designed to let a daemon, script
program or similar pretend that it is on the outside of the machine
looking in through a serial interface, receiving exactly the same
bytes as would be passed over a serial line. And, barring wire-cutters
and over-voltage, a serial line is very open-ended and EOF-free.

However, a serial interface may have some control lines, such as DTR
or RTS. These will typically be asserted when the corresponding UNIX
device file is opened; they may be negated when it is closed (but only
if the HUPCLS flag is set, I think). So, apart from the HUPCLS
business, the EIO error on a pty master corresponds to ``DTR not
asserted'', not to EOF.

--
Lars Mathiesen, DIKU, U of Copenhagen, Denmark      [uunet!]mcsun!diku!thorinn
Institute of Datalogy -- we're scientists, not engineers.      thorinn@diku.dk