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<TITLE>The Answer Guy 33:
Psuedo tty Becomes Unusable</TITLE>
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tty help
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<H4>"Linux Gazette...<I>making Linux just a little more fun!</I>"
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<H4 align="center">By James T. Dennis,
<a href="mailto:linux-questions-only@ssc.com">linux-questions-only@ssc.com</a>
<BR>Starshine Technical Services, <A HREF="http://www.starshine.org/">http://www.starshine.org/</A>
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<H3><img src="../../gx/dennis/qbub.gif" alt="(?)"
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>Psuedo tty Becomes Unusable</H3>
<p><strong>From Scott R. Every on 21 Sep 1998</strong></p>
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<p><strong>
i have a system which has been running for a while(actually a
number of systems) after a bit the ttyp0 port is no longer
available when telnetting in. it doesn't list anywhere as being
used, but it doesn't work!
</strong></p>
<p><strong>
can you offer any suggestions?
</strong></p>
<p><strong>
thanx
<br>s
</strong></p>
<blockquote><img src="../../gx/dennis/qbub.gif" alt="(?)"
width="50" height="28" align="left" border="0"
>Try the '<tt>lsof</tt>' command. That should find out which
process is using it.
</blockquote>
<blockquote>
The <tt>/dev/ttyp*</tt> devices are for "psuedo" tty's --- these
are used by rlogind, telnetd, xterms, screen and many
other programs. There are usually <EM>many</EM> of these
psuedo tty's on a system.
</blockquote>
<blockquote>
Normally a daemon that uses a psuedo tty searches through
the list and uses the first one that it can open. There
is another approach used by some other forms of Unix --- and
supported in recent kernel whereby the daemon makes a request
of a sort of "dispatcher" device which then provide it with
the number of the next available pty/ttyp device. This is
referred to as "Unix '98 PTYs Support" in the linux kernel
-- and I've heard it referred to as "ptmx" (psudo-tty
multiplexing, or something like that). In the case of the
Linux implementation the pty's can be dynamically generated
under the "pts" virtual filesystem (which is a bit like
the <tt>/proc</tt> filesystem in that it doesn't exist on a "disk"
anywhere --- it simply provides a filesystem abstraction of
the system's in memory data structures). Linux 2.2 will
also probably support a "/devfs" --- another virtual
filesystem which makes all of the entries under /dev into
dynamic entries.
</blockquote>
<blockquote>
Of course, none of that applies to your situation. That's
just the "vaporware report" on the future of the Linux
kernel.
</blockquote>
<blockquote>
If there really is no process that still owns the ttyp0
in your case then it might be a bug in your kernel.
I'd check the permissions of the device node to see if
they are changing (or to see if there is something that's
just blowing the device node away), then I'd look through
the "Change Logs" for the recent 2.0.3x kernels. It might
be that you are bumping into one of the bugs that Alan
Cox and crew have been fixing. If you aren't running a
2.0.35 or 2.0.36 kernel --- consider trying it to see
if that solves the problem.
</blockquote>
<blockquote>
To be honest I haven't seen a good description of the
whole pty*/ttyp* mess or a decent explanation of what
problems the Unix '98 ptmx design is supposed to solve.
I've heard that pty's and ttyp's are paired off in
"master/slave" pairs that have something to do with
providing different device nodes for control (ioctl()?)
and communications over the channel. If any of our
readership knows of a good treatise on the topic, please
pass me a pointer or mail me a copy.
</blockquote>
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<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
>Copyright ©</a> 1998, James T. Dennis <BR>
Published in <I>Linux Gazette</I> Issue 33 October 1998</H5>
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