uugetty & lock files

[~XT6561110~Marc Mengel~C25~M27~6184~]

In article <384 at fe2o3.UUCP> michael at fe2o3.UUCP (Michael Katzmann) writes:
>This program looks to see if a lockfile is present /usr/spool/locks/LCK..tty1,
>before blindly writting out to the line.
>
>The problem:
>	uugetty seems to create a lockfile ...
>Any ideas anyone?

Your confusion is common; actually a uucp lock file is only valid if
the process whose id is written in the lock file is still running.
What usually happens is that a uugetty sees some data, locks the port
to let someone log in, then times out and exits, and a new uugetty is
respawned.  The LCK..ttyxx file lis left there, but has the process id
of the old uugetty in it.  This is important to have happen because
a uugetty will exec a login who will exec a shell, none of whom have
any idea the lock file is there; but they still need it locked.  Since
they are exec-ed however, they will have the same process id and the
lock will be valid.

To check if a uucp lock file is valid you have to read the pid from the 
file, and try a kill(pid,0) to see if the process exists.  If it doesn't,
you unlink() the lock file and open(...O_EXCL) your own, and write your
own pid in it.

On the other hand, to do things like chat with a modem, you can write
a real simple shell script with "cu" (which knows all the lock file 
conventions, etc.) by just doing something like:

	+-----------------------
	|cu -l /dev/ttyxx <<EOF
	|ATS0=1
	|~!sleep 1
	|EOF
	+-----------------------

and save yourself lots of programming...
-- 
 Marc Mengel					mmengel at cuuxb.att.com
 						attmail!mmengel
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