Path: utzoo!attcan!uunet!aplcen!uakari.primate.wisc.edu!uflorida!mlb.semi.harris.com!thrush.mlb.semi.harris.com!del
From: del@thrush.mlb.semi.harris.com (Don Lewis)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: sockets vs. streams
Message-ID: <1990Sep7.043303.21129@mlb.semi.harris.com>
Date: 7 Sep 90 04:33:03 GMT
References: <2350@cirrusl.UUCP> <1990Sep5.020521.803@mlb.semi.harris.com> <2364@cirrusl.UUCP>
Sender: news@mlb.semi.harris.com
Organization: Harris Semiconductor, Melbourne FL
Lines: 53

In article <2364@cirrusl.UUCP> dhesi%cirrusl@oliveb.ATC.olivetti.com (Rahul Dhesi) writes:
>Don Lewis writes:
>
>     Now, if the printer is on a terminal server somewhere...
>	     cat somefile >/dev/tcp/x.x.x.x-y.y
>
>     Specifically, you could use this for the lp= parameter in
>     /etc/printcap...
>
>This would be nice.  A possible solution is to write a program "netsend":
>     
>     cat somefile | netsend tcp:x.x.x.x-y.y
>
>Since printcap allows sending to a filter rather than a specific
>device, the solution is general enough.

This is not enough if you need to use the `of' filter.  Lets say
you specify both the `if' and `of' filters and lpr a simple text
file.  Lpd opens the printer device, forks and execs a copy of
the `of' filter with its stdout connected to the printer and its
stdin connected to a pipe back to lpd.  Lpd then squirts the banner
down the pipe to the `of' filter followed by ^Y^A.  When the `of'
filter sees the ^Y^A, it flushes stdout and sends itself a STOP
signal.  Lpd sees the `of' filter stop and then forks and execs
a copy of the `if' filter which has its stdout connected to
the printer and its stdin connected to the file to be printed.
Once the `if' filter exits, lpd sends a CONT signal to the `of'
filter and writes either the banner for the next job or the trailer
string to it.  For different print file formats, substitute
the appropriate filter for the `if' filter.

If the particular device you are using needs the functionality
of the `of' filter, you can't just put a shell wrapper that
pipes its output to netsend because there is no way for the
`if' filter to make a connection to the printer as long as
the `of' filter is running (also, the link between `of' and
the printer may need to be bidirectional).  If you don't need
`of', you may be able to do this with `if', but you will
not be able the banner and trailer because lpd wants to
send these to the printer before/after running `if'.

There are possible workarounds using pty's or named pipes with
daemons on the other side or passing the fd to the socket through
unix domain sockets, but YUCK!  If the device is something wierd
and you don't have the source for the filters then you are in
pretty deep trouble.

Why should it take a wizard to switch a printer from a serial
port to a terminal server?
--
Don "Truck" Lewis                      Harris Semiconductor
Internet:  del@mlb.semi.harris.com     PO Box 883   MS 62A-028
Phone:     (407) 729-5205              Melbourne, FL  32901