Very Computer

I've been asked a couple of times by customers if Linux
supports "transparent printing".  AFAICT from looking at tty
and serial drivers, the answer is no.

My understanding of "transparent printing" (gleaned from SCO
manuals) is:

 1) You've got a serial terminal with an aux serial port (I
    remember ADM5's used to have them) which si connected to a
    serial printer.

 2) That terminal's "main" serial port is hooked to a serial
    port on a Unix machine. For example's sake: /dev/ttyS0.

 3) The serial driver provides a second device interface, for
    example /dev/ttySp0.

 4) Data written to /dev/ttyS0 is sent un*erated to the
    terminal (and hence shows up on the screen).

 5) Data written to /dev/ttySp0 is enclosed between two escape
    sequences that route the data to the printer.

Thus the printer can be used by anybody on the system while the
terminal is used simultaneously for a normal interactive
session (apparently the printer-boud data is chopped up into
reasonable sized chunks so that response time on the terminal
doesn't get too bad).

Has anybody seen anything like that under Linux?

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Consider writing a program that reads from a pseudo-terminal,
or, even better, from its standard input and adds those
escape sequences. Any decent spooling system ought to be
able to incorporate that. I do not see a cause for a kernel
mode involvement here.

-- Pete

Perhaps I phrased my question poorly.  I just wanted to know if
support currently existed.  I'm not interested in implementing
the functionality. I just wanted to make sure I was giving my
customers the correct answer to the question.

In order to do it in user space, you'd have to use two pty's
one for lpd and one for getty/login/shell.  The tricky bit is
that you can't just shove the encapsulated printer data into
the serial data stream at any arbitrary point.

FWIW, this functionality was provided by at least one serial
driver available for SCO.  It was apparently used quite
frequently by people writing POS applications.

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