cat /dev/ttya > /dev/null &

[Robert Perlberg]

In article <343 at brazos.Rice.edu>, dpointer at uicsrd.csrd.uiuc.edu (David B. Pointer) writes:
> No one here understands why this fix works. Does anyone on the net have
> any ideas why this works?  This really bothers me.  I mean, UNIX at times
> seems like black magic, but this looks like REAL voodoo.  Thanks for any
> ideas.

"A sufficiently advanced technology is indistinguishable from magic.",
or words to that effect.

This is a very old problem found on many UNIX systems.  The crux of the
problem is that when you close a tty port, the tty driver drops DTR and
abandons processing on the port.  Since you almost always write data to
the tty faster than the serial port can pump it out at whatever baud rate,
a considerable queue builds up in the tty driver.  By the time your
application is finished writing and closes the port, there is still quite
a bit of data in the queue which has not yet gone out.  Whatever data is
still in the queue when the port is closed is thrown away.  This results
in the "missing pages" problem  reported by many lpr users.  On some
systems, the driver will continue to send characters, but will stop
honoring XON/XOFF.  The solution is to make sure that the tty port is not
closed until after all of the data has been transmitted.  One way of doing
this, as suggested by your guru, is to have another process hold the port
open all the time.  I remember there being a program on the ONYX called
/etc/openup which did nothing but open the specified device and pause().
The solution I use in my lpr filters is to wait for the tty queue to empty
before exiting.  I use the following code for that purpose:

#include <stdio.h>
#include <sgtty.h>

outputwait(fd)
int fd;
{
	int outchars;

	while(ioctl(fd, TIOCOUTQ, &outchars) == 0 && outchars > 0)
	{
		sleep(1);
	}
}

Robert Perlberg
Dean Witter Reynolds Inc., New York
phri!{dasys1 | philabs | mancol}!step!perl
	-- "I am not a language ... I am a free man!"