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From: csch@netcs.UUCP (Clemens Schrimpe)
Newsgroups: comp.unix.xenix
Subject: Re: Select(S) in Xenix 386 2.3.2
Keywords: select, serial, pipes
Message-ID: <172@prmmbx.UUCP>
Date: 23 Jun 89 23:35:30 GMT
References: <117@accsys.acc.uu.no>
Reply-To: csch@netcs.UUCP (Clemens Schrimpe)
Organization: netCS Kommunikationstechnik GmbH, Berlin
Lines: 63

ivar@acc.uu.no (Ivar Hosteng) writes:
<> I have experienced some problems using the select call in Xenix 386 V2.3.2.
<> It does not seems to detect when a pipe gets ready to been read from.
This is, because there is no provision to select on pipes!
Why? The stuff is almost totally ported 1:1 from the Berkeley code and
in BSD pipes should consist of AF_UNIX sockets, on which you can naturally
select.
I was very angry, when I found this out after hours of digging with adb in
the kernel. But I also tried the same on a SUN under SunOS 4.0 and it doesn't
work either ... seems to be a common illness ???
(I wonder, because the code for that is very simple ... ??? ...)

<> I also 
<> have trouble using select on a serial port.  When I do that the input
<> turns into garbage.  This does not occur when I use select on the
<> multiscreen ttys (tty01-tty12).
Hehe - we had just the same!
Here is the solution (thanks to my colleague Stefan Koehler, who took one
look at my screen, into which I had starred for hours, to find it ...)

Select is implemented by an undocumented ioctl
	(0xFFFF == IOC_SELECT -> [sys/slect.h])
which is handled by ttiocom() for all devices using the standard
SYS-V linediscipline!

The ioctl-routine for the serial devices [sioioctl()] just calls 
ttiocom() [after some undefinable VPIX stuff ???] and
if it returns NON-ZERO it calls sioparam(), which adjusts certain
parameters and garbles the output!
OK so far. Now: The Bug lies in the ttiocom-code within the check
for IOC_SELECT. After detecting the IOC_SELECT, the ttiocom calls
the select-code and returns NOTHING, which means that if EAX is
non-zero (randomly) sioparam() is called and garbles the output.

The Fix: (quick and dirty)
Write a routine called "ttiocom", which might look like this:

ttiocom(ttyp, com, arg, flag)
struct tty *ttyp;
int com, arg, flag;	/* there should be better types for this :-) */
{
	if (com == IOC_SELECT)
	{
		ttselect(ttyp, flag);
		return(0);	/*** THIS IS IMPORTANT ***/
	}
	return(Ttiocom(ttyp, com ,arg, flag));
}

Compile something like this, then use whatever you have (GNU-Emacs is
great in patching strings in binaries) to patch /usr/sys/sys/libsys.a
to change the original ttiocom into Ttiocom !
Link in your code and -by some magic reason- experience a full blown
select on your System V / Xenix machine!!!

Have fun playing around with it -

	Clemens Schrimpe, netCS Informationstechnik GmbH Berlin
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