Re: X server sets VMIN? (was Re: Under /bin/script, characters get printed in four-character chunks)

[Denis Excoffier <cygwin at Denis-Excoffier dot org>]

On 2015-03-24 20:53, Corinna Vinschen wrote:
> 
> On Mar 24 19:59, Denis Excoffier wrote:
>> On 2015-02-28 16:30, Corinna Vinschen wrote:
>>> I can not reproduce this in mintty, nor in a Cygwin xterm started on a
>>> remote X server running under Linux.  I can reproduce this with a local
>>> xterm started via startxwin.  But, and that's the problem, I can
>>> reproduce it with the current 1.7.35-0.5 test release, with 1.7.34, and
>>> last but not least also with a debug version of the Cygwin DLL in which I
>>> backed out all PTY-related changes since last November.
>>> 
>>> I'm not sure this is a giveaway, but from that it seems this problem
>>> is not directly related to a Cygwin change in the last months.
>>> 
>>> So, jturney and I are wondering when exactly you encountered this problem
>>> for the first time.  Did it coincide with a certain Cygwin release,
>>> or a certain X server?  Or new X libs, perhaps?
>>> 
>>> Anything you can provide to narrow down the potential culprit would be
>>> helpful.
>>> 
>> Well. Here is some more inputs.
>> 
>> This is connected with the "min" option of stty. When this occurs,
>> 'stty -a' says '4' for min. If i change into 'stty min 5' the characters
>> come by chunks of 5.
>> 
>> I had a look into the sources of xterm, xinit, coreutils, tcsh and cygwin and
>> i definitely don't understand where the 4 comes from. In any case, 4 should not be
>> the problem, because 'stty min 4' is perfectly legitimate.
>> 
>> The doc of stty says that 'min' (and 'time') are used in case of '-icanon'.
>> However, i found in fhandler_tty.cc that it seems not to be always the case.
>> After i applied the following patch:
>> 
>> diff -uNr cygwin-snapshot-20150317-1.original/winsup/cygwin/fhandler_tty.cc cygwin-snapshot-20150317-1.patched/winsup/cygwin/fhandler_tty.cc
>> --- cygwin-snapshot-20150317-1.original/winsup/cygwin/fhandler_tty.cc	2015-03-17 11:42:16.000000000 +0100
>> +++ cygwin-snapshot-20150317-1.patched/winsup/cygwin/fhandler_tty.cc	2015-03-24 19:32:42.000000000 +0100
>> @@ -715,7 +715,7 @@
>> 
>>   if (is_nonblocking () || !ptr) /* Indicating tcflush(). */
>>     time_to_wait = 0;
>> -  else if ((get_ttyp ()->ti.c_lflag & ICANON))
>> +  else if (!(get_ttyp ()->ti.c_lflag & ICANON))
> 
> No, this is wrong.  You're switching the code for icanon with the
> code for -icanon.  -icanon in stty means ICANON is switched off.
> 
> I just gave it another try and the behaviour is perfectly valid.
> 
> The real problem is that "something" is setting VMIN to 4.  And that's
> somehow inside the X server, if I'm not completely wrong:
> 
> - If you start an xterm from mintty like this:
> 
>    xterm -display :0
> 
>  and then call `stty -a' in it, you'll see that min is 1, and then
>  script will behave as desired.
> 
> - However, if you start xterm from the X server tray icon and then
>  call `stty -a' in it, min is set to 4 and script will misbehave.
>  If you call `stty min 1' before calling script, script will work
>  as expected again.
> 
> So, why does the X server (or whatever controls starting applications
> from the X server tray icon) set VMIN to 4?
> 

It seems that this has something to do with tcsh (and not with XWin).
If you arrange your environment in order that the xterm launches /bin/bash
(instead of tcsh), you get min=0 under 'stty -a' and /bin/script behaves
as expected. If afterwards, in such an xterm, you run '/bin/csh -f', you get
min=4.

Consider the following:

diff -uNr tcsh-6.18.01-original/ed.init.c tcsh-6.18.01-patched/ed.init.c
--- tcsh-6.18.01-original/ed.init.c	2006-08-24 22:56:08.000000000 +0200
+++ tcsh-6.18.01-patched/ed.init.c	2015-03-25 15:56:33.000000000 +0100
@@ -65,7 +65,7 @@
 	(uc)CDSWTCH,	(uc)CERASE2,	 (uc)CSTART, 	   (uc)CSTOP,
 	(uc)CWERASE, 	(uc)CSUSP, 	 (uc)CDSUSP, 	   (uc)CREPRINT,
 	(uc)CDISCARD, 	(uc)CLNEXT,	 (uc)CSTATUS,	   (uc)CPAGE,
-	(uc)CPGOFF,	(uc)CKILL2, 	 (uc)CBRK, 	   (uc)CMIN,
+	(uc)CPGOFF,	(uc)CKILL2, 	 (uc)CBRK, 	   (uc)1,
 	(uc)CTIME
     },
     {

In the original code, CMIN is set to CEOF and CEOF is set to Control-D, hence
min=4. With the patch above, all seems to go well. But this does not
explain why the min=4 is not permanent.

Regards,

Denis Excoffier.



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