stty problem

[John Polstra]

In article <3081 at brazos.Rice.edu> mhickey at sherwood.prime.com (Mark E. Hickey x4149 x4417 ) writes:
>X-Sun-Spots-Digest: Volume 8, Issue 198, message 11 of 18
>
>>I can't seem to alter the options of /dev/ttya or /dev/ttyb on my SPARC
>>station 1.  The command I use is
>> 
>>	 stty parodd cs8 > /dev/ttya
>>	  
>> [results of this command failing deleted]
>>
>>		      What am I doing wrong?
>
>Just out of curiousity, I looked into this problem on our SPARCstation 1
>(run- ning 4.0.3). I found that the device must be turned on in the file
>/etc/ttytab (ie: the fourth field of the entry for ttya must read "on");
>by default it is turned off.  Also, if you turn the port on while the
>system is running, you must tell init to re-read the file by sending it
>SIGHUP (eg: kill -1 1 on most systems, do a "ps -eax | grep init" to be
>sure of the process id).  If the port is turned off, getty apparently
>resets the parameters no matter what you set them to using stty ...

Good guess, but that's not what is going on.

When you do an "stty" under Unix, the new modes stick only as long as the
tty port is held open (by one or more processes).  When the last process
closes the tty, the modes revert to some kernel-defined default.  You can
see this by comparing the effects of the following two lines:

    stty parodd cs8 >/dev/ttya; stty everything >/dev/ttya

    (stty parodd cs8; stty everything) >/dev/ttya

In the first line, the tty gets closed between the two "stty" commands,
and the second "stty" will show the default modes regardless of what was
set by the first "stty".

In the second line, the tty is held open from the beginning of the first
"stty" through the end of the second "stty".  Here, the second "stty" will
show the modes as set by the first "stty".

If you mark a tty as "on" in "/etc/ttytab", then "getty" will hold that
tty open all the time.  Then, any "stty" that you do will stick.  This has
the unfortunate side-effect that any incoming characters on the tty will
be swallowed by "getty".

If you want to change the tty modes for a single command (e.g., "foo"),
the easiest way is like this:

    (stty parodd cs8; foo) >/dev/ttya

If you want to change the modes more-or-less permanently, this trick will
work:

    sleep 100000000 >/dev/ttya &
    stty parodd cs8 >/dev/ttya

The "sleep" just holds the tty open for a few years, so that subsequent
"stty" commands will stick.

   John Polstra               jdp at polstra.UUCP
   Polstra & Co., Inc.        ...{uunet,sun}!practic!polstra!jdp
   Seattle, WA                (206) 932-6482