tty layer and lbolt sleeps

[Terry Lambert]

Mike Durian wrote:
> I'm trying to implement a serial protocol that is timing sensitive.
> I'm noticing things like drains and reads and blocking until the
> next kernel tick.  I believe this is due to the lbolt sleeps
> in the tty.c code.
> 
> It looks like I can avoid these sleeps if isbackground() returns
> false, however I can't figure out how to make this happen.  The
> process is running in the foreground and my attempts to play
> with the process group haven't helped.
> 
> Can anyone explain what is happening and nudge me towards a fix?

You need your process to become a process group leader, and then
you need the serial port you are interested in to become the
controlling tty for your process.

The first is accomplished with setpgid(2); the second is accomplished
with setsid(2) and open(2) (the open must not specify O_NOCTTY).  You
can move around after that by calling tcsetpgrp(3).

You can only have one controlling tty per process, so if you wanted
to, for example, have a terminal emulation program that would quit
when you turned off your terminal (on-to-off transition of DTR) *and*
you *also* wanted it to receive SIGHUP when you got an on-to-off DCD
transition from a modem, you would need two processes.

See also the source code for getty(8) and the library utility
function login_tty(3).

-- Terry