Q: how to control the TTY output queue in real time?

[Stas Sergeev <stsp@list.ru>]

Hello.

I am writing an app that needs to control the
serial xmit in real-time. What I need is a notification
that the TTY output queue fillup (returned by TIOCOUTQ
ioctl) have dropped below the specified value.
I haven't found anything that can help implementing
this. If I can't get an async notification, the sync
notification will do too, like, for instance, the tcdrain()
call, but with the argument to specify the needed fillup,
below which the function will return.
If there is nothing like this, then even the notification
on every transmitted char will do.
But I've found nothing of the above. :(

Any suggestions how the real-time control can be
implemented?

(please CC me the replies)