> I'd like to propose a new ioctl, to be called FIONWRITE.

Sounds like a good idea to me.  I did something similar, for sockets,
by adding a SO_SNDQLEN option for getsockopt().  Making it parallel to
FIONREAD and making it work on things other than sockets (eg, ttys)
would be a clear improvement.

> I want this call because I have an application which needs to do
> scheduling flow control in userland.

And I wanted it because I wanted to be able to tell whether data were
moving on a TCP connection without depending on back-pressure all the
way up to write() (I didn't have anything to write, so I couldn't use
back-pressure).

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