mixing O_NONBLOCK and raw tty causes portability problems

[Matthias Drochner]

[only a short reply, will be AFK until monday night]

nisse@lysator.liu.se said:
> >   - a VTIME timeout begins at the time the read() call is done,
> Sure about that? I believe the timer should be started as soon as a
> character arrives to the tty driver
> [...]
>   "In this case TIME serves as an inter-byte timer and is activated
>    after the first byte is received.

Only a few lines later:
    If the data is in the buffer at
    the time of the read(), the result will be as if the data has been
    received immediately after the read().

> > the result of poll()/select() must not depend on setting
> >     of O_NONBLOCK

Here is a reference:
http://www.opengroup.org/onlinepubs/007908799/xsh/poll.html

> If a blocking
> read wouldn't return immediately due to the VTIME value, then the fd
> is *not* readable.

If you take that and assume that the result of select()/poll()
does not depend on O_NONBLOCK, then your code with VMIN==4
can't work as expected. The callback wouldn't be called
until 4 characters are received.
(D'UNIX appearently behaves this way, but neither Linux nor NetBSD)

The only way around this I can imagine is to have VMIN==1 most
of the time; to get characters coalesced for efficiency, VMAX
could be set to 4 for the time of the read() (and the O_NONBLOCK
cleared). What a mess...
(this would effectively mean that O_NONBLOCK is useless on
the stdin tty descriptor)

> poll works on both linux and Solaris, but the way they set the
> various poll flags is very different.

I wasn't aware of this...

> Exactly which version of netbsd are you using?

Newest bleeding edge current on i386. I can do tests on older
releases if needed, and perhaps also on other architectures
(alpha, mips, m68k).

best regards
Matthias