Re: poll(): IN/OUT vs {RD,WR}NORM

[Johnny Billquist <bqt%softjar.se@localhost>]

On 2024-05-28 03:05, Greg Troxel wrote:
> Johnny Billquist <bqt%softjar.se@localhost> writes:
>
> >       POLLPRI        High priority data may be read without blocking.
> >
> >       POLLRDBAND     Priority data may be read without blocking.
> >
> >       POLLRDNORM     Normal data may be read without blocking.
>
> Is this related to the "oob data" scheme in TCP (which is a hack that
> doesn't work)?   Where do we attach 3 priority levels to data?

At the end of my quoting there were:

     The distinction between normal, priority, and high-priority data is
     specific to particular file types or devices.


So in the context of poll itself, it's undefined. But it's easy to think 
that the TCP urgent data would be something usable in this context. But 
as you note, the urgent data is a somewhat broken thing that noone ever 
really figured out how it was meant to be used or anything about it at 
all. It's not really even oob. And it should basically just go away.

But poll isn't really getting into any details about this. Just that if 
you have some sort of device of file, where you can assign multiple 
priorities to the data, then poll can make use of it.
I have no idea if any such device or file ever have had such a distinction.

  Johnny

--
Johnny Billquist                  || "I'm on a bus
                                  ||  on a psychedelic trip
email: bqt%softjar.se@localhost             ||  Reading murder books
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